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Vishay Transducer Application Notes

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Vishay Transducers technical and application information for load cells
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Application Note VPG-05 VPG TRANSDUCERS Current Calibration APPLICATION NOTE Load Cells For technical support, contact in Americas [email protected], in Europe [email protected], in China [email protected], in Taiwan [email protected] www.vpgtransducers.com 1 Document Number: 11860 Revision 12-Dec-2011 Conventional Calibration The conventional method of rationalizing load cell outputs creates problems when load cells are connected in parallel. Multiple load cell systems normally require the individual adjustment of each load cell output to ensure that weight measurements are within tolerance for weight placements within prescribed areas. The individual load cell adjustments are very time-consuming, particularly for high-capacity systems or in hostile environments where containers may need to be emptied and filled several times during calibration. Traditionally, load cell specification sheets quote the rated output of each load cell in voltage, usually mV/V, with a "rationalized" tolerance of 0.1% ( 2 ± 0.002 mV/V ). However when connected in parallel, each load cell will be loaded with the output impedance of the other lo ad cells. As a result the system needs further adjustment in the field to be accurate. The figure opposite shows the electrical diagram of four load cells, connected in parallel. Each load cell can be represented as a voltage source " U" with resistance " R o " (output resistance). Calculations are better understandable when the Norton equivalent circuit is used. The load cell is now represented as a current source, driving current through the parallel combination of the load cell source impedances, where I = U / R o . Example, the following four conventional calibrated load cells are connected in parallel and supplied with an excitation voltage of 10 Vdc: LC Capacity Rated Output (mV/V) Output (mV) R out (Ω) Current (mA) 1 1000 2.001 20.01 350.50 0.0571 2 1000 2.001 20.01 352.00 0.0569 3 1000 2.000 20.00 351.50 0.0569 4 1000 2.002 20.02 351.00 0.0570 Total 4000 2.001 (1) 20.01 (1) 87.81 (2) 0.2279 1) The combined load cell output equals the arithmetic mean value of the individual load cell outputs 2) 1/R t = 1/R 1 + 1/R 2 + 1/R 3 + 1/R 4 ated load cells are connected in parallel and
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Page 1: Vishay Transducer Application Notes

Application Note VPG-05

VPG TRANSDUCERS

Current Calibration

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Load Cells

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

www.vpgtransducers.com1

Document Number: 11860Revision 12-Dec-2011

Conventional Calibration

The conventional method of rationalizing load cell outputs creates problems when load cells are connected in parallel. Multiple load cell systems normally require the individual adjustment of each load cell output to ensure that weight measurements are within tolerance for weight placements within prescribed areas. The individual load cell adjustments are very time-consuming, particularly for high-capacity systems or in hostile environments where containers may need to be emptied and filled several times during calibration.

Traditionally, load cell specification sheets quote the rated output of each load cell in voltage, usually mV/V, with a "rationalized" tolerance of 0.1% ( 2 ± 0.002 mV/V ). However when connected in parallel, each load cell will be loaded with the output impedance of the other lo ad cells. As a result the system needs further adjustment in the field to be accurate.

The figure opposite shows the electrical diagram of four load cells, connected in parallel. Each load cell can be represented as a voltage source "U" with resistance "Ro" (output resistance).

Calculations are better understandable when the Norton equivalent circuit is used. The load cell is now represented

as a current source, driving current through the parallel combination of the load cell source impedances, where I = U / Ro.

Example, the following four conventional calibrated load cells are connected in parallel and supplied with an excitation voltage of 10 Vdc:

LC Capacity Rated Output (mV/V) Output (mV) Rout (Ω) Current (mA)

1 1000 2.001 20.01 350.50 0.0571

2 1000 2.001 20.01 352.00 0.0569

3 1000 2.000 20.00 351.50 0.0569

4 1000 2.002 20.02 351.00 0.0570

Total 4000 2.001(1) 20.01(1) 87.81(2) 0.2279

1) The combined load cell output equals the arithmetic mean value of the individual load cell outputs2) 1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4

CURRENT CALIBRATION

CONVENTIONAL CALIBRATION

The conventional method of rationalising load cell outputs creates problems when load cells are connected in parallel. Multiple load cell systems normally require the individual adjustment of each load cell output to ensure that weight measurements are within tolerance for weight placements within prescribed areas. The individual load cell adjustments are very time-consuming, particularly for high-capacity systems or in hostile environments where containers may need to be emptied and filled several times during calibration.

Traditionally, load cell specification sheets quote the rated output of each load cell in voltage, usually mV/V, with a "rationalised" tolerance of 0.1% ( 2 ± 0.002 mV/V ). However when connected in parallel, each load cell will be loaded with the output impedance of the other load cells. As a result the system needs further adjustment in the field to be accurate.

The figure opposite shows the electrical diagram of four load cells, connected in parallel. Each load cell can be represented as a voltage source "U" with resistance "Ro" (output resistance).

Calculations are better understandable when the Norton equivalent circuit is used. The load cell is now represented as a current source, driving current through the parallel combination of the load cell source impedances, where I = U / Ro.

Example, the following four conventional calibrated load cells are connected in parallel and supplied with an excitation voltage of 10 Vdc:

LC Capacity Rated output (mV/V) Output (mV) Rout ( )ΩΩ Current (mA)

1 1000 2.001 20.01 350.50 0.0571

2 1000 2.001 20.01 352.00 0.0569

3 1000 2.000 20.00 351.50 0.0569

4 1000 2.002 20.02 351.00 0.0570

Total 4000 2.0011) 20.011) 87.812) 0.2279

1) The combined load cell output equals the arithmetic mean value of the individual load cell outputs.2) 2) 1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 10/06-08/02 Page 1

Page 2: Vishay Transducer Application Notes

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Document Number: 11860Revision 12-Dec-2011

www.vpgtransducers.com2

Current Calibration

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

The combined output can also be calculated by multiplying the total current with the combined resistance:

U = It * Rt = 0.2279 * 87.81 = 20.012 ≈ 20.01 mV

The reading when applying a test load of 500 kg on each individual load cell will be:

where:

It=T*S*E / Ro*Emax

Uo=It*Rt

M=Uo*N*Emax / Uoc

It Total current (mA)

T Test load (kg) = 500

S Rated output LCx (mV/V)

E Excitation voltage (V) = 10

Ro Output resistance LCx (Ω)

Emax Rated capacity load cell (kg) = 1000

Uo Total output (mV)

Rt Combined resistance (Ω) = 87.81

M Reading (kg)

N Number of load cells = 4

Uoc Combined output (mV) = 20.01

The readings are based on a full scale calibration. The zero balance (output at no-load) is considered to be 0 mV/V. Hence, if the load cell is not loaded, the current will also be 0 mA.

The example above considers a test load which only acts on one of the four load cells. In practice the test load will be unequally divided over all load cells because of the structure (platform/hopper) of the system. The absolute errors will therefore be smaller, but still considerable.

If all load cells were loaded with 500 kg, the total reading will be 501.05 + 498.91 + 499.39 + 500.61 = 1999.96 ≈ 2000 kg.

These calculations show clearly that the system needs further "corner" adjustment to be accurate. This is usually done in a junction box (signal- or excitation trim), using fixed or variable resistors. But this method has major disadvantages:

Additional temperature-sensitive resistors are being introduced into the system.

Selection of these resistors can be very time-consuming and require the use of deadweight’s.

The process of adjustment must be repeated each time a load cell is exchanged.

A solution used by some load cell manufacturers to improve the overall result is to supply separate resistors with each load cell for use in the output lines to balance up the output resistances. However this does not solve the problem of fitting extra resistors and again these must be changed when any load cell is exchanged.

In General

Typical conventional calibration specifications are:

• Toleranceonratedoutput: ±0.1% (absolute error 0.2%)

• Toleranceonoutputresistance: ±1.0% (absolute error 2.0%)

By combining the three formulas above, it can be recognized that the maximum corner difference is based only on the tolerance on rated output and output resistance:

M=(T*E*Rt*N / Uoc)*(S / Ro)=Const*(S / Rt)

Hence, the maximum corner difference will be:

√(0.22 + 2.02) = 2.01%

Current Calibration makes external balancing resistors unnecessary; allows much quicker on-site set up and calibration; and enables load cells to be replaced in the field without any need to readjust the system.

Current calibrated load cells are rationalized in terms of current output, rather than in terms of voltage output.

During production of load cell "LCx", the output resistance "Rx" is measured. The desired output is then calculated by:

Ux = Iref * Rx

After this calculation the required value for "Ux" is obtained by means of the internal calibration resistors to an accuracy of 0.05%, resulting in identical output current tolerances for each load cell.

Load Applied on LC 1-2-3-4

Total Current It

Total Output Uo

Reading M

500-0-0-0 0.028545 2.5056 501.05

0-500-0-0 0.028423 2.4958 498.91

0-0-500-0 0.028450 2.4982 499.39

0-0-0-500 0.028519 2.5043 500.61

Page 3: Vishay Transducer Application Notes

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Current Calibration

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

www.vpgtransducers.com3

Document Number: 11860Revision 12-Dec-2011

The total output can be calculated by multiplying the total current with the combined resistance:

U = It * Rt = 0.2276 * 87.81 = 19.986 mV

The total output when applying a test load of 500 kg on each individual load cell will be:

The above calculations show clearly that the system needs NO further "corner" adjustment to be accurate.

In General

Typical current calibration specifications are:

• Toleranceonratedoutput:±1.0%

• Toleranceonoutputresistance:±1.0%

• Toleranceonoutputcurrent,Iref: ±0.05% (absolute error 0.1%)

This results in a maximum corner difference of 0.1%, approximately 20 times better than conventional calibrated load cells.

The manner in which the load is transmitted through the load cell has a major impact on the accuracy and repeatability. Current calibrated load cells only perform without corner load differences in a multiple cell system when they are correctly installed:

• Allloadcellsshouldbeplacedonthesamehorizontallevel (corrections can be made by placing thin plates underneath the load cell with minor output).

• Theloadshouldbetransmittedverticallythroughtheload cell (2° out of the perpendicular is already causing an error of approximately 0.061%).

Load Cell Replacement

Although current calibrated load cells remove the need for corner adjustment, calibration should always be checked after replacing a load cell. If the load cell as a current source is considered to be a constant factor, it can be recognized that the calibration change is directly related to the change of combined resistance;

Uo=It*Rt=Const*Rt

Hence, the change of calibration can be calculated by:

(M / N)*a (%)

Where:

M Number of load cells to be replaced

N Number of load cells in the system

a Resistance change in percentages: ((ΣmRnew - ΣmRold) / ΣmRold) * 100%

Example, a load cell with an output resistance of 350.5Ω will be replaced by a load cell with an output resistance of 353.0Ω. The application has a total of four load cells. The resistance change will be:

(353.0-350.5 / 350.5)*100% = 0.71%

The calibration change will be:

(M/N)*0.71% = (1/4)*0.71% = 0.18%

Load Applied on LC 1-2-3-4

Total Current It

Total Output Uo

Reading M

500-0-0-0 0.028450 2.4982 499.99

0-500-0-0 0.028450 2.4982 499.99

0-0-500-0 0.028450 2.4982 499.99

0-0-0-500 0.028450 2.4982 499.99

LC Capacity Rated Output (mV/V) Output (mV) Rout (Ω) Current (mA)

1 1000 1.9943 19.943 350.50 0.0569

2 1000 2.0029 20.029 352.00 0.0569

3 1000 2.0000 20.000 351.50 0.0569

4 1000 1.9972 19.972 351.00 0.0569

Total 4000 1.9986 19.986 87.81 0.2276

Example, the following four current calibrated load cells are connected in parallel and supplied with an excitation voltage of 10 Vdc:

Page 4: Vishay Transducer Application Notes

Application Note VPG-08

VPG TRANSDUCERS

Load Cell Troubleshooting

Load Cells

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

www.vpgtransducers.com1

Document Number: 11867Revision 14-Dec-2011

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Scope

Load cells are designed to sense force or weight under a wide range of adverse conditions; they are not only the most essential part of an electronic weighing system, but also the most vulnerable.

Load cel ls might be damaged because of (shock) overloading, lightning strikes or heavy electrical surges in general, chemical or moisture ingress, mishandling (dropping, lifting on cable, etc.), vibration or internal component malfunction. As a direct result the scale or system might (zero) drift, provide unstable/unreliable readings or not register at all.

This application note is written to assist our customers with potential load cell problems. It describes basic field tests which can be performed on site, and provides the information necessary to interpret the results.

Proper field evaluation is absolutely critical to prevent similarly induced damage in the future! Under no circumstances should fault location, as described below, be attempted on load cells installed in a hazardous area!

In General

Carefully check the system integrity before evaluating the load cells:

• Checkforforceshunts(mightbecausedbydirt,mechanical misalignment or accompanying components such as stay- or check rods).

• Checkfordamage,corrosionorsignificantwearintheareas of load introduction.

• Checkcableconnectionstojunctionboxandindicator.

• Checkthemeasuringdeviceorindicatorwithanaccurate load cell simulator.

Visually inspect the load cells before performing the tests as described on the following pages. Pay particular attention to signs of corrosion (especially around the critical gauge area), the integrity of the cable (might be compromised due to cuts, abrasions, etc) and the condition of the cable entry.

The following test equipment is required to properly evaluate a load cell:

• Ahighquality,calibrated,digitalvolt-andohmmeterwith a measuring accuracy of ±0.5Ω and ±0.1 mV, to measure the zero balance and integrity of the bridge circuit.

• Amegohmmeter,capableofreading5000MΩwithanaccuracyof500MΩat50volts,tomeasuretheinsulation resistance. Do not use megohm meters which supply more than 50 volts to the load cell, in order to prevent permanent damage!

• Ameanstoliftthedeadload(weighbridge,tank,hopper, conveyor, etc.) off the load cell to be able to measure the zero balance or to remove the load cell(s), i.e.acrane,hydraulicjack,etc.

Load cells are produced according to specifications and tolerances which are described in the applicable data sheet.More detailed information can be found on thecalibration certificate which is packed with each load cell.Thecalibrationcertificatementionstheexactvaluesfor the input and output resistance, insulation resistance, zero balance, rated output and the correct wiring code; it provides an important reference for the values which can be measured and should be f iled with the system documentation set.

Page 5: Vishay Transducer Application Notes

VPG-08VPG Transducers

Document Number: 11867Revision 14-Dec-2011

www.vpgtransducers.com2

Load Cell Troubleshooting

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

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Test Procedures and Analysis

The diagram below represents a proposed sequence for testing load cells after a particular system malfunction. Isolate the fault location by moving a relatively small deadweight over each load cell, or by disconnecting load cell by load cell.

Test #1: Zero Balance

The Zero Balance is defined as the load cell output in a "no-load" situation. Therefore, all weight (including deadload) has to be removed from the load cell. Low capacity load cells should be measured in the position in which the load cell is designed to measure force to prevent the weight of the element giving wrong results.

The load cell should be connected to a stable power supply, preferablyaloadcellindicatorwithanexcitationvoltageof at least 10 volts. Disconnect any other load cell for multiple load cell systems.

Measure the voltage across the load cell's output leadswith a millivoltmeter and divide this value by the input orexcitationvoltagetoobtaintheZeroBalanceinmV/V.Compare the Zero balance to the original load cell calibration certificate ( if available ) or to the data sheet.

Analysis

Changes in Zero Balance usually occur if the load cell has been permanently deformed by overloading and/or excessive shocks.Load cells that experienceprogressive

zero output changes per time period are most likely undergoing a change in the strain gauge resistance because of chemical or moisture intrusion. However, in this case the insulation resistance and/or the bridge integrity will also be compromised.

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TEST PROCEDURES AND ANALYSIS

The diagram below represents a proposed sequence for testing load cells after a particular system malfunction. Isolate the fault location by moving a relatively small deadweight over each load cell, or by disconnecting load cell by load cell.

TEST #1: ZERO BALANCE

The Zero Balance is defined as the load cell output in a "no-load" situation. Therefore, all weight (including deadload) has to be removed from the load cell. Low capacity load cells should be measured in the position in which the load cell is designed to measure force to prevent the weight of the element giving wrong results.The load cell should be connected to a stable power supply, preferably a load cell indicator with an excitation voltage of at least 10 volts. Disconnect any other load cell for multiple load cell systems.Measure the voltage across the load cell's output leads with a millivoltmeter and divide this value by the input or excitation voltage to obtain the Zero Balance in mV/V. Compare the Zero balance to the original load cell calibration certificate ( if available ) or to the data sheet.

Input

Output

ANALYSISChanges in Zero Balance usually occur if the load cell has been permanently deformed by overloading and/or excessive shocks. Load cells that experience progressive zero output changes per time period are most likely undergoing a change in the strain gauge resistance because of chemical or moisture intrusion. However, in this case the insulation resistance and/or the bridge integrity will also be compromised.

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 03/6-12/01 Page 3

Test #1 Test #3Test #2 Test #4Zero Balance

OKInsulation

Resistance

Moistureor chemical

ingress

BridgeIntegrity

ShockResistance

OK OK OK

Wrong Wrong Wrong Wrong

Mechanicaloverload

Failed electricalconnection

Broken wireor component

Short circuitto housing / screen

Electrical overloador internal

short circuit

R1kΣ R4

Sudden changein Zero point

Unstable readings,random change

in Zero point

Scale reads overload,incorrect or not at all

Erratic readingswhen load is

applied or removed

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TEST PROCEDURES AND ANALYSIS

The diagram below represents a proposed sequence for testing load cells after a particular system malfunction. Isolate the fault location by moving a relatively small deadweight over each load cell, or by disconnecting load cell by load cell.

TEST #1: ZERO BALANCE

The Zero Balance is defined as the load cell output in a "no-load" situation. Therefore, all weight (including deadload) has to be removed from the load cell. Low capacity load cells should be measured in the position in which the load cell is designed to measure force to prevent the weight of the element giving wrong results.The load cell should be connected to a stable power supply, preferably a load cell indicator with an excitation voltage of at least 10 volts. Disconnect any other load cell for multiple load cell systems.Measure the voltage across the load cell's output leads with a millivoltmeter and divide this value by the input or excitation voltage to obtain the Zero Balance in mV/V. Compare the Zero balance to the original load cell calibration certificate ( if available ) or to the data sheet.

Input

Output

ANALYSISChanges in Zero Balance usually occur if the load cell has been permanently deformed by overloading and/or excessive shocks. Load cells that experience progressive zero output changes per time period are most likely undergoing a change in the strain gauge resistance because of chemical or moisture intrusion. However, in this case the insulation resistance and/or the bridge integrity will also be compromised.

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 03/6-12/01 Page 3

Test #1 Test #3Test #2 Test #4Zero Balance

OKInsulation

Resistance

Moistureor chemical

ingress

BridgeIntegrity

ShockResistance

OK OK OK

Wrong Wrong Wrong Wrong

Mechanicaloverload

Failed electricalconnection

Broken wireor component

Short circuitto housing / screen

Electrical overloador internal

short circuit

R1kΣ R4

Sudden changein Zero point

Unstable readings,random change

in Zero point

Scale reads overload,incorrect or not at all

Erratic readingswhen load is

applied or removed

R≤1 kΩ R∞

Page 6: Vishay Transducer Application Notes

VPG-08VPG Transducers

Load Cell Troubleshooting

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

www.vpgtransducers.com3

Document Number: 11867Revision 14-Dec-2011

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Test #2: Insulation Resistance

The insulation resistance is measured between the load cell circuit and element or cable shield. Disconnect the load cell from the junctionboxor indicator and connect allinput, output and sense (if applicable) leads together.

Measure the insulation resistancewith amegohmmeterbetweenthesefourorsixconnectedleadsandtheloadcellbody. Repeat the measurement between the same 4 or 6 leads and the cable shield. Finally measure the insulation resistance between the load cell body and cable shield.

Never use a megohmmeter to measure the input or output resistance, as it normally operates at a voltage which exceeds the maximum excitation voltage by far!

Analysis

The insulation resistance of all load cells should be 5000MΩormore for bridge circuit to housing, bridgecircuit to cable screen and housing to cable screen.

A lower value indicates electrical leakage, which is usually caused by moisture or chemical contaminations within the

loadcellorcable.Extremelylowvalues(≤1kΩ)indicateashort circuit rather than moisture ingress.

Electrical leakage results usually in unstable load cell or scale reading output. The stability might vary with temperature.

Test #3: Bridge Integrity

The bridge integrity is verified by measuring the input and output resistance as well as the bridge balance. Disconnect theloadcellfromthejunctionboxormeasuringdevice.

The input and output resistance is measured with an ohmmeter across each pair of input and output leads. Compare the input and output resistance to the original calibration certificate (if available) or to the data sheet specifications.

The bridge balance is obtained by comparing the resistance from -output to -input, and -output to +input. The difference between both values should be smaller than, or equal to 1Ω.

Analysis

Changes in bridge resistance or bridge balance are most often caused by a broken or burned wire, an electrical component failure or internal short circuit. This might

result from over-voltage (lightning or welding), physical damage from shock, vibration or fatigue, excessivetemperature, or from production inconsistencies.

TEST #2: INSULATION RESISTANCE

The insulation resistance is measured between the load cell circuit and element or cable shield. Disconnect the load cell from the junction box or indicator and connect all input, output and sense (if applicable) leads together.Measure the insulation resistance with a megohmmeter between these four or six connected leads and the load cell body. Repeat the measurement between the same 4 or 6 leads and the cable shield. Finally measure the insulation resistance between the load cell body and cable shield.Never use a megohmmeter to measure the input or output resistance, as it normally operates at a voltage which exceeds the maximum excitation voltage by far!

Input

Output

ANALYSISThe insulation resistance of all load cells should be 5000 M or more for bridge circuit to housing, bridge circuit toΩΩ cable screen and housing to cable screen.A lower value indicates electrical leakage, which is usually caused by moisture or chemical contaminations within the load cell or cable. Extremely low values (≤ 1k ) indicate a short circuit rather than moisture ingress.ΩΩElectrical leakage results usually in unstable load cell or scale reading output. The stability might vary with temperature.

TEST #3: BRIDGE INTEGRITY

The bridge integrity is verified by measuring the input and output resistance as well as the bridge balance. Disconnect the load cell from the junction box or measuring device. The input and output resistance is measured with an ohmmeter across each pair of input and output leads. Compare the input and output resistance to the original calibration certificate (if available) or to the data sheet specifications.The bridge balance is obtained by comparing the resistance from -output to -input, and -output to +input. The difference between both values should be smaller than, or equal to 1 .ΩΩ

Input

Output

ANALYSISChanges in bridge resistance or bridge balance are most often caused by a broken or burned wire, an electrical component failure or internal short circuit. This might result from over-voltage (lightning or welding), physical damage from shock, vibration or fatigue, excessive temperature, or from production inconsistencies.

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 03/6-12/01 Page 4

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TEST #2: INSULATION RESISTANCE

The insulation resistance is measured between the load cell circuit and element or cable shield. Disconnect the load cell from the junction box or indicator and connect all input, output and sense (if applicable) leads together.Measure the insulation resistance with a megohmmeter between these four or six connected leads and the load cell body. Repeat the measurement between the same 4 or 6 leads and the cable shield. Finally measure the insulation resistance between the load cell body and cable shield.Never use a megohmmeter to measure the input or output resistance, as it normally operates at a voltage which exceeds the maximum excitation voltage by far!

Input

Output

ANALYSISThe insulation resistance of all load cells should be 5000 M or more for bridge circuit to housing, bridge circuit toΩΩ cable screen and housing to cable screen.A lower value indicates electrical leakage, which is usually caused by moisture or chemical contaminations within the load cell or cable. Extremely low values (≤ 1k ) indicate a short circuit rather than moisture ingress.ΩΩElectrical leakage results usually in unstable load cell or scale reading output. The stability might vary with temperature.

TEST #3: BRIDGE INTEGRITY

The bridge integrity is verified by measuring the input and output resistance as well as the bridge balance. Disconnect the load cell from the junction box or measuring device. The input and output resistance is measured with an ohmmeter across each pair of input and output leads. Compare the input and output resistance to the original calibration certificate (if available) or to the data sheet specifications.The bridge balance is obtained by comparing the resistance from -output to -input, and -output to +input. The difference between both values should be smaller than, or equal to 1 .ΩΩ

Input

Output

ANALYSISChanges in bridge resistance or bridge balance are most often caused by a broken or burned wire, an electrical component failure or internal short circuit. This might result from over-voltage (lightning or welding), physical damage from shock, vibration or fatigue, excessive temperature, or from production inconsistencies.

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 03/6-12/01 Page 4

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Page 7: Vishay Transducer Application Notes

VPG-08VPG Transducers

Document Number: 11867Revision 14-Dec-2011

www.vpgtransducers.com4

Load Cell Troubleshooting

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

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Test #4: Shock Resistance

The load cell should be connected to a stable power supply, preferablyaloadcellindicatorwithanexcitationvoltageof at least 10 volts. Disconnect all other load cells for multiple load cell systems.

With a voltmeter connected to the output leads, lightly rap on the load cell with a small mallet to mildly shock it. Exercise extreme care not to overload low capacity load cells while testing their shock resistance.

Watch the readings during the test. The readings should not become erratic, should remain reasonably stable and return to original zero readings.

Analysis

Erratic readings may indicate a failed electrical connection or a damaged glue layer between strain gauge and element as a result of an electrical transient.

TEST #4: SHOCK RESISTANCE

The load cell should be connected to a stable power supply, preferably a load cell indicator with an excitation voltage of at least 10 volts. Disconnect all other load cells for multiple load cell systems. With a voltmeter connected to the output leads, lightly rap on the load cell with a small mallet to mildly shock it. Exercise extreme care not to overload low capacity load cells while testing their shock resistance. Watch the readings during the test. The readings should not become erratic, should remain reasonably stable and return to original zero readings.

Input

Output

ANALYSISErratic readings may indicate a failed electrical connection or a damaged glue layer between strain gauge and element as a result of an electrical transient.

LOAD CELL EVALUATION FORM

A load cell evaluation form is included in this application note. The form should be used as a guide for testing and evaluating load cells. We recommend this form is included in the customer dossier and its use as a basis to discuss the test results and diagnostics with third parties.If a load cell is returned to Vishay Revere Transducers, the Evaluation Form will assist our repair department in further diagnostics and repairing the cell.

Customer support:

The Vishay Revere Transducers combines sixty years of load cell manufacturing with sixty years of application know how. For further information, please contact our manufacturing operation or any one of our regional sales offices.

Vishay Revere Transducers B.V.Ramshoorn 7Postbus 6909, 4802 HX BredaThe NetherlandsTel. (+31)76-5480700Fax. (+31)76-5412854

Website: www.revere.nlEmail: [email protected]

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 03/6-12/01 Page 5

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Load Cell Evaluation Form

The following load cell evaluation form should be used as a guide for the testing and evaluation of load cells. We recommend that this form be included in the customer dossier and used as the basis to discuss the test results and diagnostics with third parties.

If a load cell is returned to VPG Transducers, the Evaluation Form will assist our repair department in further diagnostics and repair of the cell.

Page 8: Vishay Transducer Application Notes

VPG-08VPG Transducers

Load Cell Troubleshooting

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Load Cell Evaluation Form

Company: _______________________________________________ Contact person: __________________________________Address: __________________________________________________ City / Country: __________________________________Tel./Fax.: _________________________________________________ Repair order: _________ Date: ___________________

Load cell type: _____________________________________________ Serial number: __________________________________Capacity: ________________________________________________ Accuracy grade: __________________________________

Short description of system failure and application: ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Electrical InspectionBridge Measurements Actual Specification Conclusion

Zero balance mV/V ≤±1% of rated span OK Wrong

Input resistance Ω Ω±1% OK Wrong

Output resistance Ω Ω±1% OK Wrong

Output – to input – Ω OK Wrong

Out put – to input + Ω difference ≤±1% OK Wrong

Insulation Resistance Actual Specification Conclusion

Bridge to housing MΩ ≥5000 MΩ OK Wrong

Bridge to shield MΩ ≥5000 MΩ OK Wrong

Shield to housing MΩ ≥5000 MΩ OK Wrong

Visual Inspection

Label OK Unreadable Missing

Condition Like new Broken welds

Cable cut Visual mechanical overload

J-boxdamage Dents/cracks in parts

Corroded parts Weld(s) Housing/element

J-box/cableentry Top/bottom plate

Diaphragm Bellow/tube/cubs

Affected by chemicals No Unknown Yes: _______________

Expected Reason for Failure Moistureingress Short circuit Broken wire/component Excessiveheat

Electrical transients Mechanicaloverload Excessivecorrosion Broken cable

Other: ____________________________________________

Recommendation

Return load cell to supplier for further evaluation and repair (if possible) Return load cell to supplier for warranty Load cell beyond (economic) repair ________________________________________ ________________________________________

Page 9: Vishay Transducer Application Notes

Application Note VPG-04

VPG TRANSDUCERS

Environmental Conditions

Load Cells

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Document Number: 11861Revision 04-Dec-2011

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Scope

Most of the process industries rely heavily upon strain gage based load cells for accurate and consistent weight data and expect them to perform under a wide range of adverse conditions, including mechanical and chemical attack.

The problem is that premature failure of load cells can have far reaching effects on the overall processes, and have a consequent impact on cost, safety and product reputation. Replacing a load cell in the field will involve not only the cost of the component itself, but also the expenses associated with labour, downtime and re-calibration.

There are several factors that can cause load cell failure, one of the most important is the environment. This application note takes a look at the effects the environment can have on load cells and offers guidelines on how to minimize these effects through proper selection and application. In addition,existing load cell classification standards are overviewed.

Classification Standards

No area of load cell operation causes more confusion and contention than that of environmental protection and sealing standards. Although the weighing and load cell industries have in-depth standards and test procedures to define load cell and weighing system performance, no standards have been developed to cover product suitability for specific environmental conditions.

In the absence of such standards, most manufacturers have adopted the IP classification (Ingress Protection by IEC/EN60.529 or DIN 40.050) or National Electrical Manufacturers Assoc iat ion Standards (NEMA) Publication 250 classifications to define the level of sealing for their products. Both standards are good test procedures for environmental sealing when applied to the products for which they were intended - those being electrical enclosures, but they are not very well suited to load cells.

IP Classification

• Protectionofpersonsagainstaccesstohazardousparts inside the enclosure.

• Protectionoftheequipmentinsidetheenclosureagainst the ingress of solid foreign objects.

• Protectionofequipmentinsidetheenclosureagainstharmful effects due to the ingress of water.

The IP code consists of five categories or brackets identified by a number or letter that indicate the degree of some element to the standard. The first characteristic number relates to access to the hazardous part by persons or solid foreign objects. A number from 0 - 6 defines the physical size of the accessing object.

Numbers 1 and 2 relate to solid objects and parts of the human anatomy, while 3 to 6 relate to solid objects such as tools, wire and dust particles. As shown in the accompanying table on the next page, the higher the number, the smaller the accessing object.

Most load cell manufacturers use the number 6 for this category to signify that their products are dust tight. However the effectiveness of this classification depends on what constitutes an enclosure. Of particular significance here are load cells of a more open nature, such as single point cells, where the introduction of a tool, such as a screwdriver, could have catastrophic results even though the load cells are dust tight with regard to the critical components.

The second characteristic number relates to the entrance of water with what is described as harmful effects. Unfortunately, the standard does not define harmful. Presumably, for electrical enclosures, the main problem with water could be one of electrical shock to persons in contact with the enclosures, rather than the malfunctioning of the unit. The characteristic describes conditions ranging from vertically dripping water, through spraying and jetting, to continuous immersion.

Load cell manufacturers usually adopt either the 7 or the 8 designation for their products. However the standard clearly states that "An enclosure designated with a second characteristic number 7 or 8 is considered unsuitable for exposure to water jets (designated by the second characteristic 5 or 6) and need not comply with requirements for number 5 or 6 unless it is dual coded, e.g. IP66/IP68". In other words, under certain conditions and for certain product designs, a product that passed a half-hour immersion test may not necessarily pass one that involves high pressure water jets from all angles.

Page 10: Vishay Transducer Application Notes

VPG-04VPG Transducers

Document Number: 11861Revision 04-Dec-2011

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Environmental Conditions

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As with IP66 and IP67, IP68 conditions are set by the manufacturer of the product, but must be at minimum more severe than for IP67 (i.e. longer duration or greater depth of immersion). The requirements for IP67 are that the enclosure can withstand immersion to a maximum depth of 1 meter for 30 minutes.

While the IP standard is an acceptable starting point there are shortcomings:

• TheIPdefinitionofenclosureistooloosetobemeaningful for load cells.

• TheIPsystemonlyrelatestowaterentranceandignores moisture, chemicals etc.

• TheIPsystemcannotdifferentiatebetweenloadcellswith different constructions with the same IP rating.

• Nodefinitionisgivenfortheterm"harmfuleffects",so the effect on load cell performance is open to interpretation.

NEMA Classification

Classifications in the NEMA system run from NEMA 1 to NEMA 12, but load cell manufacturers concern themselves with NEMA 4 and NEMA 6. Unlike the IP system, NEMA does concern itself with environmental conditions such as corrosion, rust, icing, oil and coolants.

NEMA 4 enclosures are intended for indoor and outdoor

use, and provide a degree of protection against windblown dust and rain, splashing water, and hose directed water. However, no consideration is given for the effects of internal condensation. NEMA 4X enclosures meet the same standards as NEMA 4 and are constructed of 304 stainless steel or other material offering equal corrosion resistance.

NEMA 6 enclosures are used where there is a chance of temporary immersion. The standard calls for the highest part of the enclosure to remain submerged in water, with its highest point 1.83 metres below the surface for 30 minutes. NEMA 6P enclosures are used where prolonged immersion may occur and resistance to corrosion is needed.

While it may seem that NEMA standards offer some advantages over the IP system for corrosion resistance, they only relate to external corrosion of enclosures. This is very limited when applied to the more complex load cell construction and the different effects of corrosion or water. Also, neither system concerns itself with internal condensation or the subject of cable entry into the enclosures.

Damp Heat Cycling

The IP standard clearly states that it does not deal with internal condensation or moisture within the enclosure, saying that this is the responsibility of the relevant product

IP First Number Protection Against Solid Objects

IP Second Number Protection Against Liquids

0 No protection 0 No protection

1Protected against solid objects up to 50 mm e.g. accidental touch by hands

1Protected against vertically falling drops of water (e.g. condensation)

2Protected against solid objects up to 12 mm e.g. fingers

2Protected against direct sprays of water up to 15º from the vertical

3Protected against solid objects more than 2.5 mm e.g. tools and small wires

3Protected against direct sprays of water up to 60º from the vertical

4Protected against solid objects more than 1 mm e.g. small wires

4Protected against water sprayed from all directions, limited entrance allowed

5Protected against dust-limited entrance (no harmful deposit)

5Protected against low pressure jets of water from all directions, limited entrance allowed

6 Totally protected against dust 6Protected against strong jets of water e.g. for use on ship decks , limited entrance allowed

7Protected against the effects of immersion between 15 cm and 1 m

8Protected against long periods of immersion under pressure

Page 11: Vishay Transducer Application Notes

VPG-04VPG Transducers

Environmental Conditions

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standard. However, moisture or condensation is of vital importance in load cell operation.

Moisture may enter the inside of the load cell over a long period and have a catastrophic effect, especially when acids or alkalis are present. One test used to determine a load cells ability to withstand moisture or condensation is the Damp Heat Cycling Test. Although there are several versions of the test, the one most universally accepted is(IEC) 68-2-30.The object of the IEC standard is "Todetermine the suitability of components, equipment, or other articles for use and storage under conditions of high humidity when combined with cyclical temperature changes".

It is obvious that this standard is a much more useful classification than the IP rating when it comes to defining load cell environmental suitability.

Load cells certified to OIML R-60 are tested to withstand 12 damp heat cycles of 24 hours each. Load cells which are not suited to withstand this test should be marked with "NH"(non-humidity) behind the appropriate accuracygrade.

Load Cell Construction

Besides a given IP-rating or NEMA-classification load cells should also be classified according to their design in terms of cable entry, material of construction and gages sealing method. Load cells can be divided into six main groups in terms of sealing:

1009080

70

+ 50

+ 2 5

Time ( hr )0 3 12 13.5 18 24

Am

bien

t tem

pera

ture

°CR

elat

ive

hum

idity

%

1 Open IP64 The gages have a minimum basic coating, but no formal potting

2 Potted IP65 Critical areas are covered with potting compound, but no mechanical protection

3 Enclosed IP67Critical areas are fully potted and mechanically protected with rubber bellows or side plates; have standard cable entry

4 Enclosed IP66/IP67Critical areas (are potted) and protected with welded covers (bellows, cups, etc.); have standard cable entry

5 Enclosed IP66/IP68Critical areas (are potted) and protected with welded covers and have water block cable entry

6Hermetically Sealed

IP66/IP68Critical areas (are potted) and protected with welded covers and have glass to metal sealed cable entry

Page 12: Vishay Transducer Application Notes

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Document Number: 11861Revision 04-Dec-2011

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Environmental Conditions

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Whether or not the products 4 to 5 in the above listing meet the IP66 classification may depend upon how the load cells are used and whether additional mechanical protection is provided. In parallel with these six classifications, different load cells are constructed from different materials. The main ones are aluminium; copper beryllium; tool steel-painted;tool steel-nickel plated or stainless steel. Certain products may be a combination of these e.g. tool steel-nickel plated body with stainless steel bellows or cups.

The Cable Entry

Whilst it is relatively common to weld-seal critical areas on a load cell body, one potential problem area is the cable entry. A variety of methods are used to make sure cells are properly sealed at this point.

1) In some load cells the main cable enters through a conventional cable gland directly into the gage area. Regardless of how well the gage area is sealed, however moisture and solvents can penetrate either around the gland or through the centre of the cable itself. Often, temperature changes cause a pumping action to occur, pushing moisture down the inside of the cable. Entry also can be via a leaking junction box or through a damaged part of the cable. This can take some time to reach critical areas, but once there it will become sealed in place to do its damage.

2) An improvement on the cable gland is a water block at the point of cable entry. Here, the main cable terminates at for example a small circuit board with on-going wires leading tot he gage area. The block is fully potted to prevent moisture or other contaminants from reaching the critical areas.

3) The best solution is the use of a glass-to-metal cable entrance. This prevents any contamination from reaching the gage or other critical areas. In addition, the manufacturing process used must keep the load cell free from residue contaminations. The problem of residuals is usually solved by purging the internal cavity with helium. VPG Transducers model RLC is first filled with helium, which allows leaks to be found with conventional leak detection equipment and just before closing the load cell the helium will be replace by argon.

Corrosion Resistance

The corrosion resistance of load cells is a very complex subject, one that is further complicated by the variety of available configurations. As a result it is only possible to use standard corrosion charts as guidance for load cells. In addition, the following factors must be considered:

• Surfacefinish

• Weldareasaroundseals,bellowsandcups

• Thicknessofseals

• Varyingconstructionmaterials

• Highstresslevelsatloadingpoints

The environment itself plays a large part in how a particular load cell type behaves in practice. Salt water, for example,has different corrosion effects depending on the local circumstances. Stainless steel in stagnant salt water is subject to crevice corrosion and a regular wash down is necessary to avoid degradation.

Unfortunately the term stainless steel has become synonymous with "no corrosion, no problem and nomaintenance".While stainless steel load cells usuallyoffer optimum protection in most environments, other factors should be taken into account. In certain applications,painted or plated load cells may offer better long-term protection.

Additional Coatings

Protective coatings are the oldest and most widely used method of corrosion control. Special paints are often used after installation to protect load cells and mounting hardware. However, the effectiveness of these is dependant upon initial surface preparation as well as the specific environment (care should be exercised when preparing the surface to prevent any damage to the load cell itself; if in doubt, please contact our manufacturing operation):

• Onethatprovidesaninertbarrieragainstattack(paint)

• Onethatprovidesreactive(galvanic)protection,suchas cadmium plating

An alternative is wrap-around protective covers. These can provide good environmental protection, but can be self-destructive if corrosive material is trapped inside the cover. Paint or plating can not always protect the load application point on certain load cell designs.

The suitability of any paint-like protective coating should be checked with the end-user of the load cell as well as the supplier of the coating. The influence on accuracy after coating is usually negligible small, but the coating must be"f lexible" enough towithstand the def lection of theload cell. Very suitable flexible coatings with an excellent resistance to most harsh environments can often be found at a car-shop.

Page 13: Vishay Transducer Application Notes

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Environmental Conditions

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Preventative Maintenance

Preventative maintenance is often overlooked or ignored by both load cell users and service companies. However, the regular service and maintenance of load cells in a weighing system will greatly improve their long-term reliability and performance as well as greatly reduce their sensitivity to corrosion. Maintenance inspections can be divided into two categories:

• Routine:Performedatperiodicintervals,itincludesthe removal of any material or debris build-up from around the load cells and mounting fixtures. Serious damage can occur to the load cells if mounting systems do not function correctly. Any damage or degradation of surface coatings should be remedied and all cables and junction-boxes should be checked. To minimize the effects of flooding,any drainage systems in the pit should be free from debris. Where required, regular wash down of the load cell should be carried out to prevent chemical attack.

• Adhoc:Madeimmediatelyafteranyadverseorunexpected events such as flash floods, gales, seismic activity or electrical storms.

In general, careful consideration must be given to any reason for failure. If this has occurred as a result of ingress of water or chemicals, then continued deterioration of any other load cell(s) in the system can be expected, resulting in mechanical failure. This failure can have serious safety and cost consequences.

The chart on the right should only be used as guidance. Acids, bases and salts in a solution of water. More information about specific substances is available on request. Resistance designation:

0 Not affected

1 Slightly affected, additional protection recommended

2 Severely affected, additional protection necessary

3 Not applicable

Corrosion Chart

con (%)

T (ºC)

1.4542 1.4301

1.4403 1.4568

H2O4 (sulfuric acid)

0.0 60 0 0

1 20 1 0

2 60 1 0

5 35 2 1

10 20 1 0

20 20 1 0

20 35 2 2

25 25 1 2

40 20 2 1

>40 >20 3 3

HCI (hydrogen chloride)

0.2 20 1 0

0.2 50 2 0

1 20 2 0

2 20 1 1

>2 >20 3 3

HNO3 (nitric acid)

50 65 0 0

60 20 0 0

60 Boil 1 1

65 Boil 2 2

>90 Boil 3 3

H3PO4 (phosphoric acid)

30 102 0 0

50 108 1 0

50 Boil 2 1

60 20 0 0

60 Boil 3 3

80 20 1 0

80 Boil 3 3

CH3-COOH

10 20 0 0

10 Boil 1 0

80 20 0 0

80 Boil 2 1

HCOOH (formic acid)

10 20 1 0

10 Boil 2 1

40 65 2 0

H2CO3 <100 <Boil 0 0

Chart continues next page

Page 14: Vishay Transducer Application Notes

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Document Number: 11861Revision 04-Dec-2011

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Environmental Conditions

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Corrosion Chart (cont.) Conclusion

Selecting the wrong load cell for a particular application in terms of environmental compatibility can have far reaching consequences in terms of costs, safety and product reputation. Current classifications fall well short of defining adequate environmental standards for load cells. As a result,this subject needs careful review by both load cell users and manufacturers to ensure that clear guidelines are available.

Users should be able to compare like-for-like features when selecting products from different manufacturers. If in doubt,they should ask pertinent questions relating to:

• Constructionoftheloadcell

• Cableentrymethod

• Pastexperiences(long-termenvironmental success stories)

For applications in harsh environments, additional protection for the load cells may be needed to assure their reasonable working life. This can be achieved with enhanced scale designs and the use of additional coatings on the load cell, such as paints, greases and plating. The scale or system design should minimize the possibility of material build-up around the cells. If appropriate, the design should also provide mechanical protection from the effects of direct water and solvents whilst cleaning. Sealing compounds and rubbers used on some load cells can deteriorate when exposed to chemicals or direct sunlight. Because they embrittle rubber, chlorine-based compounds are a particular problem.

Load cells correctly selected and regularly maintained should be capable of a working life in excess of ten years. There are always exceptions, but the engineer needs to be able to obtain the optimum performance out of his or her selected load cells.

con (%)

T (ºC)

1.4542 1.4301

1.4403 1.4568

NaOH (sodium hydroxide)

25 Boil 0 0

30 Boil 2 0

34 20 0 0

34 Boil 2 2

50 20 1 1

60 Boil 3 3

Ca(OH)2 <100 <Boil 0 0

NH4OH <100 <Boil 0 0

NaNO3 <100 <Boil 0 0

NA2CO3<100 20 0 0

100 820 3 3

NaCI (sodium chloride)

<100 30 0 0

100 >30 2 0

NH4CI

10 20 0 0

10 Boil 1 0

25 20 1 0

25 Boil 2 1

(NH4)2SO4 (ammonium sulfate)

5 20 0 0

10 20 1 0

10 Boil 2 0

FeCI2 10 25 0 0

FeCI31 20 2 1

5 20 3 3

K2CO3 30 65 0 0

HBr/HF 3 3

Acetone 100 <Boil 0 0

Ether 100 <Boil 0 0

Page 15: Vishay Transducer Application Notes

Application Note VPG-06

VPG TRANSDUCERS

Vessel Weighing

Load Cells

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Scope

Load cells may be used to weigh vessels in various installation configurations. The installation of load cells into a practical field application requires following several basic rules as well as careful design attention if the system has to be accurate and provide a long, maintenance free span of operation.

This application note describes the options and external influences, applicable for the design of a weighing vessel, such as type and number of load cells to use, mode of operation, overall accuracy required and piping.

Accuracy

Accuracy requirements for load cells used in scales for trade are clearly defined by Weights and Measures Authorities. For process weighing applications it is more difficult to define accuracy and usually it is requested for a system "to be as accurate as possible".

Calculating true system accuracy is possible by adding the individual errors of the external influences and should be done in the very early stage of design. Determined by the application, weighing systems can be divided into the following categories:

The maximum achievable system accuracy equals approximately 5000 divisions, i.e. 1 kg divisions for a weighing system with a capacity of 5 tons. However the accuracy of most process weighing applications is limited to approximately 750 divisions due to external influences.

Mechanical Considerations

It is a common misconception that a load cell can be considered as a solid piece of metal on which vessels, silos or hoppers can be supported. The performance of a load cell depends primarily on its ability to deflect under highly repeatable conditions when load is applied or removed. More importantly, if more than one load cell is used then

the deflection and output of each load cell should be equal for equal loading.

The general considerations to design a weighing vessel are:

• Usearigidfoundationformaximumaccuracy.

• Avoidforceshuntsbetweenthefoundationandthevessel as much as possible.

• Keepclearancearoundthevesselandsufficientclearance between the foundation and vessel.

• Trytoincorporateacalibrationfacilityontothevessel.

• Avoidsloshingofliquids,bydividingthevesselintocompartments.

• Payattentiontomaterialentryandexit;avoidimpactforces due to material flow. Realize the air flow and air pressure due to material flow.

• Pipeconnectionsandotherexternalequipmenttothevessel should be as flexible as possible.

• Foroutdoorinstallation;realizethewindinfluence.

• Systemintegrityisvirtuallyimportant;usesafetysystems if necessary.

• Realizetheinfluenceoftemperaturedifferencesofthevessel and its connections.

Mode of Operation

Loadcellsmeasureforceinoneoftwodirections;tensionor compression. In the tensile mode the vessel is suspended from one or more load cells. In the compressive mode a vessel is supported by load cells.

The use of a tension or compression system depends upon the mechanical structure around the vessel and the ease of making the system. If a vessel must be placed on an open concrete pad, compression will be a logical way to operate, because a tension system would require an expensive additional overhead supporting structure.

As a general rule, if a suitable structure for a tension application is available, it is usually easier, more acceptable and less costly to suspend the vessel from one or more load cells up to a vessel capacity of 15 tons. When the vessel capacity exceeds this value, the physical size of the load cells and the tension rods become large, installation

High accuracy ±0.02% to ± 0.10% Scales for trade

Medium accuracy ±0.10% to ± 0.50% General purpose

Low accuracy ±0.50% to ± 5.00% Level detection

Page 16: Vishay Transducer Application Notes

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Document Number: 11873Revision 07-Dec-2011

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Vessel Weighing

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becomes more difficult and there is more cost involved in making the required hardware than providing an adequate base for compression assemblies. Furthermore, a large tension system has a low natural frequency, which might cause the indicator to bounce up and down objectionable. Stiff, lowdeflection supportingmembers are thereforedesirable.

In theory, suspension of a vessel by a single load cell may be the ideal solution, but such tension installations are not usually feasible. Three of four point supports are the most commonly used configurations.

The Number of Load Cells

The number of load cells to support a vessel is usually fixed by the design of the vessel, especially for an existing system. The most ideal situation is to support a vessel by three load cells. If a weighing vessel is supported by four or more load cells and the stiffness of the vessel is to high, the construction might be statically undefined. In this case three or in the worse case only two load cells will bear the total weight. A high vertical vessel, especially with a closed top is very stiff.

When only two load cells bear the total weight, an overload situation on these cells might occur. By measuring the output of every individual load cell (before filling the

vessel), such a situation can be recognized and corrected by placing shim plates underneath the cells with minor output.

The load cells should be positioned in such a way that each load cell will bear the same amount of weight. This can be established by calculating the sum of moments on each side of the Centre of Gravity (C of G) which should be equal. The moment of each individual load cell equals the product of the force and the perpendicular distance of that load cell to the center of gravity.

The load cells should be positioned in such a way that each load cell will bear the same amount of weight. This can be established by calculating the sum of moments on each side of the Centre of Gravity ( C of G ) which should be equal. The moment of each individual load cell equals the product of the force and the perpendicular distance of that load cell to the centre of gravity.

Horizontal and vertical vessel, supported by four load cells.

Horizontal and vertical vessel, supported by three load cells.

PIVOT WEIGHING

In certain applications it is possible to weigh only half the vessel, the other half is supported on dummy load cells or flexure beams acting as pivots. Such a system can only be used when weighing a symmetrical vessel containing liquids. Solid materials will pile-up on the sides and will cause a shift of the centre of gravity. The accuracy that can be obtained with these systems is less than with an entire supported vessel. In practice, accuracies of ±0.5% can be achieved. Pivot weighing provides an excellent, low cost level detection system. In fact not the weight, but the force is measured by the load cell(s). The force on the load cell(s) can be calculated by :

Flc = (d*Ftot)/l

The distance "l" between the live and dummy cells should be as long as possible to achieve maximum accuracy. Horizontal forces on the vessel out of the plane of the pivots must be avoided (Wind forces on vertical outdoor vessel!).

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 07/07-06/03 Page 3

C of G

1/2 l1/2 l

1/2 l 1/2 l

1/2 b

1/2 b1/2 b

1/2 b

2/3 l

2/3 l

1/3 l

1/3 l

1/2 b

1/2 b1/2 b

1/2 b

d

l

C of G

Live cellDummy cell

The load cells should be positioned in such a way that each load cell will bear the same amount of weight. This can be established by calculating the sum of moments on each side of the Centre of Gravity ( C of G ) which should be equal. The moment of each individual load cell equals the product of the force and the perpendicular distance of that load cell to the centre of gravity.

Horizontal and vertical vessel, supported by four load cells.

Horizontal and vertical vessel, supported by three load cells.

PIVOT WEIGHING

In certain applications it is possible to weigh only half the vessel, the other half is supported on dummy load cells or flexure beams acting as pivots. Such a system can only be used when weighing a symmetrical vessel containing liquids. Solid materials will pile-up on the sides and will cause a shift of the centre of gravity. The accuracy that can be obtained with these systems is less than with an entire supported vessel. In practice, accuracies of ±0.5% can be achieved. Pivot weighing provides an excellent, low cost level detection system. In fact not the weight, but the force is measured by the load cell(s). The force on the load cell(s) can be calculated by :

Flc = (d*Ftot)/l

The distance "l" between the live and dummy cells should be as long as possible to achieve maximum accuracy. Horizontal forces on the vessel out of the plane of the pivots must be avoided (Wind forces on vertical outdoor vessel!).

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 07/07-06/03 Page 3

C of G

1/2 l1/2 l

1/2 l 1/2 l

1/2 b

1/2 b1/2 b

1/2 b

2/3 l

2/3 l

1/3 l

1/3 l

1/2 b

1/2 b1/2 b

1/2 b

d

l

C of G

Live cellDummy cell

Try to incorporate a calibration facility on to the vessel. Avoid sloshing of liquids, by dividing the vessel into compartments. Pay attention to material entry and exit; avoid impact forces due to material flow. Realize

the air flow and air pressure due to material flow. Pipe connections and other external equipment to the vessel should be as flexible as

possible. For outdoor installation; realize the wind influence. System integrity is virtually important; use safety systems if necessary. Realize the influence of temperature differences of the vessel and its connections.

MODE OF OPERATION

Load cells measure force in one of two directions; tension or compression. In the tensile mode the vessel is suspended from one or more load cells. In the compressive mode a vessel is supported by load cells.The use of a tension or compression system depends upon the mechanical structure around the vessel and the ease of making the system. If a vessel must be placed on an open concrete pad, compression will be a logical way to operate, because a tension system would require an expensive additional overhead supporting structure.As a general rule, if a suitable structure for a tension application is available, it is usually easier, more acceptable and less costly to suspend the vessel from one or more load cells up to a vessel capacity of 15 tons. When the vessel capacity exceeds this value, the physical size of the load cells and the tension rods become large, installation becomes more difficult and there is more cost involved in making the required hardware than providing an adequate base for compression assemblies. Furthermore, a large tension system has a low natural frequency, which might cause the indicator to bounce up and down objectionable. Stiff, low deflection supporting members are therefore desirable.In theory, suspension of a vessel by a single load cell may be the ideal solution, but such tension installations are not usually feasible. Three of four point supports are the most commonly used configurations.

THE NUMBER OF LOAD CELLS

The number of load cells to support a vessel is usually fixed by the design of the vessel, especially for an existing system. The most ideal situation is to support a vessel by three load cells. If a weighing vessel is supported by four or more load cells and the stiffness of the vessel is to high, the construction might be statically undefined. In this case three or in the worse case only two load cells will bear the total weight. A high vertical vessel, especially with a closed top is very stiff.When only two load cells bear the total weight, an overload situation on these cells might occur. By measuring the output of every individual load cell ( before filling the vessel ), such a situation can be recognized and corrected by placing shim plates underneath the cells with minor output.

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 07/07-06/03 Page 2

Horizontal and vertical vessel, supported by four load cells

Horizontal and vertical vessel, supported by three load cells

Page 17: Vishay Transducer Application Notes

VPG-06VPG Transducers

Vessel Weighing

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Pivot Weighing

In certain applications it is possible to weigh only half the vessel, the other half is supported on dummy load cells orflexurebeamsactingaspivots.Suchasystemcanonlybe used when weighing a symmetrical vessel containing liquids.Solidmaterialswillpile-upon the sidesandwillcause a shift of the center of gravity. The accuracy that can be obtained with these systems is less than with an entire supported vessel. In practice, accuracies of ±0.5% can be achieved.Pivotweighingprovides an excellent, low costlevel detection system.

In fact not the weight, but the force is measured by the load cell(s). The force on the load cell(s) can be calculated by:

Flc = (d*Ftot)/l

The distance "l" between the live and dummy cells should be as long as possible to achieve maximum accuracy.

Horizontal forces on the vessel out of the plane of the pivots must be avoided (wind forces on vertical outdoor vessel!).

Besides dummy load cells, flexure beams are often used to provide the vessel to pivot with the load cell deflection. It is essential to align the beam webs very carefully for maximum accuracy.

The main advantage of flexure beams is their ability to take up horizontal side forces. Therefore no constrainers are necessary to get a stable construction.

The selection of the flexure beam must be based on the ability of the beam to bear the weight of the vessel without bending of the web (buckling effect).

Excessive bending of the pivoting beams, during installation should be avoided.

Load Cell and Mount Selection

VPGTransducersoffersawiderangeofindustrialloadcellsand mounts, with a capacity from 6 kg to approximately 200t.Theloadcellelementsaremadeofnickel-platedtoolsteel or (more suitable for the process industry) stainless steel.

The selection of which capacity to use in a weighing application should be based on the following factors :

• Determinethemaximumweightoftheappliedload,or "Live Load".

• Calculatetheweight,"tare",oftheconstruction,or"Dead Load".

• Determinethenumberofloadcellstobeusedinthestructure (N).

• Checkthepossiblepresenceofunequalloadingconditions ( factor fa). This factor is an allowance for low tare estimates and unequal load distribution. Standard:fa = 1,3.

• Checkonextrafactorsasvibration,shocketc( factor fb).Thisfactorisadynamicloadfactor;forstaticweighing fb = 1.

• Foroutdoorvessels,calculatethewindforceFw (applicationnote09/3-01/01).

The individual minimum load cell capacity can be calculated by :

Fw+( fa* fb*(LiveLoad+DeadLoad)/N)

The load cells should be positioned in such a way that each load cell will bear the same amount of weight. This can be established by calculating the sum of moments on each side of the Centre of Gravity ( C of G ) which should be equal. The moment of each individual load cell equals the product of the force and the perpendicular distance of that load cell to the centre of gravity.

Horizontal and vertical vessel, supported by four load cells.

Horizontal and vertical vessel, supported by three load cells.

PIVOT WEIGHING

In certain applications it is possible to weigh only half the vessel, the other half is supported on dummy load cells or flexure beams acting as pivots. Such a system can only be used when weighing a symmetrical vessel containing liquids. Solid materials will pile-up on the sides and will cause a shift of the centre of gravity. The accuracy that can be obtained with these systems is less than with an entire supported vessel. In practice, accuracies of ±0.5% can be achieved. Pivot weighing provides an excellent, low cost level detection system. In fact not the weight, but the force is measured by the load cell(s). The force on the load cell(s) can be calculated by :

Flc = (d*Ftot)/l

The distance "l" between the live and dummy cells should be as long as possible to achieve maximum accuracy. Horizontal forces on the vessel out of the plane of the pivots must be avoided (Wind forces on vertical outdoor vessel!).

VISHAY REVERE TRANSDUCERS APPLICATION NOTE 07/07-06/03 Page 3

C of G

1/2 l1/2 l

1/2 l 1/2 l

1/2 b

1/2 b1/2 b

1/2 b

2/3 l

2/3 l

1/3 l

1/3 l

1/2 b

1/2 b1/2 b

1/2 b

d

l

C of G

Live cellDummy cell

Besides dummy load cells, flexure beams are often used to provide the vessel to pivot with the load cell deflection. It is essential to align the beam webs very carefully for maximum accuracy.The main advantage of flexure beams is their ability to take up horizontal side forces. Therefore no constrainers are necessary to get a stable construction.

The selection of the flexure beam must be based on the ability of the beam to bear the weight of the vessel without bending of the web (buckling effect). Excessive bending of the pivoting beams, during installation should be avoided.

LOAD CELL AND MOUNT SELECTION

Vishay Revere Transducers offers a wide range of industrial load cells and mounts, with a capacity from 6 kg to approximately 200 t. The load cell elements are made of nickel-plated tool steel or (more suitable for the process industry) stainless steel. The table below is representing a part of the offered product families.

SHB

xR

9102

SSB

AC

B

9103

C(S

)P-M

ASC BSP

9363

RL

CRated load :5 kg50 kg 100 200 kg→500 1000 kg→2 t5 t10 t25 100 t→Stainless steel Yes Yes Yes Yes Yes Yes Yes Yes Yes YesIP gradeEN IEC 60529

IP66/IP68

IP66/IP68

IP66/IP68

IP66/IP68

IP67 IP66/IP68

IP66/IP68

IP66/IP68

IP67 IP66/IP68

Deflection (mm)

0,3 0,8 0,5 0,3 0,9 0,4 0.5 0,3 0,4 0,1

Operation mode B B SC SC DS C C S B/SC C

B : Beam type load cell.SC : Shear beamDB : Double ended beam type load cell.C : Compression type load cell.S : S-shape tension type load cell.

Page 18: Vishay Transducer Application Notes

VPG-06VPG Transducers

Document Number: 11873Revision 07-Dec-2011

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Vessel Weighing

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The manner in which the load is transmitted through a load cell has a major impact on the accuracy and repeatability that can be achieved from the system. As a result, the mounting system around the load cell is of paramount importance.

The load should always be transmitted vertically through the load cell in the way which it was designed and tested to measure force. Load cell supports have to be designed avoiding the following effects to the load cell:

• Lateralforces

• Bendingmoments

• Torsionmoments

• Offcenterloadingtotheloadcell

These effects may be caused by expansion of the vessel due to temperature decrease or deflection of the vessel's construction due to loading. Further, for high outdoor vessel, an overturn protection has to be incorporated within the mount. All mounts/load cells must be placed on the same horizontal level.

Load cells should be protected against direct sunlight or drippingaggressiveliquidsbyprotectivescreens.Preventtheloadcellsfrombeingsubmerged;i.e.inapit.

Avoid electric welding after installation of the load cells. If welding is necessary and the load cells can not be

removed then disconnect each individual load cell cable from the indicator ormeasuring instrument. Place theclamp earthing electrode of the welding apparatus in the very neighborhood of the weld to avoid a current path through the load cells. Further, connect a flexible cupper lead of at least 16 mm2 cross section between the vessel and foundation over each load cell.

External Connections

From an accuracy point of view, a weighing system should be free from its surroundings. However in most industrial applications a contact between the weighing object and its surroundings is present.Examples are; pipes, tubes,pneumatic/hydraulic hoses, electrical cables, bellows and constrainers.

Usually theweightofpipesorcablescanbe treatedasapart of the dead load of the vessel. If the influence of pipes orcablesisnotconstant,non-repeatabilityandhysteresiscan be introduced, e.g. a pipe with changing contents or stiff pipes (1), thermal expansion of the vessel (2) or a friction-effectcreatedintheclampingpoints(3).

When one of these situations is likely to be present, it is recommended first to calculate the error and to relate this to the required system's accuracy, before any (expensive) solutions are considered.

The selection of which capacity to use in a weighing application should be based on the following factors :

Determine the maximum weight of the applied load, or "Live Load". Calculate the weight, "tare", of the construction, or "Dead Load". Determine the number of load cells to be used in the structure (N). Check the possible presence of unequal loading conditions (factor fa). This factor is an

allowance for low tare estimates and unequal load distribution. Standard : fa = 1,3. Check on extra factors as vibration, shock etc (factor fb). This factor is a dynamic load

factor; for static weighing fb = 1. For outdoor vessels, calculate the windforce Fw ( application note 09/3-01/01 ).

The individual minimum load cell capacity can be calculated by :

Fw+( fa* fb*(LiveLoad+DeadLoad)/N)

The manner in which the load is transmitted through a load cell has a major impact on the accuracy and repeatability that can be achieved from the system. As a result, the mounting system around the load cell is of paramount importance.The load should always be transmitted vertically through the load cell in the way which it was designed and tested to measure force. Load cell supports have to be designed avoiding the following effects to the load cell:

Lateral forces Bending moments Torsion moments Off centre loading to the load cell

These effects may be caused by expansion of the vessel due to temperature decrease or deflection of the vessel's construction due to loading. Further, for high outdoor vessel, an overturn protection has to be incorporated within the mount. All mounts/load cells must be placed on the same horizontal level.

SSB load cell + mount

CSP load cell + mount 5(9)103 load cell +

mount

Load cells should be protected against direct sunlight or dripping aggressive liquids by protective screens. Prevent the load cells from being submerged; i.e. in a pit. Avoid electric welding after installation of the load cells. If welding is necessary and the load cells can not be removed then disconnect each individual load cell cable from the indicator or measuring instrument. Place the clamp earthing electrode of the welding apparatus in the very neighbourhood of the weld to avoid a current path through the load cells. Further, connect a flexible cupper lead of at least 16 mm2 cross section between the vessel and foundation over each load cell.

SSB load cell + mount CSP load cell + mount DESB load cell + mount

EXTERNAL CONNECTIONS

From an accuracy point of view, a weighing system should be free from its surroundings. However in most industrial applications a contact between the weighing object and its surroundings is present. Examples are; pipes, tubes, pneumatic/hydraulic hoses, electrical cables, bellows and constrainers.Usually the weight of pipes or cables can be treated as a part of the dead load of the vessel. If the influence of pipes or cables is not constant, non-repeatability and hysteresis can be introduced, e.g. a pipe with changing contents or stiff pipes (1), thermal expansion of the vessel (2) or a friction-effect created in the clamping points (3).

When one of these situations is likely to be present, it is recommended first to calculate the error and to relate this to the required system's accuracy, before any ( expensive ) solutions are considered.

1. THE STIFFNESS OF PIPES

The stiffness of the pipes in relation to the stiffness of the weighing system plays an important role in the error development. The stiffness of the weighing system (Cs) can be calculated by:

Cs=(n*Emax*g) / f

n The number of load cellsEmax The individual load cell capacityf The deflection of the load cellg gravitation ( approximately 9.8 m/s2 )

The stiffness of the pipes Ct can be calculated by the sum of the stiffness of each individual pipe Ca:

Ca=(0.05*K*E*(D4-d4)) / l3

K Clamping factorD Outer diameter of piped Inner diameter of pipel Length of pipeE Elasticity modulus, for steel: E = 210.000 N/mm2

for copper: E = 110.000 N/mm2

for aluminium: E = 70.000 N/mm2

The clamping factor K equals K=12 for a pipe clamped rigidly at both ends. The following K-values are valid for a pipe with constant diameter, bend in the vertical plane (1) and in the horizontal plane (2) clamped rigidly at both ends:

A B K

Page 19: Vishay Transducer Application Notes

VPG-06VPG Transducers

Vessel Weighing

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

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Document Number: 11873Revision 07-Dec-2011

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1. The Stiffness of Pipes

The stiffness of the pipes in relation to the stiffness of the weighing system plays an important role in the error development. The stiffness of the weighing system (Cs) can be calculated by:

Cs=(n*Emax*g) / f

n The number of load cells

Emax The individual load cell capacity

f The deflection of the load cell

g gravitation ( approximately 9.8 m/s2 )

The stiffness of the pipes Ct can be calculated by the sum of the stiffness of each individual pipe Ca:

Ca=(0.05*K*E*(D4-d4)) / l3

K Clamping factor

D Outerdiameterofpipe

d Inner diameter of pipe

l Length of pipe

E Elasticity modulus,

for steel: E = 210.000 N/mm2 for copper: E = 110.000 N/mm2 for aluminium: E = 70.000 N/mm2

The clamping factor K equals K=12 for a pipe clamped rigidly at both ends. The following K-values below arevalid for a pipe with constant diameter, bend in the vertical plane (1) and in the horizontal plane (2) clamped rigidly at both ends.

The influence on span (e) can now be calculated by:

e=(Ct/Cs)*100%

The error which is caused by the stiffness of the pipes is a typical span-error and can be reduced by the calibration procedure. However, stiffness of the pipes are no stable values and can change during operation.

Example:

A vessel is supported by four load cells, with a capacity of 2 t and a deflection of 0.5 mm.

Two pipes are connected to the vessel, one bend in the vertical plane as in the opposite drawing.

The pipes are made of steel with an inner diameter of 30 mm and an outer diameter of 40 mm.

The stiffness Cs of the weighing system equals:

Cs=(4*2000*9.8) / 0.5=156800N/mm

The stiffness Ca1 of the pipe, bend in the vertical plane equals:

Ca1=(0.05*8*210000*(404-304)) / 40003=2.30N/mm

0.2 l 8.0

0.5 l 6.0

1.0 l 4.8

5.0 l 3.4

0.2 l 7.1

0.5 l 4.3

1.0 l 1.8

5.0 l 0.06

The influence on span (e) can now be calculated by:

e=(Ct/Cs)*100%

The error which is caused by the stiffness of the pipes is a typical span-error and can be reduced by the calibration procedure. However, stiffness of the pipes are no stable values and can change during operation.

Example:A vessel is supported by four load cells type SSB, with a capacity of 2 t and a deflection of 0.5 mm.Two pipes are connected to the vessel, one bend in the vertical plane as in the opposite drawing.The pipes are made of steel with an inner diameter of 30 mm and an outer diameter of 40 mm.

The stiffness Cs of the weighing system equals:

Cs=(4*2000*9.8) / 0.5=156800N/mm

The stiffness Ca1 of the pipe, bend in the vertical plane equals:

Ca1=(0.05*8*210000*(404-304)) / 40003=2.30N/mm

The stiffness Ca2 of the straight pipe equals:

Ca2=(0.05*12*210000*(404-304)) / 15003=65.33N/mm

The total stiffness Ct of the pipes equals Ca1 + Ca2 = 67.33 N/mm. The influence on span (e) can now be calculated:

e=(67.63/156800)*100%=0.043%

2. THERMAL EXPANSION

The height of the clamping point of the pipe can change with any change in ambient temperature by expansion of the vessel. Stiff pipes will try to counteract this movement, causing a zero-shift and non-reproducibility.

ΔL=Lo+ΔT*α

The change in height can be calculated by:

l

lA

B

1 2

0.2 l 8.0

0.5 l 6.0

1.0 l 4.8

5.0 l 3.4

0.2 l 7.1

0.5 l 4.3

1.0 l 1.8

5.0 l 0.06

The influence on span (e) can now be calculated by:

e=(Ct/Cs)*100%

The error which is caused by the stiffness of the pipes is a typical span-error and can be reduced by the calibration procedure. However, stiffness of the pipes are no stable values and can change during operation.

Example:A vessel is supported by four load cells type SSB, with a capacity of 2 t and a deflection of 0.5 mm.Two pipes are connected to the vessel, one bend in the vertical plane as in the opposite drawing.The pipes are made of steel with an inner diameter of 30 mm and an outer diameter of 40 mm.

The stiffness Cs of the weighing system equals:

Cs=(4*2000*9.8) / 0.5=156800N/mm

The stiffness Ca1 of the pipe, bend in the vertical plane equals:

Ca1=(0.05*8*210000*(404-304)) / 40003=2.30N/mm

The stiffness Ca2 of the straight pipe equals:

Ca2=(0.05*12*210000*(404-304)) / 15003=65.33N/mm

The total stiffness Ct of the pipes equals Ca1 + Ca2 = 67.33 N/mm. The influence on span (e) can now be calculated:

e=(67.63/156800)*100%=0.043%

2. THERMAL EXPANSION

The height of the clamping point of the pipe can change with any change in ambient temperature by expansion of the vessel. Stiff pipes will try to counteract this movement, causing a zero-shift and non-reproducibility.

ΔL=Lo+ΔT*α

The change in height can be calculated by:

l

lA

B

1 2

A B C

0.21 8.0

0.51 6.0

1.01 4.8

5.01 3.4

0.21 7.1

0.51 4.3

1.01 1.8

5.01 0.06

Page 20: Vishay Transducer Application Notes

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Document Number: 11873Revision 07-Dec-2011

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Vessel Weighing

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The stiffness Ca2 of the straight pipe equals:

Ca2=(0.05*12*210000*(404-304)) / 15003=65.33N/mm

The total stiffness Ct of the pipes equals Ca1 + Ca2 = 67.33 N/mm. The inf luence on span (e) can now be calculated:

e=(67.63/156800)*100%=0.043%

2. Thermal Expansion

The height of the clamping point of the pipe can change with any change in ambient tempera ture by expansion of thevessel.Stiffpipeswilltrytocounteractthismovement,causingazero-shiftandnon-reproducibility.

ΔL=Lo+ΔT*α

The change in height can be calculated by:

ΔL Change in length (mm)

Lo Originallength(mm)

ΔT Changeinambienttemperature:T–To(K)

α Linearexpansion(K-1),

for steel α = 1.2*10-5 for copper α = 1.7*10-5 for aluminium α = 2.4*10-5

The reaction force of the pipe can be calculated by:

F=ΔL*Ca

F Reaction force of the pipe

Ca Stiffnessofthepipe

The error to the system can be calculated by:

e=(F/scale capacity*g)*100%

The error which is caused by thermal expansion is a typical zero-error.Weighing systemswithout connections tothe outer world are not affected by temperature effects, provided that a well designed mounting system is used.

Load cells are manufactured to operate within a certain temperature range,normally from -40 to+80°C.A loadcell is compensated for a part of this temperature range tooperatewithin specifications, normally -10 to+40°C.Shieldsorinsulationpathsmustbeestablishedtokeeptheload cell within the operating range and for high accuracy systems within the compensated temperature range.

Example:

A vessel is supported on four load cells, by a supporting struc ture made of steel. The scale capacity equals 10 tons.

The vessel is made of aluminium. A pipe with a stiffness Ca of 75N/mm is connected to the vessel. The critical dimensions are indicated in the figure opposite. During the daytheambienttemperaturedecreasesfrom15to25°C.

The height of the supporting structure will decrease with:

ΔL=3000*(25-15)*1.2*10-5=0.35mm

The height of the vessel will decrease with:

ΔL=3000*(25-15)*2.4*10-5=0.72mm

The height of the clamping point of the pipe will change with 0.35 + 0.72 = 1.07 mm. This will cause a reaction force of the pipe of:

F=1.07*75=80.25N

The error to the system, caused by the temperature decrease will be:

e=((80.25/(10000*9.8)*100%=0.08%

3. Friction-Effects

Friction-effectscreatedintheclampingpointsareleadingto an undefined error, causing non-repeatability andhysteresis.Pipesupports,especiallythefirstsupportsawayfrom the vessel should be attached to the same structure as to which the vessel is supported.

ΔL Change in length (mm)Lo Original length (mm)ΔT Change in ambient temperature: T - To (K)α Linear expansion (K-1), for steel = 1.2*10α -5

for copper = 1.7*10α -5

for aluminium = 2.4*10α -5

The reaction force of the pipe can be calculated by:F=ΔL*Ca

F Reaction force of the pipeCa Stiffness of the pipe

The error to the system can be calculated by:

e=(F/scale capacity*g)*100%

The error which is caused by thermal expansion is a typical zero-error. Weighing systems without connections to the outer world are not affected by temperature effects, provided that a well designed mounting system is used.Load cells are manufactured to operate within a certain temperature range, normally from -40 to +80 °C. A load cell is compensated for a part of this temperature range to operate within specifications, normally -10 to +40 °C. Shields or insulation paths must be established to keep the load cell within the operating range and for high accuracy systems within the compensated temperature range.

Example:A vessel is supported on four load cells, by a supporting structure made of steel. The scale capacity equals 10 tons. The vessel is made of aluminium. A pipe with a stiffness Ca of 75N/mm is connected to the vessel. The critical dimensions are indicated in the figure opposite. During the day the ambient temperature decreases from 15 to 25 °C.

The height of the supporting structure will decrease with:ΔL=3000*(25-15)*1.2*10-5=0.35mm

The height of the vessel will decrease with:

ΔL=3000*(25-15)*2.4*10-5=0.72mmThe height of the clamping point of the pipe will change with 0.35 + 0.72 = 1.07 mm. This will cause a reaction force of the pipe of:

F=1.07*75=80.25N

The error to the system, caused by the temperature decrease will be:

e=((80.25/(10000*9.8)*100%=0.08%

3. FRICTION-EFFECTS

Friction-effects created in the clamping points are leading to an undefined error, causing non-repeatability and hysteresis. Pipe supports, especially the first supports away from the vessel should

Page 21: Vishay Transducer Application Notes

VPG-06VPG Transducers

Vessel Weighing

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Document Number: 11873Revision 07-Dec-2011

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Compensators

When the influence of pipes exceed the allowed error then the following solutions should be considered:

• Decreasethelengthofpipe(s).

• Designtheclampingtobelessrigid.

• Introducecompensatorsinthepipe.All piping tends to sag from its theoretical design position due to its own dead weight. This effect will decrease with

the length of the pipe. It is therefore important to check all piping runs between the vessel and the first pipe support for adequate clearance.

Flexible piping devices or compensators should be selected based on their f lexibility and their process chemistry suitability i.e. High or low pressure systems, temperature, aggressive chemicals.

Flexible devices of non-metallicmaterials offermoreflexibility in less space and with less vibration transmission

be attached to the same structure as to which the vessel is supported.

Care should be paid to less obvious sources of deflection which are often ignored, such as deflection of the floor or roof and

two weighing vessels with pipe connections.

Large horizontal side forces may arise by thermal linear expansion of rigidly clamped pipes.

T 6 F

be attached to the same structure as to which the vessel is supported.

Care should be paid to less obvious sources of deflection which are often ignored, such as deflection of the floor or roof and

two weighing vessels with pipe connections.

Large horizontal side forces may arise by thermal linear expansion of rigidly clamped pipes.

T 6 F

be attached to the same structure as to which the vessel is supported.

Care should be paid to less obvious sources of deflection which are often ignored, such as deflection of the floor or roof and

two weighing vessels with pipe connections.

Large horizontal side forces may arise by thermal linear expansion of rigidly clamped pipes.

T 6 F

Large horizontal side forces may arise by ther mal linear expansion

of rigidly clamped pipes.

Care should be paid to less obvious sources of deflection which are often ignored, such

as deflection of the floor or roof and...

two weighing vessels with pipe connections.

COMPENSATORS

When the influence of pipes exceed the allowed error then the following solutions should be considered:

Decrease the length of pipe(s). Design the clamping to be less rigid. Introduce compensators in the pipe.

All piping tends to sag from its theoretical design position due to its own dead weight. This effect will decrease with the length of the pipe. It is therefore important to check all piping runs between the vessel and the first pipe support for adequate clearance.

Flexible piping devices or compensators should be selected based on their flexibility and their process chemistry suitability i.e. High or low pressure systems, temperature, aggressive chemicals.Flexible devices of non-metallic materials offer more flexibility in less space and with less vibration transmission than the metal counterparts. These benefits plus, variously, increased wear, corrosion and fatigue resistance makes non-metallic materials highly attractive when the process pressure and temperature requirements can be met.When large displacements must be accommodated with low force, consider using two compensators in series or a bent U-shape flexible hose. This is particularly important for low capacity systems were even small piping forces will disturb weigh system stability.Do not stretch or compress compensators excessively to compensate for initial piping misalignments at fitup, to prevent their stiffness characteristics from being altered.

Elbow Stub Flexible piping devices

When multiple pipes are connected to a weighing vessel, then the connections should be made symmetrical if possible.

Avoid Avoid if possible Correct installation

COMPENSATORS

When the influence of pipes exceed the allowed error then the following solutions should be considered:

Decrease the length of pipe(s). Design the clamping to be less rigid. Introduce compensators in the pipe.

All piping tends to sag from its theoretical design position due to its own dead weight. This effect will decrease with the length of the pipe. It is therefore important to check all piping runs between the vessel and the first pipe support for adequate clearance.

Flexible piping devices or compensators should be selected based on their flexibility and their process chemistry suitability i.e. High or low pressure systems, temperature, aggressive chemicals.Flexible devices of non-metallic materials offer more flexibility in less space and with less vibration transmission than the metal counterparts. These benefits plus, variously, increased wear, corrosion and fatigue resistance makes non-metallic materials highly attractive when the process pressure and temperature requirements can be met.When large displacements must be accommodated with low force, consider using two compensators in series or a bent U-shape flexible hose. This is particularly important for low capacity systems were even small piping forces will disturb weigh system stability.Do not stretch or compress compensators excessively to compensate for initial piping misalignments at fitup, to prevent their stiffness characteristics from being altered.

Elbow Stub Flexible piping devices

When multiple pipes are connected to a weighing vessel, then the connections should be made symmetrical if possible.

Avoid Avoid if possible Correct installation

Elbow

Avoid

Stub

Avoid if possible

Flexible piping devices

Correct installation

ΔT = ΔF

Page 22: Vishay Transducer Application Notes

VPG-06VPG Transducers

Document Number: 11873Revision 07-Dec-2011

www.vpgtransducers.com8

Vessel Weighing

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than the metal counterparts. These benefits plus, variously, increased wear, corrosion and fatigue resistance makes non-metallicmaterialshighlyattractivewhentheprocesspressure and temperature requirements can be met.

When large displacements must be accommodated with low force, consider using two compensators in series or a bentU-shapeflexiblehose.Thisisparticularlyimportantfor low capacity systems were even small piping forces will disturb weigh system stability.

Do not stretch or compress compensators excessively to compensate for initial piping misalignments at fitup, to prevent their stiffness characteristics from being altered.

When multiple pipes are connected to a weighing vessel, then the connections should be made symmetrical if possible.

Pressurized Vessels

If the content of the vessel is under gas pressure and the pipe connection is made with a vertical compensator (bellow), a vertical disturbance force can arise. The compensators should be located in horizontal piping runs adjacent to the weigh vessel to avoid these vertical thrust forces from varying internal pressures associated with material flow andprocesschemistry.Atemporaryover-pressurecanalsobe created by filling a vessel with a dusty material.

The disturbing force can be calculated by:

F=(ΔP*π*D2) / 4

ΔP Overorunder-pressure(N/m2)

D Effective diameter of the bellow (m)

Example:

Avessel is pressurizedwith 2barover-pressure and thepipe connection is made with a vertical bellow having an effective diameter of 150mm.

The maximum disturbing force can be calculated by:

F=(2*105*π*(150*10-3)2 / 4=3534N

The flexibility of the bellow will cause the indicator to bounce between the actual weight and the actual weight plus the maximum disturbing force.

Gas pressure in a vertical pipe gives minor influence if the pipe is connected to the vessel with a stiff part as indicated in the last drawing of the previous page.

Restraining Devices

Load cells should be protected against side forces by the use of restraining devices. These assem blies are designed to allow ample vertical freedom for weight sensing, while simultaneously eliminating inaccuracies caused by side loading.

Accuracy and reliability of systems not protected in this way would be greatly reduced in the presence of extraneous forces, which might even result in damaged to the load cell in extreme cases.

Two types of restraining devices are used:

• Stayrods

• Limitingstops

MostmountsofferedbyVPGTransducersareself-aligningwith an in-build limiting stops. Thesemounts do notrequire further restraining devices in most applications. Stayrodsmustbeusedwhenavibratorormixerisusedinthe vessel!

Stay rods should not essentially transfer any forces tothe container in the vertical direction, but have sufficient strength in the horizontal direction to be able to absorb the maximum horizontal forces arising. The length of the stay rods should be chosen as long as possible, as this has a favorable effect on reducing vertical forces.

The arrangement of the stay rods depends on the plan view geometry of the structure. In most cases four stay rods give the best results. Figure 3 below represents a basic stay rod arrangement for a vessel under thermal expansion. More information about the arrangement of stay rods for specific applications is available on request.

PRESSURIZED VESSELS

If the content of the vessel is under gas pressure and the pipe connection is made with a vertical compensator (bellow), a vertical disturbance force can arise. The compensators should be located in horizontal piping runs adjacent to the weigh vessel to avoid these vertical thrust forces from varying internal pressures associated with material flow and process chemistry. A temporary over-pressure can also be created by filling a vessel with a dusty material.The disturbing force can be calculated by:

F=(ΔP*π*D2) / 4

ΔP Over or under-pressure ( N/m2 )D Effective diameter of the bellow ( m )

Example:

A vessel is pressurized with 2 bar over-pressure and the pipe connection is made with a vertical bellow having an effective diameter of 150mm.

The maximum disturbing force can be calculated by:

F=(2*105*π*(150*10-3)2 / 4=3534N

The flexibility of the bellow will cause the indicator to bounce between the actual weight and the actual weight plus the maximum disturbing force.

Gas pressure in a vertical pipe gives minor influence if the pipe is connected to the vessel with a stiff part as indicated in the last drawing of the previous page.

RESTRAINING DEVICES

Load cells should be protected against side forces by the use of restraining devices. These assem-blies are designed to allow ample vertical freedom for weight sensing, while simultaneously eliminating inaccuracies caused by side loading.Accuracy and reliability of systems not protected in this way would be greatly reduced in the presence of extraneous forces, which might even result in damaged to the load cell in extreme cases.Two types of restraining devices are used:

Stay rods Limiting stops

Most mounts offered by Vishay Revere Transducers are self-aligning with an in-build limiting stops. These mounts do not require further restraining devices in most applications. Stay rods must be used when a vibrator or mixer is used in the vessel!

D

Po + P PoPo + ΔP Po

D

Page 23: Vishay Transducer Application Notes

VPG-06VPG Transducers

Vessel Weighing

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EStay rods should not essentially transfer any forces to the container in the vertical direction, but have sufficient strength in the horizontal direction to be able to absorb the maximum horizontal forces arising. The length of the stay rods should be chosen as long as possible, as this has a favourable effect on reducing vertical forces.The arrangement of the stay rods depends on the plan view geometry of the structure. In most cases four stay rods give the best results. Figure 3 below represents a basic stay rod arrangement for a vessel under thermal expansion. More information about the arrangement of stay rods for specific applications is available on request. Note:

Placing stay rods as represented in figure 1 will cause high stresses in the stay rods and should be avoided. The arrangement in figure 2 will cause a rotation of the vessel. This configuration should be avoided if there are stiff connections to the vessel. Stay rods should be placed in a exactly horizontal level.

This application note is written as a short guide in understanding the considerations which must be taken into account for vessel weighing. For more information we specially refer to the following application notes:

10/06-01/01 Windforces10/06-02/02 Load cell cabling10/06-03/02 Shunt diode barriers10/06-04/02 Potentially Explosive Atmospheres10/06-07/02 Environmental Conditions

Customer support:

Vishay Revere Transducers combines sixty years of load cell manufacturing with sixty years of application know how. For any further question, please contact our manufacturing operation or any one of our regional sales offices.

Vishay Revere Transducers B.V.P.O.box 6909, 4802 HX BredaThe NetherlandsTel. (+31) 76-5480700 Website: www.vishaymg.comFax. (+31) 76-5412854 Email: [email protected]

Figure 1 Figure 2 Figure 3

Note: Placing stay rods as represented in figure 1 will cause high stresses in the stay rods and should be avoided. The arrangement in figure 2 will cause a rotation of the vessel. This configuration should be avoided if there are stiff

connections to the vessel. Stay rods should be placed in a exactly horizontal level.

Page 24: Vishay Transducer Application Notes

Application Note VPG-03

VPG TRANSDUCERS

Legal Metrology

Load Cells and Weigh Modules

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Scope

This application note is meant to provide the user with a reasonably quick reference to a fairly complex subject; the load cell requirements for legal-for-trade non automatic weighing instruments, according to section 4.12 of OIML Recommendation R76 (EN45501).

Every effort has been made to include most of the important topics and to enable the user to select load cells for a particular approved non automatic weighing instrument. The type of weighing instruments covered are: single range instruments, multiple range instruments and multi-interval instruments.

The first pages cover the metrological terms used to describe load cell features according to OIML Recommendation R60. These terms are implemented in section 4.12, and should therefore be explained.

Standardized Metrology - OIML

The establishment of a worldwide standardized metrology or measurement system has two main functions. One related to scientif ic activities assuring world-wide consistency and repeatability of critical scientific units, while the other is concerned with legal metrology which is the name given to all applied metrology or measurement subjected to regulations by law or governmental degree. In most countries, legal metrology covers measurements in protection of individuals from a financial, health and environmental point of view.

In order to harmonize and standardize on an international basis, a convention was held in Paris on October 12th, 1955, and the participating States (countries) agreed to set up an international organization of legal metrology - the OIML was born. Because the official language of the OIML was French, the name of the organization is Organisation Internationale de Metrologie Legale.

The OIML is a worldwide inter-governmental organization whose main task is that of harmonizing the regulations and metrological controls applied by the Weights and Measures of its Member States. Because it is a Treaty Organization, membership of a country is subject to the signature of a convention through diplomatic channels. Once a member, a country has moral and ethical obligations to harmonize with the beliefs and output of the OIML.

The aim of such harmonization is to facilitate free trade and commerce between countries not only for measuring instruments, but for all commodities and services whose value is determined by measurements.

OIML Recommendations and Documents relate to specific measuring instruments and technology. International Recommendations (OIML R) are model regulations generally establishing the metrological characteristics required of the measuring instruments concerned and specifying methods and equipment for checking their conformity. OIML member states are expected to implement these Recommendations as far as possible.

Metrological Terms For Load Cells

The metrological terms most frequently used in the load cell field can be divided into two main categories; load related terms or accuracy related terms.

1. Load Related Terms

Minimum dead load (Emin):The smallest value of a quantity (mass) which may be applied to a load cell without exceeding the maximum permissible error.

Maximum capacity (Emax):The largest value of a quantity (mass) which may be applied to a load cell without exceeding the maximum permissible error.

Load cell measuring range:The range of values of the measured quantity (mass) for which the result of measurement should not be affected by an error exceeding the maximum permissible error.

Safe load limit:The maximum load that can be applied without producing a permanent shift in the performance characteristics beyond those specified.

Ultimate load limit:The maximum load that can be applied without physical destruction of the load cell.

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Legal Metrology

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2. Accuracy Related Terms

Load cell interval:

Part of the load cell measuring range into which that range is divided.

Load cell verification interval (v):

The load cell interval, expressed in units of mass, used in the test of the load cell for accuracy classification

Number of verification intervals (n):

The number of verification intervals, used in the test of the load cell for accuracy classification.

Accuracy class:

A class of load cells which are subjected to the same conditions of accuracy.

Load cells are ranked, according to their overall performance capabil ities, into four classes whose designations are "Class A", "Class B", "Class C" and "Class D". A load cell is classified by the alphabetical classification and the maximum number of load cell intervals stated in units of 1000; for example C3 represents class C, 3000v.

The number of verification intervals (n) into which the measuring range of a class C load cell can be divided is fixed between 500 and 10000. VPG Transducers offers a wide range of class C industrial load cells from 1000v

to 6000v. Class C load cells are suited for class and weighing systems.

Minimum verification interval (vmin):

The smallest value of a quantity (mass) which may be applied to a load cell without exceeding the maximum permissible error. Specified as Emax/γ or as a percentage of the measuring range.

The minimum verification interval is inextricably linked to the utilization of the load cell. The utilization can be defined as the minimum measuring range (MMR) for a particular load cell over which full specification will be maintained. The following formulas can be applied:

MMR(kg) = vmin * nmax or MMR(%) = nmax * 100 / γ

For example a 1t load cell, with vmin= Emax/10000 has a minimum measuring range of

1000 * 4000 / 10000 = 400 kg or 4000 * 100 / 10000 = 40%The minimum measuring range can apply over any part of the measuring range between Emin and Emax. In practice, certain accuracy parameters (linearity, hysteresis) will improve when a smaller part of the load cell rated capacity is utilized. However, temperature effect on zero load output is a fixed error percentage of the rated output, and

Maximum Measuring Range

Measuring Range

E minNo Load maxE Safe Load Ultimate Load

D min D max

Maximum Measuring Range

Measuring Range100kg

0kg 500kg 750kg 1500kg

400kg

The terms that appear above the central horizontal line are parameters that are fixed by the design of the load cell. The terms that appear below that line are parameters that are variable, depending on the conditions of use and the quality of the load cell as measured during tests.

As a specific example, an "S" type load cell with a capacity of 500 kg is used to weigh a hopper with a dead load of 100 kg (Dmin) and a live load of 300 kg (Dmax = Dmin + 300 = 400 kg). Emin =0% of Emax, the safe load limit is 150% of Emax and the ultimate load limit is 300% of Emax.

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must be tightly controlled to achieve lower vmin values for a particular grade of load cells.

The effect of the utilization factor on creep will depend on which part of the load cell range is being used for the scale. For example, creep will be more significant in a scale where its working range is at the top end of the load cell's rated capacity than when it is at the bottom.

Load cells having a small value for vmin are most suitable for applications with a relatively high dead load. The above calculation applies to a single load cell when used on its own. The requirements for multiple load cell weighing instruments are specified further on.

Non-linearity:

The deviation of the increasing load cell calibration curve from a straight line which passes through minimum load output and the load cell output at 75% of the measuring range, at 20°C.

Hysteresis error:

The difference between load cell output readings for the same applied load, one reading obtained by increasing the load from minimum load and the other by decreasing the load from maximum load.

Creep:

The change in load cell output occurring with time while under constant load (> 90% of the load cell capacity) and with all environmental conditions and other variables also remaining constant.

Minimum dead load output return:

The difference in load cell output at minimum dead load, measured before and after load application.

Temperature effect on minimum dead load output:

The change in minimum dead load output due to a change in ambient temperature.

Temperature effect on sensitivity:

The change in sensitivity due to a change in ambient temperature.

Combined error:

The approach taken by R60 recognizes that several load cell errors must be considered together when fitting load cell performance characteristics to the error envelope permitted. It is possible to have low non-linearity and hysteresis and moderate temperature errors or, conversely, to have moderate non-linearity and hysteresis errors and low temperature errors.

Thus, it is not considered appropriate to specify individual error limits for given characteristics (non-linearity, hysteresis and temperature effect on sensitivity), but rather to consider the total error envelope allowed for a load cell as the limiting factor. The use of an error envelope concept allows balancing individual contributions to the total error of measurement while still achieving the intended result.

Maximum permissible load cell errors

The maximum permissible load cell errors for each accuracy class, the indicated load cell output having been adjusted to zero at minimum dead load, are related to the maximum number of verif ication intervals. The table below shows the error limits as represented on the VPG Transducers datasheets. "Temperature effect on Sensitivity" and" Combined error" are combined in such a way that the load cells meet the OIML R60 tolerance envelope. S equals Rated Output and corresponds directly with Emax.

Accuracy Designation C1 C2 C3 C4 C5

Combined Error %S 0.0300 0.0230 0.0200 0.0150 0.0100

Non-Repeatability %S 0.0200 0.0100 0.0100 0.0090 0.0070

Minimum Dead Load Output Return %S 0.0500 0.0250 0.0167 0.0125 0.0100

Creep Error (30 minutes) %S 0.0490 0.0245 0.0245 0.0184 0.0147

Creep Error (20-30 minutes) %S 0.0105 0.0053 0.0053 0.0039 0.0032

Temperature Effect on Sensitivity %S/5ºC 0.0085 0.0060 0.0055 0.0045 0.0035

Note: OIML recommendation R60 specifies the maximum permissible errors in terms of verification intervals or minimum verification intervals. The maximum permissible errors for combined error are similar to those of non automatic weighing instruments, when a factor of 0.7 (pi-factor) is used. A copy of the recommendation is available on request.

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EN 45501, Requirements For Load Cells

Section 4.12 of EN 45501 (OIML R76) requires that load cells have been tested in conformity with International Recommendation OIML R60. These load cells can be applied in three groups of weighing instruments:

1. Single interval instruments: Instrument having one weighing range.

2. Multiple range instruments: Instrument having two or more weighing ranges with different maximum capacities and different scale intervals for the same load receptor, each range extending from zero to its maximum capacity.

3. Multi-interval instruments: Instrument having one weighing range which is divided into partial weighing ranges, each with different scale intervals, with the weighing range determined automatical ly according to the load applied, both on increasing and decreasing loads.

The most important metrological terms for weighing instruments are:

Reduction ratio:

The reduction ratio of a load transmitting device is given by: R=FM / FLWhere: FM is the load acting on the load measuring device

(total number of load cells). FL is the load acting on the load receptor (scale).

Maximum capacity (Max):

Maximum weighing capacity, not taking into account the additive tare capacity.

Minimum capacity (Min):

Value of the load below which the weighing results may be subject to an excessive relative error.

Actual scale interval (d):

Value expressed in units of mass of:

• Thedifferencebetween the values corresponding totwo consecutive scale marks, for analogue indication,or

• The difference between two consecutive indicatedvalues, for digital indication.

Verification scale interval (e):

Value, expressed in units of mass, used for the classification and verification of an instrument.*

Number of verification scale intervals (n):

Quotient of the maximum capacity and the verification scale interval, for a single-interval instrument:

n = Max / e

1. Requirements For Single Interval Instruments

1.1 The maximum capacity of the load cell shall satisfy the condition:

Emax ≥ Q * Max * R / N

Where: Emax: maximum capacity of the load cell N: Number of load cells R: Reduction ratio Q: Correction factor

The correction factor Q > 1 considers the possible effects of eccentric loading, dead load of the load receptor (scale), initial zero setting range and non uniform distribution of the load.

To be precise: the total capacity of all load cells should be larger or equal to the maximum capacity of the scale, the dead weight of the construction and the overall effect on zero-setting and zero-tracking devices. The overall effect of zero-setting and zero-tracking devices shall be not more than 4% and of the initial zero-setting device not more than 20%, of the maximum capacity. Further to this, the following eccentric loading conditions should be considered:

• Onaninstrumentwithaloadreceptorhavingnpointsof support, with n4, the fraction 1/(n-1) of the sum of the maximum capacity and the maximum additive tare effect shall be applied to each point of support.

• On an instrument with a load receptor subject tominimal off-center loading (e.g. tank, hopper) a test load corresponding to one-tenth of the sum of the maximum capacity and the maximum additive tare effect shall be applied to each point of support.

If the above considerations are applied on a platform scale with a capacity of 1500 kg and a dead load of 100 kg, the individual load cell capacity if four load cells are used can be calculated by:

Eccentricity behavior tested with 1/(n-1) * Max = 1/3* 1500 = 500 kg

Dead load weight distribution equals 100/n = 100/4 = 25 kg

* Note: "e" equals "d" in the majority of digital weighing instruments. A scale interval should be numbered in the form 1 * 10k, 2 * 10k or 5 * 10k, k being a positive or negative whole number or equal to zero.

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Zero-setting/tracking: (24% of 1500)/n = 360/4 = 90 kg

Hence, the load cell capacity (Emax) should at least be 500 + 25 + 90 = 615 kg

1.2 The maximum number of load cell intervals shall satisfy the condition:

nlc ≥ n

For each load cell, the maximum number of load cell intervals nlc shall not be less than the number of verification scale intervals n of the instrument, e.g. a 3000d class weighing instrument should have at least class C3 load cells.

1.3 The minimum load cell verification interval shall satisfy the condition:

vmin ≤ e * R / √ N

The minimum load cell verification interval vmin shall not be greater than the verification scale interval e multiplied by the reduction ratio R of the load transmitting device and divided by the square root of the number N of load cells. This formula can be rewritten as:

e ≥ vmin * √ N / R

For example, a platform scale with a capacity of 1500 kg is built with four load cells, type SSB-C3-1t, with vmin =Emax/8333.

1) The load cell capacity is in agreement with point 1.1 (see calculation example).

2) The maximum number of scale intervals should be smaller or equal to the maximum number of load cell verification intervals. Hence, the maximum number of scale intervals is 3000.

3) By applying the formula given at point 1.3, the minimum value for e can be calculated:

e ≥ vmin* √ N / R, e ³ 1000 * 2 / 8333 * 1 e ³ 0.240 => e = 0.5 kgIt is important to verify the output per scale division with the required minimum signal level for the measuring device to ensure compatibility. The output per scale division (in μV)can be calculated by:

UE * S * Max * 1000 / (N * Emax * n)

Where:

UE: Excitation voltage S: Rated output load cell N: Number of load cells n: Number of scale divisions

The SSB load cell has a rated output of 2mV/V. The output per verification scale interval at an excitation voltage of 10V for the example above will be:

10 * 2 * 1500 * 1000 / (4 * 1000 * 3000) =2.5 μV

The graph below represents the minimum value for e, in relation to the load cell capacity (Emax) when the instrument is constructed with four load cells (Reduction ratio R = 1).

2. Requirements for Multiple Range Instruments

On multiple range instruments, each range is treated basically as an instrument with one range. Switching while the instrument is loaded, from one weighing range to another is only allowed if the verification scale interval increases. Furthermore, it is not allowed to enter a lower range after a tare setting, or by using a preset tare value. An example of a multiple range instrument with three ranges is given in the diagram below:

400 800 1000

0.1

0.2

0.5

e1

e2

e3

Max1

Max2

Max3

(kg)

(kg)

1 2 3 4 5 6 7 8 9 10

0.1

0.2

0.5

1.0

0.3

0.4

0.6

0.7

0.8

0.9

1.1

1.2

1.3

1.4

1.5

Emax (t)

γ = 8

333

γ = 10

000

γ = 13500

γ = 15000

γ = 28000e

(kg

)

γ = 20000

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The requirements for load cells are:

2.1 The maximum capacity shall satisfy the condition: Emax ≥ Q * Maxr * R / N2.2 The maximum number of load cell intervals shall satisfy

the condition: nlc ≥ n2.3 The minimum load cell verification interval shall satisfy

the condition: vmin ≤ e1 * R / √ N2.4 The minimum dead load output return of the load cell

shall satisfy the condition: DR ≤ e1 * R / N Or, where DR is not know (specifically specified on the

R60-certificate) the following acceptable solution should be satisfied:

nlc ≥ 0.4 * Maxr / e1

Where:

ni: the number of verification scale intervals for range i (i= 1, 2, etc.)

Maxr: the maximum capacity of the highest weighing range

e1: the verification scale interval of the smallest weighing range

For example, a platform scale with a capacity of 1000 kg is built with four load cells, 0.5t, with vmin = Emax / 13500.

1) The eccentricity behavior will be tested with: Maxr / (n - 1) = (1/3) * 1000 ≈ 325 kg This is well below the capacity of the load cell, hence,

acceptable.

2) The number of scale verification intervals should be smaller than or equal to 4000.

3) The minimum scale verif ication interval can be calculated by:

e1 ≥ vmin * √ N / R, e1 ≥ 500 * 2 / 13500 e1 ≥ 0.074 kgHence, e1 will be 0.1 kg

4) As there is no value specified for the DR in the certi-ficate, the use of the following formula is acceptable:

nlc ≥ 0.4 * Maxr / e1, nlc ≥ 0.4 * 1000 / 0.1 nlc ≥ 4000

Hence, the following ranges are allowed to use:

0 - 400 kg with e = 0.1 kg (4000 divisions)

0 - 800 kg with e = 0.2 kg (4000 divisions)

0 - 1000 kg with e = 0.5 kg (2000 divisions)

These values are used in the diagram on the previous page.

Multiple range instruments shall satisfy the following conditions:

On a multiple range instrument the deviation on returning to zero from Max1 shall not exceed 0.5 e1. Furthermore, after returning to zero from any load greater than Max1 and immediately after switching to the lowest weighing range, the indication near zero shall not vary by more than e1 during the following 5 minutes.

Zero setting in any weighing range shall be effective also in the greater weighing ranges, if switching to a greater weighing range is possible while the instrument is loaded.

The tare operation shall be effective also in the greater weighing ranges, if switching to a greater weighing range is possible while the instrument is loaded.

A preset tare value may only be transferred from one weighing range to another one with a larger verification scale interval but shall then be rounded to the latter.

MR-load cells

Load cells suitable for multiple range instruments should preferably have a high γ value (Emax /vmin). VPG has distinguished special MR versions in the latest certificates.

3. Requirements for Multi-Interval Instruments

A multi-interval weighing instrument has one weighing range, which is divided into partial weighing ranges, each with different scale intervals. The weighing range is determined automatically according to the load applied, both on increasing and decreasing loads.

A multi-interval instrument offers the end-user more flexibility with its partial weighing ranges in comparison to multiple range instruments. It is possible to enter a lower partial range after a tare setting or by using a preset tare-value. An example of a multi-interval weighing instrument with two partial ranges is given in the following diagram:

400 800 1000

0.1

0.2

0.5

e1

e2

e3

Max1

Max2

Max3

(kg)

(kg)

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The requirements for load cells are:

3.1 The maximum capacity shall satisfy the condition: Emax ≥ Q * Maxr * R / N3.2 The maximum number of load cell intervals shall satisfy

the condition: nlc ≥ n3.3 The minimum load cell verification interval shall satisfy

the condition: vmin ≤ e1 * R / √ N

3.4 The minimum dead load output return of the load cell shall satisfy the condition:

DR ≤ 0.5 * e1 * R / N Or, where DR is not know (specifically specified on the

R60-certificate) the following acceptable solution should be satisfied:

nlc ≥ Maxr / e1

Where:

ni: the number of verification scale intervals for partial range i (i = 1, 2, etc.)

Maxr: the maximum capacity of the highest partial weighing range

e1: the verification scale interval of the smallest partial weighing range

MI-load cells

VPG Transducers model RLC has two versions, specially certified for multi-interval instruments. These versions are coded RLC-C3MI6 or RLC-C3MI7.5. Specifications:

C3MI6DR = 2Emax / 6000vmin = Emax / 7000 C3MI7.5DR = 2Emax / 7500vmin = Emax / 7000

For example, a platform scale with a capacity of 10t is built with four load cells type RLC-C3MI7.5-3.5t, with vmin = Emax/ 7000 and DR = 2Emax / 7500.

1) The eccentricity behavior will be tested with: Maxr / (n - 1) = 10000 / 3 ≈ 3250 kg This is well below the capacity of the load cell, hence,

acceptable.

2) The number of scale verification intervals for each partial range should be smaller or equal to 3000.

3) The minimum scale verif ication interval can be calculated by:

e1 ≥ vmin * √ N / R, e1 ≥ 3500 * 2 / 7000 e1 ≥ 1 kgHence, e1 should be greater or equal to 1.0 kg

4) DR is specified as 2Emax /7 500, hence the following formula should be applied:

DR ≤ 0.5 * e1*R / N, 1750 / 7500 ≤ 0.5 * e1*1 / 4 e1 ≥ 1.87 => e1 = 2 kgHence, the following partial ranges are allowed to be used: 0 - 6000 kg with e1 = 2 kg (3000 divisions) 6000 - 10000 kg with e2 = 5 kg (2000 divisions)

These values are used in the diagram on the previous page. The error envelope for the above mentioned scale will be:

0 e1 - 500 e1 ~ 0 - 1000 kg max. error, 0.5 e1 ~ 1 kg

500 e1 - 2000 e1 ~1000 - 4000 kg max. error, 1.0 e1 ~ 2 kg

2000 e1 - 3000 e1~ 4000 - 6000 kg max. error, 1.5 e1 ~ 3 kg

1200 e2 - 2000 e2 ~ 6000 - 10000 kg max. error, 1.0 e2 ~ 5 kg

1

2

3

5

-1

-2

-3

-5

01000 4000 6000 10000

(kg)

(kg)

By applying formula 3.3 and 3.4, the following values for e1 can be obtained:

Load cell capacity Emax

(kg)

type C3MI6; 2EMAX/6000 type C3MI7.7; 2EMAX/7500 Recommended capacity Maxr (kg)

e1 (kg) 3LC e1 (kg) 4LC e1 (kg) 3LC e1 (kg) 4LC 3LD-hopper 4LC-platform

500 0.5 0.5 0.2 0.5 1000 1000

1000 0.5 1 0.5 1 2500 2500

2000 1 2 1 2 5000 5000

3500 2 5 2 2 8000 10000

5000 5 5 2 5 12000 12500

Page 31: Vishay Transducer Application Notes

VPG-03VPG Transducers

Document Number: 11862Revision 29-Nov-2011

www.vpgtransducers.com8

Legal Metrology

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

AP

PL

ICA

TIO

N N

OT

E

As a second example, a hopper with a capacity of 5000 kg is built with three load cells type RLC-C3MI6-2t, with vmin =Emax / 7000.

The smallest verification scale interval (e1) can be obtained from the table above and equals 1 kg. Hence, the following ranges are allowed to be used:

0 - 3000 kg with e = 1 kg (3000 divisions) 3000 - 5000 kg with e = 2 kg (1000 divisions)

Multi-interval instruments shall satisfy the following conditions:

On a multi-interval instrument, the deviation on returning to zero as soon as the indication has stabilized, after the removal of any load which has remained on the instrument for one half hour, shall not exceed 0.5 e1.

The maximum preset tare value shall not be greater than Max1 and the indicated or printed calculated net value

shall be rounded to the scale interval of the instrument for the same net weight value.

It should be realized that the field of application for multi-interval weighing instruments has become smaller by the introduction of more accurate load cells and more sensitive measuring devices (indicators).

The hopper-application on the previous page could for example also be served with three load cells type RLC-C5-2t.

However the advantage of multi-interval instruments are a stronger signal per division (μV/d) and larger error limits at the high end of the measuring range (of particular interest at applications with the presents of force-shunts, i.e. hoppers).


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