IC211_Lecture1

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IC211

Experimental and Measurement Laboratory

Instructors:

N K Khosla H Arya

V M Gadre Bhaskaran Raman* (Coordinator)

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Measurements in everyday life

Car and Scooter Dash Board

LevelSpeed Engine RPM

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Tire pressure gauge

Blood pressure gauge

Clinical Thermometer

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Kitchen Scale Bathroom Scale Chronometer

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Measurements in everyday lifeopen loop control processes: Measurement of ?

Toaster Clothes Iron Table fan

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Engineering measurements

Furnace Temperature

Battery Voltagemeasurement

pH Measurement

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For evaluation & comparison: Hard disk access time and datatransfer rate

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Data gathering and analysis

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Determining characteristics of Light emitting diode (LED)

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Ability to generate reliable, useful dataRequires an appreciation of-

1. Quantity to be measured

2. How good should the measurement be

3. Data presentation

4. How should I measure (sensor/An/Dig)

5. How representative is the measured data

Course Objectives

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Quantity to be measuredIs the quantity directly measurable: toasting of bread or Ironing of clothes

manufacturing bulletproof armor.

Bullet proof

helmet

Required

microstructure

Heat treatment cycle

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Quantity to be measured

`

Is the true value

clearly definable

Thermal gradient across the thermometer stem

ensure that correct temperature may never be

read unless proper immersion practice is followed

What about

Surfaceroughness

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Choice of best methodfor measuring temperature

Effect of temperature on solid/ liquid/ gasexpansion (expansion thermometry)

Effect of temperature on change in

conductivity (Resistance thermometry) Other effects such as thermoelectric

(Thermoelectric thermometry)

Dependence of radiation on temperature(Radiation thermometry)

Sensitivity

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Choice of best technique

Perturbation to

the system

Non-contact

measurement

Measurement of Temperature

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Measurement of strain:

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Potential divider or bridge


Measures resistancedirectly

Measured voltage isused to computeresistance

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Dummy gauge forTemperaturecompensation

Temperaturecompensation &

increased signaloutput

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Cantilever load cell with strain gauge half bridge

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Higher sensitivity with temperature compensation

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Principles of measurements

To state sub-systems in a measurement system

To understand main function in each sub-system

To understand the basic properties of measurementsystems

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Basic components of a

measurement system

Basic components in a measurement system are-

It is also important to mention that a power supplyis an important element for the entire system.

Amplification and Conditioning

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Characteristics of an instrument Show the expected performance of the

instruments.

Divided into two categories: static and dynamiccharacteristics.

Static characteristics refer to the comparisonbetween steady output and ideal output when theinput is constant.

Dynamic characteristics refer to the comparison

between instrument output and ideal output whenthe input changes continuously with time.

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Accuracy is the ability of an instrumentto show the true value.

Normally related to the extent of the

wrong reading/ non accuracy.

Normally expressed as percentage ofthe full scale reading, or span of the

instrument.

Static Characteristics1. ACCURACY

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Example :

A pressure gauge with a range between 0-1bar with an accuracy of 5% fs (full-scale)has a maximum error of:

5 x 1 bar = 0.05 bar

100

Note: It is essential to choose anequipment which has a suitable operatingrange.

Static Characteristics

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Example :

A pressure gauge with a range between 0 -10 bar is found to have an error of 0.15bar when calibrated by the manufacturer.

Calculate :

a. The error percentage of the gauge.

b. The error percentage when thereading obtained is 2.0 bar.


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Answer :

a. Error Percentage = 0.15 bar x 100 = 1.5%

10.0 bar

b. Error Percentage = 0.15 bar x 100 = 7.5 %

2.0 bar

The gauge is not suitable for use in the lowpressure range.

Alternative : use a gauge with a suitable range,or better precision (lower error).


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Example :

Two pressure gauges (pressure gauge A and B) have afull scale accuracy of 5%. Sensor A has a range of 0-1bar and Sensor B, 0-10 bar. Which of these gauges ismore suitable to be used if the reading is 0.9 bar?

Answer :Sensor A :

Equipment max error = 5 x 1 bar = 0.05 bar

100

Equipment accuracy@ 0.9 bar ( in %) = 0.05 bar x 100 = 5.6%

0.9 bar


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Sensor B :

Equipment max error = 5 x 10 bar = 0.5 bar100

Equipment accuracy

@ 0.9 bar ( in %) = 0.5 bar x 100 = 55%0.9 bar

Conclusion :

Sensor A is more suitable to use at a reading of 0.9 barbecause the error percentage ( 5.6%) is smaller compared to

the percentage error of Sensor B ( 55%).


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2. PRECISION

An equipment which is precise is not

necessarily accurate. Defined as the capability of an

instrument to show the same reading

when used each time (reproducibility of

the instrument).


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Accuracy and Precision

Low accuracyhigh precision

High Accuracy

low precision

High accuracy

high precision

Accuracy: Accuracy is how close a measured value is to theactual (true!!) value

Precision: Precision is how close the measured values are to

each other.

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High Precision, butlow accuracy.

So, if you are playing soccerand you always hit the leftgoal post instead of scoring,then you are not accurate,but you are precise!


There is a systematic erroralso called BIAS.

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High accuracy means that the mean is close to the true value, whilehigh precision means that the standard deviation 1 is small.


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Not to be confused with accuracy in readout:

Accuracy in reading a measurement depends upon theinstrument you are measuring with, and the approximationsmade by the observer.

If the instrument measures with aresolution of 1 unit, any valuebetween 6.5 & 7.5 may bemeasured as 7.

Alternatively, any value between6 & 7 may be measured as 6.Likewise, any value between 7 &

8 may be measured as 7.

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Repeatability and Reproducibility

Repeatability is the variability of the measurements obtained byone person while measuring the same item repeatedly.

More repeatable

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Repeatability and Reproducibility

Reproducibility is the variability of the measurement systemcaused by differences in operator behavior (bias).

Variability of each operator is same.

Overall variability is high

Three operators

Repeatability and Reproducibilityare computed by:

Range and Average Method

Analysis of Variability method(ANOVA)

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3. BIAS

Constant error which occurs during the measurementof an instrument.

This error is usually rectified through calibration.

Example :

A weighing scale always gives a bias reading. This

equipment always gives a reading of 1 kg even

without any load applied. Therefore, if A with aweight of 70 kg weighs himself, the given reading

would be 71 kg. This would indicate that there is a

constant bias of 1 kg to be corrected.


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4. TOLERANCE

Closely related to accuracy of an equipment

where the accuracy of an equipment is

sometimes referred to in the form of

tolerance limit.

Defined as the maximum error expected in

an instrument.

Explains the maximum deviation of the

output (from the true value) at a certain

value.


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5. RANGE and SPAN

Defined as the range of reading between

minimum value and maximum value for the

measurement of an instrument. Span always has a positive value e.g..:

The span of an instrument which has a

reading range of 100C to 100 C is 200 C.


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6. LINEARITY

Maximum deviation from linear relation between input

and output. The output of an instrument should preferably be

linearly proportionate to the measured quantity.

Normally shown in the form of full scale percentage(% fs).

The graph shows the output reading of an instrument

when a few input readings are entered. Linearity= maximum deviation from the reading of x

and the straight line.


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Linearity

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7. SENSITIVITY

Defined as the ratio of change in output, related to

the corresponding change in input, at a steady statecondition.

Sensitivity (K) =

i

: change in output; i : change in input

Example 1:

The resistance value of a Platinum ResistanceThermometer changes when the temperatureincreases. Therefore, the unit of sensitivity for thisequipment is Ohm/C.


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Sensitivity

Variation of the physical variables

Most sensitive

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Example 2:

Pressure sensor A with a value of 2 barcaused a deviation of 10 degrees. Therefore,the sensitivity of the equipment is 5

degrees/bar. Sensitivity of the whole system is (k) = k1 x k2

x k3 x .. x kn

k1 k2 k3io


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Consider a measuring system consisting of atransducer, amplifier and a recorder, with sensitivity for

each equipment given below:Transducer sensitivity 0.2 mV/C

Amplifier gain 2.0 V/mV

Recorder sensitivity 5.0 mV/VTherefore, Sensitivity of the whole system:

(k) = k1 x k2 x k3k = 0.2 mV x 2.0 V x 5.0 mV

C mV V

k = 2.0 mV/C

Static CharacteristicsExample:

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Example :

The output of a platinum resistance thermometer (RTD)PT-100 is as follows:

Calculate the sensitivity of the equipment.

Answer :

Draw an input versus output graph. From that graph, thesensitivity is the slope of the graph.

K =

slope = (174.697-98.763) ohm = 0.38 ohm/Ci (470-270) C @370 C

,QSXW. 2XWSXW2KP

137.310

174.697

210.929

246.005

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0

S e r i e s 1

S i Ch i i

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8. DEAD SPACE / DEAD BAND

Defined as the range of input reading whenthere is no change in output (unresponsivesystem, caused by friction, backlash).

Dead Space

OutputReading

MeasuredVariables

- +


St ti Ch t i ti

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9. RESOLUTION

The smallest change in input reading that

produces a measurable change in the

output (smallest measurable input change).

Specified in the form % of full scale (% fs)


St ti Ch t i ti

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10. THRESHOLD

When the reading of an input is increasedfrom zero, the input reading will reach a

certain value before change occurs in theoutput.

The minimum limit of the input reading isthreshold (smallest measurable input).


C

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Dynamic Characteristics

Explain the behaviour of instruments

when the input signal is continuouslychanging

May be demonstrated by employing

standard input functions such as step

input, ramp input and sine input.

D i Ch i i

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Step Input

Sudden change in input signal from steady

state to an increased or decreased value

The output signal for this kind of input isknown as transient response of the

instrumentInput

Time


D i Ch t i ti

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Ramp Input

The input signal changes linearly with

time

The output signal for ramp input is adelayed ramp response.

Input/Normalizedoutput

Time


D i Ch t i ti

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Sine-wave Input

Input is a sine function e.g. H Sin(t)

The output signal is a generic (out of phase) sinefunction a Cos(t) + b Sin(t)

Study of output amplitude and phase gives the

frequency response of the instrument

Input

Time


R ti

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Response time

One would like to have ameasurement systemwith fast response.

In other words, the effectof the measurementsystem on themeasurement should beas small as possible.

Example of dynamic characteristics

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Example of dynamic characteristics

Response from a 2nd order instrument:

Output

100%

90%

10%

trTime

Another kind of dynamic error

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Dip effect

Another kind of dynamic error

How to use measured data

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How to use measured data

thermistor Thermocouple

PRT

Thermocouple characteristics Thermistor & PT100 characteristics

Platinum Resistance Thermometer

Need to calibrate

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Calibration table

Calibration curve

Need to calibrate

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Linear sensors

Spring scale

LVDT

Need to calibrate

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Linear calibration

curve

What did we actually measure

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Radiation effect in thermocoupleinstallations. Thermocouple seesfurnace parts at high, low as wellas intermediate temperatures

Immersion effect. Thermalmass of the thermometerchanges the system. Heatdissipated by thermometer


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Loading effect

Voltagedropped

Cold junction compensation

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