[BS 1041-3-1989] -- Temperature Measurement. Guide to Selection and Use of Industrial Resistance...

7/25/2019 [BS 1041-3-1989] -- Temperature Measurement. Guide to Selection and Use of Industrial Resistance Thermometers

1/22

BRITISH STANDARD BS 1041-3:1989

Temperaturemeasurement

Part 3: Guide to selection and use ofindustrial resistance thermometers

UDC 536.5:536.531:001.4


2/22

BS 1041-3:1989

This British Standard, havingbeen prepared under thedirection of theIndustrial-processMeasurement and ControlStandards Committee, waspublished under the authorityof the Board of BSI andcomes into effect on31 May 1989

BSI 01-2000

First published January 1943

First revision July 1960

Second revision March 1969

Third revision May 1989

The following BSI referencesrelate to the work on this

standard:Committee reference PCL/1

Draft for comment 86/22124 DC

ISBN 0 580 16795 X

Committees responsible for thisBritish Standard

The preparation of this British Standard was entrusted by the

Industrial-process Measurement and Control Standards Committee (PCL/-)to Technical Committee PCL/1, upon which the following bodies wererepresented:

British Coal Corporation

British Gas plc

British Pressure Gauge Manufacturers Association

Department of Energy (Gas and Oil Measurement Branch)

Department of Trade and Industry (National Weights and MeasuresLaboratory)

Energy Industries Council

Engineering Equipment and Materials Users AssociationGAMBICA (BEAMA Ltd.)

Health and Safety Executive

Institution of Gas Engineers

Coopted members

Amendments issued since publication

Amd. No. Date of issue Comments


3/22

BS 1041-3:1989

BSI 01-2000 i

Contents

Page

Committees responsible Inside front cover

Foreword ii0 Introduction 1

1 Scope 1

2 Definitions 1

3 Principle of resistance thermometry 2

4 Constructional features of metallic resistancethermometer sensors 2

5 Constructional features of semiconductor resistancethermometer sensors 3

6 Characteristics of resistance thermometers 3

7 Selection of resistance thermometer sensors 4

8 Procedure for installation 6

9 Measuring circuits 7

10 Measuring instruments 9

11 Digital data-processing and logging systems 10

12 Linearization 11

Figure 1 Typical construction of resistance thermometersensor 12

Figure 2 Resistance/temperature relationships for typicalsemiconductor resistance thermometer elements 13

Figure 3 Basic bridge circuit 14

Figure 4 Circuit for 2-wire system 14

Figure 5 Circuit for 3-wire system 14

Figure 6 Circuit for 4-wire compensated system 14

Figure 7 Bridge (2-wire system) 14

Figure 8 Bridge (simple 3-wire system) 14

Figure 9 Bridge (4-wire compensated system) 14

Figure 10 Differential system 14

Figure 11 Differential system with full conductor resistancecompensation 14

Figure 12 Four-terminal sensing resistor 15

Figure 13 Kelvin double bridge (modified) 15

Table 1 Operating temperature of resistance thermometersensing resistors 5

Table 2 Approximate relationship between resistance ratioand temperature for metallic sensing resistors 6

Publications referred to Inside back cover


4/22

BS 1041-3:1989

ii BSI 01-2000

Foreword

This Part of BS 1041 has been prepared under the direction of theIndustrial-process Measurement and Control Standards Committee. It is arevision of BS 1041-3:1969 which is withdrawn, revision having proved necessary

as a result of continuing developments. This revision is intended to provideguidance on the selection and use of resistance thermometers, primarily in thesphere of plant instrumentation but also for scientific and technological use inmany other fields.

A British Standard does not purport to include all the necessary provisions of acontract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunityfrom legal obligations.

Summary of pages

This document comprises a front cover, an inside front cover, pages i and ii,

pages 1 to 16, an inside back cover and a back cover.This standard has been updated (see copyright date) and may have hadamendments incorporated. This will be indicated in the amendment table on theinside front cover.


5/22

BS 1041-3:1989

BSI 01-2000 1

0 Introduction

All materials that conduct electricity exhibit some

change of resistance with temperature. However,the magnitude and character of that changedepends upon the material used, as does thetemperature range over which it may be used. Formany years practical thermometers relied upon asmall number of pure metals, having positivechanges of resistance with temperature. However,in the past few decades semiconductor materialshave become available, enabling the production ofresistance thermometer sensing resistorspossessing much greater variation of resistancewith temperature, and with negative or positivecharacteristics. Standardization of semiconductorelements has not yet been achieved but new fields ofapplication of resistance thermometry have beenopened up by their development. However, althougha wide variety of metallic and semiconductorresistance thermometer sensors have beendeveloped for special applications, particularly atvery low temperatures, this code is concerned onlywith those which have achieved substantialindustrial usage.

In the past, resistance thermometry practiceusually favoured the use of null-balance bridges,generally resistive, but sometimes capacitive or

inductive. Nowadays, constant current circuits areavailable enabling resistance thermometer sensorsto be used with standard voltage measuringinstruments (e.g. digital voltmeters). Also, as aresult of recent advances in digital electronics andthe use of microprocessors, there are now availablea number of digital thermometers which indicatedirectly in temperature units.

1 Scope

This Part of BS 1041 gives guidance on the selectionand use of industrial resistance thermometersincorporating a metallic or semiconductor sensing

resistor, which changes in resistance withtemperature.

NOTE The titles of the publications referred to in this standardare listed on the inside back cover.

2 Definitions

For the purpose of this Part of BS 1041, thefollowing definitions apply.

2.1resistance thermometer

a measuring device for ascertaining and exhibiting,in some suitable manner, the temperature of the

thermometer sensing resistor. Essentially, itconsists of a sensing resistor together with ameasuring element and some form ofinterconnection

2.2resistance thermometer sensor

a temperature-responsive device consisting of asensing resistor within a protective sheath, internalconnecting wires and external terminals to permitthe connection of electrical measuring elements

NOTE 1 Mounting means or connection heads may be included.

NOTE 2 Typical constructions are shown in Figure 1.

2.3sensing resistor

that part of the resistance thermometer sensor ofwhich the change in resistance is used to measuretemperature

2.4

measuring elementthat part of the thermometer which responds to thechange of resistance of the sensing resistor andenables an evaluation of the temperature of thatresistor to be made

2.5internal connecting wires

that part of the thermometer which provideselectrical connection between the sensing resistorand the terminals at the head of the sensors

NOTE Compensating leads may be included.

2.6

external connecting cablethat part of the thermometer which connects theterminals at the head of the sensor to the measuringelement

NOTE In some designs of thermometers which are not fittedwith terminals, the internal and external connecting wires maybe joined together within the head of the thermometer sensor.

2.7metallic resistance thermometer sensor

a resistance thermometer sensor, the sensingresistor of which is a metallic conductor

2.8

semiconductor resistance thermometersensor

a resistance thermometer sensor, the sensingresistor of which is a semiconductor

2.9resistance ratio

the ratio of resistance at a temperature t C to thatat 0 C (expressed as Rt/R0)

2.10padding resistor

a resistor which is sometimes used in conjunctionwith the sensing resistor to bring the resistance of

the thermometer sensor within specified limits


6/22

BS 1041-3:1989

2 BSI 01-2000

3 Principle of resistance thermometry

The electrical resistance of a material varies with

any change in its temperature. In a resistancethermometer sensor, a metallic conductor or asemiconductor material with a large, reproducibleand stable change of resistance with temperature ismounted to give mechanical and chemicalprotection while maintaining good thermal contactwith its environment. Electrical connections areprovided so that the resistance may be measured.This resistance can be related to temperature oncethe characteristic has been established.

4 Constructional features of metallicresistance thermometer sensors

Platinum is predominantly used for the sensingresistors of industrial metallic resistancethermometer sensors because its refinement andproperties are well established, itstemperature/resistance characteristic isreproducible and it can be used up to about 850C.Nickel is sometimes used on the grounds of economyor because of its better sensitivity, but itscharacteristic is less linear. Copper, which has goodlinearity but sensitivity poorer than nickel, is alsosometimes used, but neither of these base metals isnormally suitable for sensing resistors which are to

be used outside the range 100C to + 180 C.In order to maintain long-term stability it isnecessary to minimize strain in the sensing resistorduring fabrication and subsequent use. It is alsodesirable that resistance thermometer sensorsshould be constructed such that:

a) thermoelectric voltages which may begenerated by the use of dissimilar metals canceleach other;

b) current flowing through them producesinsignificant self-heating;

c) the windings are non-inductive;

d) they are suitable for use in measuring systemsusing direct current or alternating current atfrequencies up to 500 Hz;

e) the transmission of heat to and from thesensing resistor by conduction along the sheath,internal wires and insulators is negligible;

f) the insulation resistance between the sensingresistor (including its internal connecting wires)and the protective sheath is adequate.

The design of a platinum wire-wound industrialsensing resistor necessarily involves a compromise

between insensitivity to vibration and stability ofcharacteristic, since to maximize its ability towithstand vibration the wire should be fullyencapsulated and cannot therefore be entirelystrain-free. (In contrast, thermometers made for useas laboratory standards of the highest accuracy areusually constructed so that the resistance wire isfree to expand and contract with the minimum ofconstraint.) For industrial thermometers wherevibration levels are such that it is essential to attachthe wire firmly to the former, the wire is usuallywound upon a glass or ceramic former, which is thencoated with glass or ceramic cement. The coating is

selected in an attempt to match the expansionproperties of the platinum, but although thethermometer is extremely robust it has somewhatpoorer stability than a partially-supported coil. Thetemperature range over which it can be used doesnot normally exceed 500 C.

In the partially-supported coil construction, helicalcoils of platinum wire are mounted in the bores of amulti-bore alumina tube. The coils are anchored bya small amount of glaze so that while the greaterpart is free, a small portion of each turn is attached.An alternative method involves embedding theplatinum coil in alumina powder to reduce the

effects of vibration. By these techniques,thermometers with stabilities of a few hundredthsof a degree can be constructed for use over therange 200 C to + 850 C.

Recent years have seen the introduction of a designof metallic resistance thermometer sensor in whichthe sensing resistor is a film of platinum depositedonto a suitable substrate. Such sensors, which canbe produced at a very modest cost, are highlyinsensitive to vibration and have stabilities similarto those of wire-wound glass-coated detectors overthe range from about 50 C to + 500 C. They areparticularly suited to applications such as surfacetemperature measurement and air temperaturemonitoring. They generally show fast time response,due to the intimate contact of the film with thesubstrate and the lower mass that needs to beheated.


7/22

BS 1041-3:1989

BSI 01-2000 3

Constructional methods similar to those describedfor platinum may be used with other metals, such as

copper and nickel. Resistance thermometer sensingresistors of all types can be fabricated in variousshapes, limited only by the need to ensure anadequate electrical resistance efficiently insulated.The surface area can be made large in relation to thevolume to provide fast response or the sensingresistor can be made compact for measuringtemperature at a point. Alternatively, it can beextended over a considerable distance so as tomeasure an average temperature.

It is sometimes permissible to immerse the sensingresistor directly in the medium of which thetemperature is being measured. This method has

the advantage that the sensor responds rapidly totemperature changes. Generally, however, someform of protection is necessary. This may be merelya ventilated cover for mechanical protection as inthe measurement of static or low-velocity airtemperatures, or a completely enclosed and sealedsheath for protection against corrosive orelectrically-conductive fluids, high pressures orabrasive media.

Where total enclosure of the sensing resistor isnecessary, special consideration should be given tothe thermal conductance between the sheath andthe sensor and precautions will be needed tominimize errors caused by conduction of heat alongthe sheath as well as along the internal connectingwires. The time of response and the self-heating ofthe sensing resistor may also be significant,especially if it is situated inside a heavy thermowell,or is being used to measure static gas temperatures.

5 Constructional features ofsemiconductor resistancethermometer sensors

The temperature-sensitive material is usually asintered metal oxide and is often encapsulated in

glass. As no support is required and as theresistivity is much higher than that of any metalused in resistance thermometry the sensingresistors can be extremely small. Typically abead, 0.25 mm to 0.5 mm diameter, is thinly glazed,and supported by its leads. To provide furtherchemical and mechanical protection and electricalinsulation it may be sealed in the tip of the glassprobe.

Sensing resistors in rod or disc form are commonlyavailable, thin discs being particularly suitable forsurface temperature measurement. However,generally, semiconductor sensors are notappropriate for use in averaging temperaturemeasurement.

6 Characteristics of resistancethermometers

6.1 General

Characteristics which are common to both metallicand semiconductor resistance thermometer sensorsinclude the following.

a) An external power supply is always required toenergize the resistance thermometer sensor.Operation may be by direct current or byalternating current at frequencies usually not inexcess of 500 Hz.

b) The energy dissipated in the sensing resistorby the current that passes through it causes a riseof temperature of the resistor above itssurroundings. The magnitude of the temperaturerise depends upon the design and construction ofthe sensing resistor, its mounting and themedium in which it is used, as well as upon themeasuring current. For example, an unmountedresistor which has a self-heating effectof < 0.01 C/(mW) when immersed in a stirredwater bath, may show an effect between 20and 40 times greater when used in unstirred air.In practice, the energy dissipated is limited so asto prevent significant errors in temperaturemeasurement, while still maintaining arelatively high output signal.

c) Thermal response time is limited by the need toprotect and insulate the temperature-sensitivematerial. Variations in construction lead towidely differing response times. A response timeof 0.5 sor less can be achieved with some sealedsensors.

d) It is possible to make circuits intrinsically safe.

e) The accuracy of the measuring elementcan be checked by substituting precisiontemperature-stable resistors for the sensingresistor.

f) Measuring circuits can be used in which nocompensation is necessary for changes inambient temperature.

6.2 Metallic resistance thermometer sensors

Metallic resistance thermometer sensing resistorscapable of providing accurate, reliable andreproducible temperature measurement in therange from about 260C to + 850 C (or higher)are available, but it should be emphasized that onlywith more specialized resistance thermometers cantemperatures below 200 C or above 600C bemeasured. Metallic sensing resistors possess thefollowing special characteristics.

a) The temperature coefficient of resistance isalways positive.


8/22

BS 1041-3:1989

4 BSI 01-2000

b) Resistance thermometer sensing resistorsmanufactured to conform to standard

characteristics are electrically interchangeablewithin defined tolerances (see BS 1904).

c) The mathematical expression relatingresistance ratio and temperature of platinum,given in BS 1904, may be employed in bridgenetworks and computer calculations.

d) The stability of a metallic resistancethermometer sensing resistor makes it suitablefor narrow temperature spans when a sufficientlysensitive measuring element is available.

e) Calibration checks are required only where thegreatest possible accuracy is necessary, or when

overheating or other misuse is suspected.f) Any number of sensors can be switched to thesame measuring element, although switchresistance may influence the accuracy attained.

6.3 Semiconductor resistance thermometersensors

Semiconductor resistance thermometer sensingresistors are normally capable of reproducibletemperature measurement over a limited part of therange 100 C to + 300 C. However, withspecialized materials and constructions,temperatures outside this range can be measured.

Semiconductor temperature sensing resistors, ingeneral, possess the following characteristics.

a) Semiconductor resistance thermometersensors have sensitivities very much higher thanthose of metallic thermometer sensors.

b) Semiconductor resistance thermometersensing resistors are usually of much higherresistance than metallic resistors, so they are lessaffected by interconnection resistances.

c) Materials with either positive or negativetemperature coefficients of resistance areavailable.

d) Semiconductor sensing resistors can be madevery cheaply when errors within about 1 C areacceptable on the replacement of a sensor. Whencloser interchangeability is required the sensorsmay be adjusted, or specially selected by themanufacturer, or compensated by a suitablenetwork.

e) Approximate linearity of resistance change of acircuit, over a range of 20 C about a nominaltemperature, can be achieved by a simple shunt.Similarly, linearity of voltage output can beobtained by the use of a series resistor. Linearityover wider ranges can be achieved by more

sophisticated electronic means.

f) The stability of a suitably processed andprotected semiconductor sensing resistor,

subjected to a limited range of temperatures, canbe comparable with that of a standard metallicsensor. Glass-protected types are the most stableand are suitable for the higher temperatures ofuse.

g) Overheating or other misuse can cause asignificant change of characteristic. The effect ofdamage is not always obvious and it is advisableto check the sensor accuracy at regular intervals,or when misuse is suspected.

h) Any number of sensors can be switched to thesame measuring element. Accuracy is thenlimited by sensor interchangeability.

7 Selection of resistance thermometersensors

There is an extremely wide range of resistancethermometer sensors available and selection of themost suitable for a particular application needscare. Where significant levels of vibration, thermalshock or nuclear radiation are likely to beencountered in service, the manufacturer should beconsulted because some designs of thermometersensors are likely to be affected less than others. Itis particularly important that the manufacturer be

consulted where thermometers are to be exposed toneutron irradiation, because as well as stabilityconsiderations, the elements of the sensor maybecome radioactive, creating long term handlingproblems. Some of the relevant parameters are asfollows.

Plastics begin to deteriorate when the accumulatedexposure to gamma radiationreaches 106Gy (108 rads) although with someplastics it is possible to exceed 107 Gy (109 rads)when the exposure is accumulated in a short time.

A wide range of materials have a significant cross

section for neutron capture and in some cases longterm radiation sources can be created by exposure tothermal neutrons for a short time. Longer exposuresresult in transmutation of the materials so that theelectrical resistance will change.

Materials in general show smaller effects fromexposure to fast neutrons. However, accumulationsof damage due to atom displacement andtransmutation, which fast neutrons cause, canproduce hardening and other effects in metallicmaterials and fragmentation of some ceramics.


9/22

BS 1041-3:1989

BSI 01-2000 5

In addition to finding a suitable type of sensor forthe particular environmental conditions, it is

necessary to consider methods of indication,compatibility with existing systems, and overalleconomy as factors in sensor selection. However, forgeneral purpose measurement of temperature at apoint, platinum resistance thermometer sensors toBS 1904 with the preferred sizes given in BS 2765will usually be found to be satisfactory and shouldbe used whenever possible.

If these specifications are not acceptable, it isadvisable to consider other established types ofresistance thermometer sensors, as special designsare expensive and can entail delay whenreplacements are required.

A semiconductor sensing resistor is most likely to bechosen where small size, high output, low cost orspecial characteristics are significant factors.

Semiconductor sensing resistors comprise threemain types.

a) Those having a negative temperaturecoefficient of resistance of exponential form,usually made from metal oxides. These sensingresistors are usually called NTC thermistorsand are the type most commonly used fortemperature measurement.

b) Those having a positive temperature

coefficient which is relatively linear over thecustomary range of use; the commonest type isthe silicon resistor. These sensing resistors areusually called PTC thermistors.

c) Those having a positive temperature coefficientwith a marked discontinuity at a characteristic

temperature at which the resistance rises verysteeply. This characteristic point depends uponthe semi-conductor material composition and canbe varied in manufacture. The material is usuallya metallic titanate. These sensors are mainlyused as switching devices for over-temperatureprotection of transformers, motors, heaters, etc.

Other semiconductors using silicon, germanium andgallium arsenide are also used in temperaturemeasurement. However, with the exception ofgermanium, which is used in resistancethermometry well below 0 C, these materials areusually used in the form of diodes over very limited

temperature ranges.Figure 2 shows the resistance/temperaturecharacteristics of typical semiconductor sensingresistors compared with platinum.

The temperature limitations of the more usualsensing resistors are given in Table 1. Othermaterials are available for special application,e.g. tungsten or molybdenum metallic sensors forhigh temperature use, rhodium-iron andgermanium sensors for low temperatures, andspecial types of semiconductor sensors for use upto 1 000 C, but these should be used only after

careful consideration of the problems involved.

Table 1 Operating temperature of resistance thermometer sensing resistors

Sensing resistorNormal minimum

operatingtemperature

Normal maximumoperating

temperature

Special maximumoperating

temperature

C C C

Metallic sensing resistors

Copper

NickelPlatinum

100

60 200

+ 100

+ 180+ 600

+ 150

+ 350+ 850

Semiconductor sensing resistors

Mixed metal oxidesSilicon

100 160

+ 200+ 160

+ 600+ 200

NOTE 1 Satisfactory measurement at temperatures above the normal maximum is possible only when special constructions andcarefully controlled environments for the sensing resistors are used.

NOTE 2 Platinum resistance thermometer sensing resistors of special construction can be used to measure temperatures downto 259 C (14 K). Below 200C, sensors have to be individually calibrated.

NOTE 3 Copper resistance thermometer sensing resistors of special construction can be used to measure temperatures downto 200 C.


10/22

BS 1041-3:1989

6 BSI 01-2000

The approximate relationship of resistance ratio totemperature for platinum, nickel and copper is

given in Table 2.Table 2 Approximate relationship betweenresistance ratio and temperature for metallic

sensing resistors

8 Procedure for installation

It is assumed in the siting of the thermometersensor and the choice of thermometer pocket thatany advice given by the suppliers of the resistancethermometer sensor has been considered. Thefollowing additional advice is applicable to mostinstallations.

a) Cables containing single-strand cores have thedisadvantage that breakage of the strand resultsin disconnection of the circuit, while multi-strandconductors give rise to the possibility of erroneousmeasurements in bridge circuits due tounsuspected changes in conductor resistancecaused by strand breakage.

b) Cables should be routed, and their totalresistance should be limited, so as to minimize

changes in conductor resistance due to ambienttemperature changes.

c) Cables should be positioned to minimize anyelectromagnetic pick-up from adjacentcurrent-carrying conductors. In particular,parallel runs should be avoided. It is usuallyadvisable to enclose the conductors in aconducting screen which may take the form ofconduit or continuous braiding. Twisting of thewires together is also effective.

d) Insecure electrical connections are a source ofvariable resistance. Where joints between cablesare essential, they should be in a junction box for

ready inspection. Conditions causingdeterioration of joints are vibration, corrosionand thermal cycling.

e) No earth connection should normally beallowed on either the sensing resistor or any partof the circuit. A test of insulation resistance toearth is usually desirable; this should be repeatedwhenever exposure to damp or corrosion issuspected. It is occasionally necessary to preventthe build up of static electricity in the system,since this can cause insulation breakdown. Aresistive earth connection from a suitable point in

the measuring circuit is then desirable.f) When a 2-wire bridge circuit is used, themeasuring element is arranged for a constantterminal input resistance at a fixed temperature.A ballast resistor, adjustable by the user, bringsthe measured external resistance of the sensorand connections up to this value.

g) When 3-wire or 4-wire bridge circuits are used,each wire should have the same resistance. Thetotal resistance should be limited to the valuerecommended for the measuring instrumentselected.

It is often convenient to connect two or morethermometer sensors in sequence to a singlemeasuring element. It is necessary to use aswitch of constant low resistance and to ensurethat the various interconnection resistances areall within the tolerances appropriate to themeasuring circuit being used.

h) Conductor materials, and the location ofelectrical connections between differentconductor materials should be chosen so as tominimize errors caused by thermoelectric effects.

Systems using a.c. excitation are not prone toerror from this source.

i) It may be necessary to take into account thecapacitance between conductors in an a.c. system.

TemperatureResistance ratioRt/Ro

Platinuma Nickel Copper

C

200 0.18

100 60 50

0.600.760.80

0.700.74

0.570.740.79

050

100

1.001.191.38

1.001.291.62

1.001.211.43

150180200

1.571.681.76

1.992.23

1.65

250300350

1.942.122.30

400500

600

2.472.81

3.14

700800850

3.453.763.90

NOTE Some thermometer sensors use padding resistors tobring the resistance of the sensor within specified limits.Generally, they are used in series with the sensing resistor, butin some types of nickel thermometers both series and shuntpadding resistors are used to enable the thermometer sensor tomatch an exponential resistance/temperature curve.

a See BS 1904.


11/22

BS 1041-3:1989

BSI 01-2000 7

9 Measuring circuits

9.1 General

Measurements are made by passing currentthrough a sensing resistor and measuring thepotential across it. If the current is known, thepotential is a measurement of the resistance andhence the temperature. If the current is not knownexactly the potential may be compared with thepotential across a known resistor; this is the basis ofthe bridge systems discussed below. Bridge circuitsmay be either the null-balance (balanced-bridge)type or the direct-deflection (fixed-bridge) type.Whichever of these methods is employed, therequirement is to determine the resistance of the

sensing resistor independently of the resistance ofthe connections.

9.2 Bridge systems

9.2.1 General.All bridge resistors, the temperaturesensing resistor excepted, are arranged to have anegligible change of resistance with temperature,and in a.c. bridges are non-inductive.

When a bridge circuit is used, it is customary toconnect the resistance thermometer sensor to themeasuring bridge by copper conductors, which mayhave an appreciable change of resistance withtemperature. Assuming that the correct installation

procedure has been followed, errors due to changesin conductor temperature are kept withinacceptable limits, partly by making the conductorresistance small in relation to the sensing resistorand partly by the circuits discussed below.

All circuits require a source of low voltage electricalsupply which is commonly a smoothed andstabilized d.c. power source. In some designs, analternating supply is used, which may have afrequency of up to 500 Hz.

9.2.2Balanced-bridge instruments.The bridgecircuit is maintained in a balanced condition bymanual or automatic adjustment of resistance inone or more arms of the bridge. In Figure 3 toFigure 11, balance means an absence of potentialbetween a and c and hence zero current throughthe balance detector, which may be a galvanometeror an electronic amplifier.

The adjustable resistance usually takes the form ofa slidewire which is linked mechanically orelectrically to an associated temperature scale.Figure 3 illustrates the simplest form of this circuit.The sensing resistor is contained within the armcd. The condition of balance occurs whenRab/Rbc = Rda/Rcdso that the value of Rdais a measure

of Rcdwhen Rab/Rbcis known. It is usual practice tomake Rab = Rbcso that Rcd = Rdaat balance.

The balanced condition of the bridge is not affectedby normal variations in the voltage supplied to it,

but it should be noted that the current in thebalance detector when the bridge is not balanced isproportional to the applied voltage. This affects theout-of-balance voltage which may occur before acorrection to the resistance Rdais required, i.e. itaffects the discrimination of the measuring system.

9.2.3 Compensation for conductor resistance.Simplecircuits illustrating the method of compensating forconductor resistance are shown in Figure 4to Figure 6. Compensation is only completelyeffective in balanced-bridge circuits.

In industrial applications of these electrical circuits,the resistance thermometer sensor can be remote

from the rest of the bridge and connected to it bycopper conductors. The resistance thermometersensor together with the conductors constitutes Rcd.

Figure 4 represents a circuit designed for a fixedmaximum value of conductor resistance, with anadjustable resistor inserted in cd to make up thismaximum value. The resistance of the copperconductors changes with variation in the ambienttemperature and when the conductors are long or ofinadequate cross section, this change in resistancemay be so large as to cause a significant error in thetemperature reading. (For example, the

temperature coefficient of resistivity of copper issuch that a copper cable of resistance 1 7willchange by 4 m7, per C change in ambienttemperature; this is equivalent to 0.02C change inresistance reading of a 100 7platinum sensingresistor, if two such leads are used to connect it tothe measuring circuit.)

The error may sometimes be kept within acceptablesystem limits by choice of conductor size,but 2-wire installations are usually restricted toa maximum of 1 7to 2 7per conductor resistance(corresponding to about 100 m of cable). Otherforms of bridge are used for cable runs in excess of

this and are satisfactory for cable runs of 10 7to 15 7per conductor (typically 1 km).

Figure 6 indicates how the effect of the resistance ofthe conductor and its variation with temperaturecan be substantially eliminated by inserting anequal length of identical conductor in da (generallyusing multi-core cable). This is commonly describedas a 4-wire compensating-cable system.

Figure 5 shows how a similar result may beobtained by connecting one conductor of the powersupply to the connecting head of the resistancethermometer sensor. This is commonly described as

a 3-wire system.In the circuits represented by Figure 5 and Figure 6it is necessary for Rabto be equal to Rbcto obtaincomplete compensation.


12/22

BS 1041-3:1989

8 BSI 01-2000

In addition to the errors introduced by theresistance of the conductors, the sliding contact

incorporated in the arm da in Figure 3 to Figure 6is capable of introducing errors, since resistance atthe contact is added into the bridge arm. Variouscircuit arrangements are employed in practice(see Figure 7 to Figure 11) to avoid such errors, byarranging that contact resistance is introduced intothe current supply or the balance detector circuit,where it cannot affect the accuracy of the bridgebalance.

In the circuit of Figure 8, the resistance of theconductors in the bridge arms should be equal but,even so, the balance position is completelyindependent of interconnection resistance at only

one position of the contact, where Rab = Rbc. By asuitable choice of values, however, adequateaccuracy can be obtained over the full range ofcontact movement.

A compensated circuit, using one slidewireonly, is provided by the use of a 4-wire system(see Figure 9). When Rab = Rbc, the position ofbalance is completely independent of conductorresistance, provided that the resistances of eachconductor pair are equal.

All the bridges systems described can be madeself-balancing by using a servo-mechanism

controlled from the balance detector.9.2.4 Inductive-ratio bridge.This is an a.c. bridgemethod incorporating precision-woundtransformers for ratio arms. It is capable of thehighest accuracy and can be made robust andtransportable; it has a negligible temperaturecoefficient and can be made very stable.

9.2.5 Fixed-bridge instruments.In a fixed-bridgeinstrument only the sensing resistor is allowed tovary, the other bridge resistance being chosen sothat the bridge is in balance for one value of Rcd.

At temperatures represented by other values of Rcd,the out-of-balance voltage developed across a-c is ameasure of the temperature, provided that thebridge supply voltage is stabilized.

2-wire, 3-wire or 4-wire circuit arrangements(see Figure 4 to Figure 6) may be used, Radbeing afixed resistor.

If a 2-wire system is used, the method of correctionfor conductor resistance is the same as that used fora balanced bridge.

If a 3-wire or 4-wire system is used, compensation isonly complete at the point when the bridge isbalanced. The error in the latter can be reduced byraising the resistance values of Raband Rbcso as tominimize changes in bridge current as thethermometer sensor resistance changes withtemperature.

Although the detector may be a simplegalvanometer with direct deflectional indication of

temperature, in practice an electronic amplifier isnormally used to provide a high input impedanceand sufficient power to drive a more robustdeflectional instrument. Alternatively, the bridgeout-of-balance voltage can be measured using adigital voltmeter or a potentiometric indicating andrecording instrument.

9.2.6Differential temperature measurement.Fordifferential temperatures a second resistancethermometer sensor is introduced into da,two 2-wire cables being used (see Figure 10); thisarrangement is suitable for conditions in which bothcables are of similar resistance. Where the two

cables are of different length or resistance and thehighest accuracy is required, improvedcompensation is effected by the use of two 4-wirecables (see Figure 11). Bridge arms cd and daeach contain a pair of wires from both cables and atthe balance point Rcd = Rda.

9.3 Potential systems

If the sensing resistor is energized from anaccurately-known and constant current source, thepotential difference developed across it can bedirectly related to resistance, and thus totemperature.

A four-terminal network is used in the mannershown in Figure 12.

The principles of a potential system are as follows.

a) During measurement negligible current flowsin the potential circuit. This requires that theinput impedance of the potential measuringdevice be considerably greater than the sensingresistor, in order to minimize circuit loadingerrors during measurement. (For example,a 0.1 % error will result from an inputimpedance 1 000 times the sensor resistance.)

b) A temperature-measurement signal in the

form of a voltage is available.c) A number of sensors can be connected in serieswith the same current source, enabling voltagesfrom each to be scanned at any speed acceptableto the measuring instrument.

d) Accurate measurements of resistance can bemade if the current is accurately known.Alternatively, accurate comparisons of resistancecan be made, since the current is constant eventhough, possibly, unknown.

e) Measurements are independent of conductorresistance and selector switch contact resistance.


13/22

BS 1041-3:1989

BSI 01-2000 9

Potential systems can be used for accuratehigh-speed work when a number of measurements

have to be made repeatedly, as in scanning anddata-handling. Such systems are readily keptaccurate by frequent checking of the voltage acrossa stable check resistor carrying the same current asthe resistance thermometer sensors. The sensorscan be connected in series with the same currentsource, the voltage across each in turn beingmeasured. Alternatively, the current source can beswitched to each sensing resistor in turn.

Small errors in current, attributable to such causesas ripple, poor resolution of the current setting orpoor regulation, appear directly as an error inread-out. Due regard should be paid to the effects of

change in load resistance, ambient temperature anddrift with time when selecting a constant currentdevice. In particular, design limits on the maximumload resistance of the current source may restrictthe number of resistance thermometer sensors thatcan be connected in series.

10 Measuring instruments

10.1 General

Clause 9describes the circuit principles mostcommonly used in resistance thermometry. Thevarious instruments available which embody these

circuits are outlined in 10.2to 10.6, but fulldescriptions of the instruments are not given.

NOTE Consideration of the accuracy of any instrument orsystem has been specifically excluded.

10.2 Instruments that include fixed-bridgecircuits

10.2.1 General.The out-of-balance potential of afixed-bridge changes progressively with changes insensing resistor temperature and offers a changingsignal to a detector.

10.2.2 Galvanometer instrument.A detector whichwas commonly used in the past, and which is still

sometimes used, is the moving-coil galvanometer. Itmay be arranged to indicate temperature directly ona graduated scale.

The galvanometer may be fitted with one or morelimit detectors which operate at pre-set deflections.Photoelectric or inductive principles are commonlyemployed. Operation of the detector may be used foron-off control and for alarms.

10.2.3D.C. amplifier or other signal-converter.Thefixed-bridge out-of-balance voltage is amplifiedwithout significant disturbance of the bridge power,to provide an analogue output of sufficient power tofeed into local or remote indicators, recorders orcontrollers.

Alternatively, an analogue-to-digital converter isused in conjunction with a linearizing circuit or

microprocessor, to provide a digital display intemperature units.

An amplifier designed with a very low powerrequirement is used in conjunction with afixed-bridge network to produce a signal-converterfor installation close to the resistance thermometersensor. As well as minimizing sensor connectingcable resistance, the converter provides a largeanalogue signal with high electromagneticinterference immunity for connection to remotecontrol or data-handling equipment. Twoconductors are used to connect the converter to itsremote power supply and to carry both supply and

measurement currents. The transmitter current isusually 4 mA d.c. to 20 mA d.c., the amplifier andfixed-bridge being adjusted so that 4 mAcorresponds to the minimum measuringtemperature.

10.2.4 Self-balancing recorder or indicator.Apotentiometric recorder is used to measure theout-of-balance voltage across the fixed-bridge. Theconstant-voltage bridge supply and fixed-bridgeresistors are usually contained within theinstrument circuit.

10.3 Instruments that include null-balance

bridge circuits10.3.1 General.A null-balance bridge requiresadjustment of the resistance or impedance value inone, two or three arms of the bridge in order toachieve a balance; a detector serves to determinethat balance has been reached.

The position of the adjusting mechanism is then ameasure of temperature. The instrument may takeone of the forms given in 10.3.2or 10.3.3.

10.3.2 Manually-adjusted bridge.This may use agalvanometer or, more usually, an amplifier andanalogue panel meter.

10.3.3Automatic self-balancing bridge.Thisnormally employs an amplifier as the detector,which reacts to any out-of-balance condition andactuates a servo-mechanism to balance the bridge.The mechanism may form part of an indicator,recorder or controller.

10.4 Instruments that include potentialsystems

The instrument is connected directly across theresistance thermometer sensor, which is energizedby a stable current source. It is important that theimpedance of the measuring element is high enough

to ensure that the current through it is negligiblecompared with the total current through thetemperature sensing resistor. The measuringelement may take one of the following forms.


14/22

BS 1041-3:1989

10 BSI 01-2000

a) Manually-operated potentiometer.

b) Self-balancing potentiometric indicator,

recorder or controller.c) Voltage amplifier or other signal-converter.This provides an analogue output of sufficientpower to feed into separate indicators orrecorders.

d) Digital voltmeter. This provides a directreading of the voltage across the resistancethermometer sensor. It may also be used to feed adigital signal into a remote display unit, acomputer or other data-handling systems.

10.5 Multi-point instruments

A multi-point instrument is one in which a single

measuring element is used for determining thetemperatures of each of a number of differentresistance thermometer sensors.

Connection to each of the sensors is made by meansof a selector switch (which may be mechanical orelectronic), and the connection is maintained forsufficient time to permit the sensing and measuringelements to respond fully. The switch is usuallydriven so that it selects thermometer sensors in aregular sequence, the response time of the combinedsensing and measuring elements imposing apractical upper limit on the frequency of selection.

The sensor selector switch is inserted directly intothe measuring circuit; with some circuits, particularcare in design is essential to minimize possibleerrors arising from switch contact resistance andswitch thermal e.m.f.s. The errors are most likely tobe significant in systems designed for rapidselection.

Solid state switching is essential for some fastscanning systems, and can lead to accuraciescomparable to those obtainable with the best type ofmechanical switching.

Multi-pen recorders are frequently used to overcomethe problems of discontinuous measurement andpossible input selector switch problems inherent inmulti-point instruments. Each measured input maybe complete with its own amplifier, measuringcircuit, servo-mechanism and recording pen,permitting a different temperature range for eachrecord.

Modern designs of instruments may have anindividual pen and step-motor servo for each input,selection of inputs and repositioning of the recordingpen being microprocessor-controlled.

10.6 Multi-range instruments

A multi-range instrument is one provided with a

means of selection which permits its use on any oneof two or more temperature ranges, the span of themeasuring element being caused to correspond witheach range thus selected.

The accuracy of the complete instrument system isoften limited by the accuracy of the measuringinstrument, stated as a percentage of span.Optimum accuracy is then obtained by choosing thenarrowest temperature range which is appropriateto the sensitivity of the measuring instrument.

In some instruments the range selection is madeby means of a switch; in others (particularly

digital-display thermometers) it is sometimesnecessary to interchange printed circuit cards.Where a range selector switch is fitted, this maygive rise to errors due to contact resistance or tothermal e.m.f.s at the switch contacts.

11 Digital data-processing and loggingsystems

11.1 General

Data-logging is the automatic measurement andrecording in digital form of a number of inputsignals. The information may be presented in

various forms, e.g. typewritten in directtemperature units, coded on magnetic tape or disc,or punched tape or cards, for subsequent processing.

A typical system comprises a multiplexer (scanner),an analogue-to-digital converter and an outputdrive unit. To this basic system may be addedmodules to provide amplification of the input signalbefore measurement, linearization of theresistance/temperature characteristics of thesensing resistor, alarm initiation, etc.

11.2 Conversion systems

The resistance change of the resistance

thermometer sensor can be converted to acorresponding d.c. voltage required by ananalogue-to-digital converter for data-loggingequipment by the following methods.


15/22

BS 1041-3:1989

BSI 01-2000 11

11.2.1Potentiometric.An accurately-knownconstant-current source is switched to each

thermometer sensor in turn, together with thepotential measuring connections to the converter.Four interconnections are required for each sensor.In some circuits direct output recording intemperature units requires zero voltage input whenthe temperature is at scale zero. This condition maybe satisfied by connecting an accurate referencevoltage (usually adjustable), corresponding to thevoltage developed across the sensor at 0 C, inopposition to the incoming signal beforepresentation to the analogue-to-digital converter.Alternatively, in some modern digital equipment,the 0 C resistance of the thermometers may be

keyed into the voltmeter, which then calculates thetemperature directly.

11.2.2 Fixed-bridge.A number of fixed-bridges, eachconnected to a separate resistance thermometersensor, are supplied from an accurateconstant-voltage d.c. source. The bridgeout-of-balance voltages are connected to themultiplexer.

The voltage supply is usually common to all bridgesand, consequently, 2-pole out-of-balance voltageselection is imperative. As open-circuit bridgepotentials are being measured the input impedanceof the measuring instrument should be high enoughto prevent circuit-loading errors.

Compensation for the resistance of the connectingwires and cable by 3-wire interconnections iscommonly used, although designs using a modifiedKelvin double bridge circuit are available(Figure 13). These require 4-wire connections toeach thermometer and further reduce conductorresistance errors.

11.2.3 Fixed-bridge with voltage amplifier.Anamplifier may be used with each fixed-bridge toprovide a higher voltage or current (0 V to 5 Vor 4 mA to 20 mA). Industrial practice is to use

a 2-wire converter (4 mA to 20 mA), as describedin 10.2.3, for each resistance thermometer input.The increased output permits modification byshunting and potential-dividing and may morereadily be made compatible with a system havingfacilities for measuring a variety of physicalquantities, all converted to a common outputsignal (4 mA to 20 mA) for logging or processing.

The higher voltages available ease the duty of themultiplexer and are preferred for use with

semiconductor switching. The location of theconverter close to the resistance thermometersensor minimizes interconnecting resistanceproblems and the high signal level electricalinterference immunity, together with highresistance capability (typically 1 000 7) iseminently suitable for industrial use. Such systemscan be designed to meet intrinsic safetyrequirements.

12 Linearization

12.1 General

Most sensing resistors use materials which havenon-linear resistance/temperature characteristics.For many applications it is necessary or convenientto have a pointer deflection, digital indication oroutput signal which varies linearly withtemperature changes.

12.2 Sensing resistor linearization

12.2.1 Metallic sensors.Many measuring elementsare themselves approximately linear, and overalllinearity may be achieved by the use of sensingresistors with linear characteristics. Copper can beconsidered to have linear characteristics over thetemperature range from 0 C to 100 C, but to relateresistance to temperature outside this range, asecond-order term needs to be introduced. Thesecond-order terms in the characteristics ofplatinum and nickel have opposite signs; acomposite sensing register of these two metals canbe made approximately linear over the range of 0 Cto 100C.

12.2.2 Semiconductor senses.In systems usingnegative temperature coefficient thermistors,optimum linearity is obtained when the thermistoris energized from a source resistance having a valuegiven by the equation:

where

Rs is the source resistance;

RT is the thermistor resistance at temperatureTm;

B is the material constant of the thermistor(in kelvins);

Tm is the temperature at the mid-point of thelinear range (in kelvins).

Rs RTB 2 Tm

B 2 Tm+------------------------=


16/22

BS 1041-3:1989

12 BSI 01-2000

The resistance may be connected across thethermistor in constant-current systems, or in series

in constant-voltage systems. The value of Rsmayhave to be adjusted to allow for the resistance of themeasuring element.

12.3 Measuring element linearization

An instrument with a null-balance bridge, or apotential system, can provide a linear temperatureoutput with special circuits or, in a few instances, bycam-corrected slidewires.

In a fixed-bridge circuit there is a non-linearrelationship between the out-of-balance voltage andthe change in value of the resistance thermometersensor. The extent of this non-linearity depends not

only on the non-linear characteristics of the sensingresistor but also on the load power which is drawnfrom the bridge by the indicator, i.e. it is inverselyrelated to the indicator input resistance.Non-linearity due to this effect may be considerablefor a galvanometric indicator but may be negligiblefor a potentiometric indicator or a d.c. amplifierwith a high input impedance.

It is easier to make a linear fixed-bridge instrumentwith a nickel resistance thermometer sensor than

with a platinum resistance thermometer sensor.The simple bridge network and a nickel sensor haveopposing departures from linearity, which can bemade to cancel one another. The departure fromlinearity of a platinum sensor and a bridge areadditive, but correction may be made by using activecircuits or non-linear components.

When the bridge out-of-balance voltage is fed to acomputer or to an instrument containing amicroprocessor, the conversion to temperature canbe made by mathematical manipulation. Thenon-linear response of the sensing resistor may becorrected by polynomial-fitting to a standard curve,

the thermometer coefficients being stored in thecomputer or in a read-only-memory in themicro-processor. Alternatively, segmentedcurve-fitting procedures may be used involving thestorage of resistance/temperature tables in thecomputer or in a read-only-memory. In both cases itis possible to store coefficients or tables which relateto an individually-calibrated thermometer sensor.

Figure 1 Typical construction of resistance thermometer sensor


17/22

BS 1041-3:1989

BSI 01-2000 13

NOTE Platinum shown for reference.

Figure 2 Resistance/temperature relationships for typical semiconductor resistancethermometer elements


18/22

BS 1041-3:1989

14 BSI 01-2000

Figure 3 Basic bridgecircuit

Figure 4 Circuit for 2-wiresystem

Figure 5 Circuit for 3-wiresystem

Figure 6 Circuit for 4-wirecompensated system

Figure 7 Bridge (2-wiresystem)

Figure 8 Bridge(simple 3-wire system)

Figure 9 Bridge (4-wirecompensated system)

Figure 10 Differentialsystem

Figure 11 Differentialsystem with full conductorresistance compensation


19/22

BS 1041-3:1989

BSI 01-2000 15

Figure 12 Four-terminal sensing resistor

Figure 13 Kelvin double bridge (modified)


20/22

16 blank


21/22

BS 1041-3:1989

BSI 01-2000

Publications referred to

BS 1904, Specification for industrial platinum resistance thermometer sensors.

BS 2765, Specification for dimensions of temperature detecting elements and corresponding pockets.


22/22

BS 1041-3:1989

BSI

389 Chiswick High Road

London

W4 4AL

BSI British Standards Institution

BSI is the independent national body responsible for preparingBritish Standards. It presents the UK view on standards in Europe and at theinternational level. It is incorporated by Royal Charter.

Revisions

British Standards are updated by amendment or revision. Users ofBritish Standards should make sure that they possess the latest amendments oreditions.

It is the constant aim of BSI to improve the quality of our products and services.We would be grateful if anyone finding an inaccuracy or ambiguity while usingthis British Standard would inform the Secretary of the technical committeeresponsible, the identity of which can be found on the inside front cover.Tel: 020 8996 9000. Fax: 020 8996 7400.

BSI offers members an individual updating service called PLUS which ensuresthat subscribers automatically receive the latest editions of standards.

Buying standards

Orders for all BSI, international and foreign standards publications should beaddressed to Customer Services. Tel: 020 8996 9001. Fax: 020 8996 7001.

In response to orders for international standards, it is BSI policy to supply theBSI implementation of those that have been published as British Standards,unless otherwise requested.

Information on standards

BSI provides a wide range of information on national, European andinternational standards through its Library and its Technical Help to Exporters

Service. Various BSI electronic information services are also available which givedetails on all its products and services. Contact the Information Centre.Tel: 020 8996 7111. Fax: 020 8996 7048.

Subscribing members of BSI are kept up to date with standards developmentsand receive substantial discounts on the purchase price of standards. For detailsof these and other benefits contact Membership Administration.Tel: 020 8996 7002. Fax: 020 8996 7001.

Copyright

Copyright subsists in all BSI publications. BSI also holds the copyright, in theUK, of the publications of the international standardization bodies. Except aspermitted under the Copyright, Designs and Patents Act 1988 no extract may bereproduced, stored in a retrieval system or transmitted in any form or by anymeans electronic, photocopying, recording or otherwise without prior writtenpermission from BSI.

This does not preclude the free use, in the course of implementing the standard,of necessary details such as symbols, and size, type or grade designations. If thesedetails are to be used for any other purpose than implementation then the priorwritten permission of BSI must be obtained.

If permission is granted, the terms may include royalty payments or a licensingagreement. Details and advice can be obtained from the Copyright Manager.Tel: 020 8996 7070.

Date post:	28-Feb-2018
Category:	Documents
Upload:	anonymous-y833nldd
View:	222 times
Download:	1 times

[BS 1041-3-1989] -- Temperature Measurement. Guide to Selection and Use of Industrial Resistance...

Documents