673

4.13 Thermocouples

T. J. CLAGGETT, R. W. WORRALL

(1969, 1982)

B. G. LIPTÁK

(1995, 2003)

Linearity:

Thermocouples are nonlinear devices. They generate from 10 to 60

µ

V/

°

C tempera-ture change.

Temperature Ranges:

−

346 to 4240

°

F (

−

210 to 2338

°

C) with nine types covering various ranges as listedin Tables 4.1a, 4.1c, and 4.13l

Temperature Spans:

See Table 4.13m. Minimum recommended span is 25

°

C (45

°

F) and is available onlyin types E, J, K, and T. For types S and R, the minimum span is 360

°

F (200

°

C).

Inaccuracy:

The total error is the sum of the thermocouple wire error (see Table 4.13m), theextension wire error and the signal conditioning or transmitter error.

For standard thermocouple, the error ranges from

±

2 to 5

°

F (1 to 2.8

°

C); for specialthermocouple, half as much.

The extension wire error varies with its length and can be equal or more than thethermocouple error.

The transmitter error is usually 0.15% of span with standard and 0.05% with intel-ligent transmitters, or the sum of the cold junction (CJ) and the minimum absoluteerror (MAE), whichever is greater. With narrower spans the sum of the CJ

+

MAEerror is likely to be the determining one and can be calculated as the sum of CJ

=

0.25

°

C (0.45

°

F) and MAE

=

0.2

°

C (0.36

°

F) for types T, J, E, and K; 0.6

°

C (1.08

°

F)for types R and S; and 0.8

°

C (1.44

°

F) for type B.

Costs:

$1 to $15 for beads; $25 to $60 for detachable probe assemblies with handles, usedin the laboratory. A thermocouple provided with a stainless steel thermowell costsabout $200, while a thermocouple element, which is integral with a transmitter, canrange from $750 to $2000, depending on its design and features

Partial List of Suppliers:

4B Components Ltd. (www.go4b.com)ABB Inc.-Instrumentation (www.abb.com/us/instrumentation)Acces I/O Products (www.accesioproducts.com)Accutech (www.savewithaccutech.com)Action Instruments (www.actionio.com)Altek Industries (www.altekcalibrators.com)Ametek Aerospace (www.ametekaerospace.com)Analab LLC (www.analab1.com)APT Instruments (www.aptinstruments.com)Ari Industries (www.ariindustries.com)Athena Controls (www.aethnacontrols.com)Automatic Timing & Controls (www.automatictiming.com)Barber Colman (www.barber-colman.com)Barnant Co. (www.barnant.com)Burns Engineering (www.burnsengineering.com)Chino Works America Inc. (www.chinoamerica.com)Chromalox (www.mychromalox.com)CMI Inc. (www.cmi-temp.com)Conax Buffalo (www.conaxbuffalo.com)Dickson (www.dicksonweb)

T1

TC

Flow Sheet Symbol

© 2003 by Béla Lipták

674

Temperature Measurement

Dresser Instrument (www.dresserinstruments.com)Dwyer Instruments (www.dwyer-inst.com)Ecom Instruments (www.ecom-ex.com)Eurotherm Controls (www.eurotherm.com)Exergen Corp. (www.exergen.com)Extech Instruments (www.extech.com)Flow Research (www.flowresearch.com)Flow-Tech Inc. (www.flowtechonline.com)Fluke Corp. (www.fluke.com)FMC Blending & Transfer (www.fmcblending.com)Foxboro-Invensys (www.foxboro.com)Gaumer Process (www.gaumer.com)Graybar Electric (www.graybar.com)Hach Co. (www.hach.com)Honeywell Industry Solutions (www.iac.honeywell.com)Honeywell Sensing and Control (www.honeywell.com/sensing)Ice Qube (www.iceqube.com)Imaging & Sensing Technology (www.istimaging.com)Instrumentation Group (www.instrumentationgroup.com)Jensen (www.jensentools.com)JMS Southeast (www.jms-se.com)Jumo Process Control (www.jumousa.com)Kobold Instruments (www.koboldusa.com)Lake Shore Cryotronics (www.lakeshore.com)Love Control (www.love-controls.com)Marsh Bellofram (www.marshbellofram.com)Martel Electronics (www.martelcorp.com)Minco Products (www.minco.com)Monarch Instrument (www.monarchinstrument.com)Moore Industries (www.miinet.com)MTI (www.mtisensors.com)National Basic Sensor (www.nationalbasicsensor.com)Newport Electronics (www.newportus.com)Ogden Mfg. (www.ogdenmfg.com)Omega (www.omega.com)Omron Electronics (www.omron.com/oei)Phonetics (www.sensaphone.com)Pyromation (www.pyromation.com)Pyrometer Instrument (www.pyrometer.com)RDF Corp. (www.rdfcorp.com)Ronan Engineering (www.ronan.com)Rosemount Inc. Div. of Emerson (www.rosemount.com)Sandelius Instrument (www.sandelius.com)Scannivalve Corp. (www.scannivalve.com)Testo Inc. (www.testo.com)Thermo Electric (www.thermo-electric-direct.com)Transmation (www.transmation.com)Triplett Corp. (www.triplett.com)TTI (www.ttiglobal.com)United Electric Controls (www.ueonline.com)Wahl Instruments (www.palmerinstruments.com/wahl)Watlow (www.watlow.com)Weed Instrument (www.weedinstrument.com)Wika Instrument (www.wika.com)Winters Instruments (www.winters.com)Yokogawa Corp. of America (www.yca.com)The most popular RTD based transmitter suppliers are: Rosemount-Emerson,

Pyromation, Honeywell, Moore Industries;Vendor list: www.temperatures.com/tcvendors.html.

© 2003 by Béla Lipták

4.13 Thermocouples

675

Imparting heat to the junction of two dissimilar metals causesa small continuous electromotive force (EMF) to be gener-ated. One of the simplest of all temperature sensors, thethermocouple (TC) depends upon the principle known as theSeebeck Effect. T.J. Seebeck discovered this phenomenon in1821, and in the ensuing years the thermocouple has becomethe most widely used electrical temperature sensor. The wordis a combination of

thermo

for the heat requirement and

couple

denoting two junctions.A TC is an assembly of two wires of unlike metals joined

at one end, designated as the hot end. At the other end, referredto as the cold junction, the open circuit voltage or Seebeckvoltage is measured. This voltage (EMF) depends on the tem-perature difference between the hot and the cold junctions andon the Seebeck coefficients of the two metal wires.

THEORY OF OPERATION

An ordinary TC consists of two different kinds of wires, eachof which must be made of a homogeneous metal or alloy.The wires are fastened together at one end to form a mea-suring junction, normally referred to as the hot junction, sincea majority of the measurements are made above ambienttemperatures. The free ends of the two wires are connectedto the measuring instrument to form a closed path in whichcurrent can flow. After the TC wires connect to the measuringinstrument, the junction inside is designated as referencejunction, or the cold junction (see Figure 4.13a)

The EMF developed at wire junctions is a manifestationof the Peltier Effect and occurs at every junction of dissimilarmetals within the measuring system. This effect involves theliberation or absorption of heat at the junction when a currentflows across it. The resultant heating or cooling depends uponthe direction of current flow. Applications of this principleare becoming increasingly useful in electric heating andrefrigeration.

A second EMF develops along the temperature gradientof a single homogeneous wire. This is the Thomson Effect.It is most important that each section of wire in a given circuitbe homogeneous. This is because if there is no change in thecomposition or physical properties along its length, the circuitEMF depends only upon the metals employed and the tem-perature of their junction. Therefore, the circuit EMFs are

independent of both length and diameter of wires. Anotherreason for requiring homogeneous wire is that thermal EMFswithin a single strand passing from a warmer to a cooler area,or vice versa, will cancel each other.

Further, if both junctions of a homogeneous metal areheld at the same temperature, the metal does not contributeto the net EMF of a circuit. Since some TCs are made ofexpensive metals, this fact can be used to cut costs by sup-plying copper extension wire for long runs.

It follows, then, that by holding temperatures constant atall junctions except one within a given circuit, we can mea-sure temperature as a function of the hot junction temperaturewith respect to the cold junction temperature.

TCs drift, because of the junction of the two dissimilarmetals that degrade. If used at low temperatures, this may onlybe a few degrees per year and can be calibrated out of the system.At higher temperatures, they degrade more quickly. Further, driftcan also be caused by long extension wires; these wires are oftenof lesser quality than the TC wires and can contribute twice theerror if subjected to harsh environmental conditions. Before theuse of transmitters, some plants have replaced their extensionwires on a regular basis to minimize this effect.

Interpreting the Generated Voltage

The TC reads the difference between the temperatures of itsmeasuring and reference junctions. (Actually, it is a generallimitation of human beings that we cannot measure anythingin the absolute; all we can do is to compare a known quantityagainst an unknown.) If we know what the reference temper-ature is, we can identify the unknown process temperatureby measuring the voltage generated by the TC:

unknown temperature

=

(voltage/Seebeck coefficient)

+

reference temperature

4.13(1)

The process temperatures can be obtained from the volt-age read by either going to a graph (Figure 4.13b) or, formore accuracy, by going to TC tables that list the voltagescorresponding to each temperature with each TC type (suchtables are provided at the end of this section).

Unfortunately, the voltage-to-temperature relationship isnot a straight-line function, and the Seebeck coefficient is nota constant (Figure 4.13c). For some TCs over certain temper-ature ranges, such as type K over the range 0 to 1000

°

C (32to 1832

°

F), the Seebeck coefficient is relatively constant (about40

µ

V/

°

C), but in general it changes with temperature. This inthe past has resulted in unique scales for each type of TC orin the need to use tables and curves to convert millivolts intotemperature. These days the memory capability of the micro-processors has resolved all these problems, and what used tobe tedious and time-consuming is now quick and easy. In short,the nonlinear nature of the TCs is no longer a problem.

The same cannot be said about the weakness of the TCsignal. As shown in Figure 4.13c, a platinum thermocouplewill generate only about 10

µ

V/

°

C. On the other hand, even

FIG. 4.13a

Thermocouple terminology.

Instrument

Reference or ColdJunction

ExtensionWires

Thermocouple

ConnectionHead

MeasuringJunction

© 2003 by Béla Lipták

676

Temperature Measurement

the best industrial transmitters have a minimum span of 1mV and a minimum absolute error of about 0.01 mV, whichis 10

µ

V. Therefore, it is difficult to obtain a measurementusing industrial transmitter and platinum TCs, which wouldhave less than a 1

°

C error or have a span that is narrowerthan 60

°

F (35

°

C). This is usually acceptable when measuringhigher temperatures but is not acceptable at low temperaturesor when the temperature span is narrow. For this reason, TCsare not recommended and resistance temperature detectors(RTDs) are used for narrow span or small temperature dif-ference measurements.

Laws of Intermediate Temperatures and Metals

The law of intermediate temperatures states that the sum of theEMFs generated by two TCs—one with its junctions at 32

°

F(0

°

C) and some reference temperature, the other with its junc-tions at the

same

reference temperature and at the measuredtemperature—will be the same as that produced by a single TC,having its junctions at 32

°

F (0

°

C) and the measured temperature.This concept is illustrated in Figure 4.13d where the mea-

sured temperature is 700

°

F (371

°

C). By adding an EMF equalto that produced by thermocouple A in Figure 4.13d (with itsjunctions at 32ºF [0

°

C] and the reference temperature) to thatof thermocouple B, a total EMF equivalent to that generatedby the hypothetical thermocouple C results. In most pyrom-eters, this is done by a temperature-sensitive resistor, whichmeasures the variations in reference junction temperaturecaused by ambient conditions, and automatically providesthe necessary EMF by means of a voltage drop producedacross it. Thus, the instrument calibration becomes indepen-dent of reference temperature variations.

The law of intermediate metals states that the introduc-tion of a third metal into the circuit will have no effect uponthe EMF generated so long as the junctions of the third metal

with the other two are at the same temperature. Any numberof different metals can be introduced, providing all the junc-tions are at the same temperature. Thus, in Figure 4.13e thecircuits shown all generate the same EMF, even though thesecond and third circuit diagrams show materials C, D, E,and F inserted between A and B.

Cold Junction Compensation

When a readout device is employed, it converts the EMFproduced by the temperature difference between the hot andcold junctions to record or otherwise display the temperatureof the hot junction. To prevent errors due to secondary EMFsproduced by variations of temperature at the cold junction

FIG. 4.13b

The millivoltage generated by thermocouples varies with wire mate-rials and is a nonlinear function of temperature.

−200 +200 +400 +600 +800 +1000 +1200 +1400 +1600 +18000 °C

°F−328 32 392 752 1112 1472 1832 2192 2552 2912 3272

(Type ‘S’)

(Type ‘R’)Platinum/10% RhodiumRhodium Versus Platinum

Platinum/13% RhodiumRhodium Versus Platinum

Copper Versus Constantan(Type ‘T’)

Type ‘E’Iron Versus Constantan

(Type ‘J’)

Chromel Versus Alumel(Type 'K')

Nicrosil

Versus N

isil

Mill

ivol

ts

−10

0

+10

+20

+30

+40

+50

+60

+70

+80

FIG. 4.13c

The Seebeck coefficient gives the amount of voltage generated (in microvolts) by a one degree change in temperature. The value of theSeebeck coefficient varies not only with thermocouple type but also with temperature.

1

Temperature

0° 500° 1000° 1500° 2000° °C32° 932° 1832° 2732° 3632° °F

R

S

Linear Region

Seeb

eck

Coe

ffic

ient

µV

/°C

20

40

60

80

100

T J

E

K

Type

SeebeckCoefficient

at RoomTemperature

( µV/°C)

J

K

ER

S

T

50

40

60

11

10

38

© 2003 by Béla Lipták

4.13 Thermocouples

677

and within the readout device, these EMFs must be com-pensated for. One method is to hold the cold junction at aconstant temperature, which can be done in laboratories withan ice bath (Figure 4.13f ). An oven can also be used,although keeping an oven temperature constant presentsanother set of problems.

Neither an ice bath nor an oven reference is practical inan industrial environment. In the temperature transmittersused in the process industry, the ice bath reference must bereplaced by a variable ambient reference junction. This isachieved by making two changes to Figure 4.13f. The first

change is to insert a short copper wire between both voltmeterterminals and the TC leads and to place these new junctionson an isothermal block (Figure 4.13g). This change elimi-nates the junctions J

3

and J

4

shown in Figure 4.13f becausein Figure 4.13g copper is joined to copper at these points. Byplacing the new J

3

and J

4

junctions on an isothermal block,as shown in Figure 4.13g, their effects cancel out as they arein opposition to each other and are at the same temperature.

The second change was to place the reference junctionnot in an ice bath (Figure 4.13f), but on the isothermal block.From the law of intermediate metals (Figure 4.13e) we knowthat when junctions in series are at the same temperature,their number makes no difference. Therefore, J

4

and J

REF

inFigure 4.13g can be replaced by J

REF

only.Figure 4.13h shows the software compensation of the

reference junction. Here the voltmeter reads the equivalentof thermocouple B in Figure 4.13d, while the thermometerR

T

reads the actual reference temperature of the isothermalblock. The thermometer used to measure T

REF

can be a ther-mistor (see Section 4.12), an RTD (see Section 4.10), or anintegrated circuit transistor. Once T

REF

is accurately measured(usually within 0.25

°

C or 0.45

°

F), the associated softwaredetermines the corresponding millivoltage that a TC

wouldhave generated

if its hot junction were at T

REF

and its coldjunction were in an ice bath (thermocouple A in Figure4.13d). The sum of A and B then represents the measuredprocess temperature (referenced to ice) and can be looked upin the type of tables that are provided at the end of thissection.

One might ask, why use a TC at all if another thermom-eter is needed to measure the reference temperature? Theanswer to that question is simple: Do not use a thermocoupleif another thermometer can measure the temperature. Unfor-tunately, the sensors, which can accurately detect the ambienttemperature (T

REF

) are not suited for the measurement of high

FIG. 4.13d

According to the law of intermediate temperatures, the EMF ofthermocouple A plus the EMF of thermocouple B is equal to theEMF of thermocouple C.

FIG. 4.13e

No harmful effect is caused by introducing any number of metalsat a thermocouple junction if all connections are at the same tem-perature.

Reference Measured

A

B

C

MV

4 MV

6.68 MV

2.68 MV

0(−17.8)

32(0)

200(93)

400(204)

°F (°C)

600(316)

800(427)

0

2

4

6

8

A

A

A

B

B

B

All ThreeCircuitsGenerateSame EMF

76 900

C

C

D

E

F

FIG. 4.13f

When an iron-constantan thermocouple measures the process (J

1

)and an identical iron-constantan thermocouple reference junction(J

2

) is placed in an ice bath, the connection to the readout voltmeterresults in two added junctions, J

3

and J

4

.

1

Iron

Constantan

HotEnd

ColdEnd @ 0°C (32°F)

Fe

Fe

Cu

Cu

Voltmeter

V+

−

J1

J1

J1J2

Ice Bath

C

© 2003 by Béla Lipták

678

Temperature Measurement

temperatures or wide spans. For these applications the naturalchoice is the TC, and when it is used, it must be compensated.

Multiplexing

The cost and complexity of software compensation is reducedwhen many TCs are multiplexed into the same readout device(Figure 4.13i). In that case, a large number of TCs can beterminated on the same isothermal block and a single ther-mometer can serve to provide software compensation for all.The disadvantages of TC multiplexing include the necessityof transporting the low-level signals over some distance to

the multiplexer (which can introduce common and normalmode noise, discussed later), the relatively high costs ofthermocouple lead wires, and the added error due to thevariable contact resistances of the multiplexer. Even withgold-plated contacts, there will be at least 1

µ

V drop throughthe contacts, which in case of platinum thermocouples cor-responds to an error of about 0.2

°

F (0.1

°

C).

Hardware Compensation

Prior to the advent of microprocessors and the associatedsoftware compensation of TCs, hardware compensation wasused. Hardware compensation can be viewed as inserting abattery that cancels out offset voltage produced by the refer-ence junction. These commercially available circuits providean electronic ice point reference for one or many TCs. Theirmain advantage relative to software compensation is speedbecause the computation time is eliminated. The main dis-advantage of hardware compensation is that each gain resistoris suited to compensate only a particular type of TC, whilesoftware compensation accepts any TC. In practice, hardwarecompensation is usually accomplished by using resistorswhose combined temperature resistance coefficient curvesmatch those of the voltage-temperature curves produced bythe reference junctions, canceling any variations in the coldjunction temperature.

MEASURING THE EMF GENERATED

The Seebeck EMF can be measured with either a millivolt-meter (Figure 4.13j) or a potentiometer (Figure 4.13k) circuit.We should remember that the thermocouple measures onlythe difference between its reference and hot junctions. Howclosely it matches the accepted EMF curve has a bearing onaccuracy. EMF tables are usually based upon 32

°

F (0

°

C)reference temperatures for convenience (see tables at the endof this section).

To relieve the control engineer of the problem of com-pensating for temperature instability at the reference junction,a copper or nickel resistor can be placed in a bridge so that

FIG. 4.13g

By using an isothermal block and by inserting copper wires at the voltmeter terminal, an equivalent circuit is arrived at which does nothave an ice bath reference.

1

Voltmeter

HI

LO

Cu

Cu

Isothermal Block @TREF

J1

J2J4

J3

Fe

Fe

C

Cu

Equivalent Circuit

Cu

J1

J4

J3

Fe

C

TREF

+

V

−

R1

FIG.4.13h

The industrial equivalent of the ice bath reference shown inFigure 4.13f.

1

FIG. 4.13i

When many thermocouples are multiplexed, a single reference ther-mometer is sufficient to provide software compensation.

1

Voltmeter

Cu

CuC

Block Temperature = TREF

J1V1

J4

J3 Fe+

V

−

+

−

R1

Voltmeter

All Copper Wires

VHI

LO

Fe

C

RT

Pt

Pt− 10%Rh

Isothermal Block(Zone Box)

+

−

© 2003 by Béla Lipták

4.13 Thermocouples

679

the TC EMF is opposed by an EMF corresponding to therequired ambient temperature correction. Operating on the null-balance principle, the resulting potentiometer (Figure 4.13k)tends to reduce any voltage difference between points A andB to zero.

Transmitter Location and Noise

As discussed earlier, TCs produce a very small microvoltoutput per degree change in temperature. This output is verysensitive to environmental influences, particularly if longextension wires are used. It is recommended to minimize thislength, which can be best achieved by mounting the trans-mitter right inside the thermocouple head.

Electromagnetic interference from motors and electricaldistribution and especially radio frequency interference (RFI)from walkie-talkies can be the cause of large errors in mea-surement. Therefore, the transmitters or other TC readoutinstruments must have rigid RFI immunity specifications tominimize these effects. TC readouts are considered to be ofgood quality if their common mode noise rejection is about100 DB, their normal mode rejection is about 70 DB, andtheir RFI immunity is 10 to 30 V/m.

Intelligent Transmitters

State-of-the-art transmitters, digital buses, and networks havebeen discussed in Sections 4.1 and 4.10. The reader isreferred to those sections and also to the concluding para-graphs at the end of this section.

THERMOCOUPLE TYPES

Based on possible combinations of metals, there could becountless numbers of thermocouples, but there are relativelyfew (see Tables 4.13l and 4.13m). Things that determine ametal’s usefulness in TC wire include:

1. Melting points2. Reaction to various atmospheres3. Thermoelectric output in combination with other metals4. Electrical conductance, the reciprocal of resistance

(listed in Table 4.13n)5. Stability 6. Repeatability7. Cost8. Ease of handling and fabrication

ISA Types J, S, and T

Iron-constantan (type J) can be used in reducing atmo-spheres. These thermocouples provide a very nearly linearEMF output. They are the least expensive commerciallyavailable type.

The platinum-platinum 90%/rhodium 10% (type S) TCis most important. It is used to define the International Tem-perature Scale between 1166.9

°

F (630.5°C), the point atwhich antimony freezes, and 1945.4°F (1063°C), the goldpoint. This TC is not limited to the above range. It can beused from about 300 to 3215°F (150 to 1768°C) with excellentresults. Industrial thermocouples (“Special” in Table 4.13m)of this material will match the standard calibration curve tobetter than ±0.25%.

Copper-constantan (type T) can be used in either oxidiz-ing or reducing atmospheres. TCs of this type exhibit a highresistance to corrosion from moisture, provide a relativelylinear EMF output, and are good from the medium to thevery low temperature range.

ISA Types B, E, K, R, and N

and platinum 30% rhodium-platinum 6% rhodium (type B),which are recommended for use in oxidizing atmospheres.They are relatively easily contaminated in other atmospheres.

Chromel-alumel (type K) can be used in oxidizing atmo-spheres. It is the most linear TC in general use.

Chromel-constantan (type E) TCs provide the highestEMF per degree of temperature change. However, it alsotends to drift more than the others. It can be used in oxidizingatmospheres.

Tungsten-tungsten 26% rhenium TCs can be used to mea-sure the highest temperatures. It cannot be used in oxidizingatmospheres, and it is also brittle and hard to handle. It isusually used in vacuum or in clean inert gas applications.

A relatively new base-metal thermocouple is designatedtype N (Nicrosil vs. Nisil). It provides stability as good as

FIG. 4.13j Millivoltmeter circuit.

FIG. 4.13kPotentiometer circuit.

ShuntResistor

GalvanometerSeriesResistor

CalibratingRheostat

Thermocouple

B Slidewire

AmplifierConstant Voltage

Power Supply

Cold JunctionCompensation

A

+−

ThermocoupleHot Junction

© 2003 by Béla Lipták

Several other TCs are commonly used (see Tables 4.13l and4.13m), including platinum-platinum 13% rhodium (type R)

680Tem

perature Measurem

ent

TABLE 4.13lThermocouple Comparison Table

ISA TypeDesignation

Positive NegativeWire Wire

Millivolts per °F

Recommended Range Limits Temp °F*

Scale Linearity

AtomosphereEnvironment

RecommendedFavorable

PointsLess Favorable

PointsNumbers = Percentages Min. Max.

B Pt70-RH30 Pt94-Rh6 .0003–.006 32 3380 Same as for type R couple

Inert or slow oxidizing

— —

E Chromel Constantan .015–.042 −300 1800 Good Oxidizing Highest EMF/°F Larger drift thanother base metalcouples

J Iron Constantan .014–.035 32 1500 Good; nearly linearfrom 300–800

Reducing Mosteconomical

Becomes brittlebelow 32°F

K Chromel Alumel .009–.024 −300 2300 Good; most linear of all TCs

Oxidizing Most linear More expensivethan T or J

R Pt87-Rh13 Platinum .003–.008 32 3000 Good at high temps. poor below 1000°F

Oxidizing Small size, fast response

More expensive than type K

S Pt90-Rh10 Platinum .003–.007 32 3200 Same as R Oxidizing Same as R More expensive than type K

T Copper Constantan .008–.035 −300 750 Good but crowded at low end

Oxidizing or reducing

Good resis. to corrosion from moisture

Limited temp.

Y Iron Constantan .022–.033 −200 1800 About same as type J

Reducing — Not industrial standard

— Tungsten W74-Re26 .001–.012 0 4200 Same as R Inert or vacuum

High temp. Brittle, hard to handle, expensive

— W94-Re6 W74-Re26 .001–.010 0 4200 Same as R Inert or vacuum

Same as above Slightly less brittle than above

— Copper Gold-Cobalt .0005–.025 −450 0 Reasonable above 60 K

— Good output atvery low temp.

Expensive lab.-type TC

— Ir40-Rh60 Iridium .001–.004 0 3800 Same as R Inert — Brittle, expensive

* CF 32

1.8° =

° −

© 2003 by Béla Lipták

4.13 Thermocouples 681

TABLE 4.13mThermocouple Errors and Spans

TC TypeMeasured Temperature

Range in °F*

TC Wire Errors for Wiresof Different Qualities*

Transmitter Error is Additional and in theCase of “Smart” Units Is ±0.05% of Span or

Value Given Below, Whichever Is Larger*

Recommended SpanLimits*

Standard Special Min. Max.

B 32–3380 NA NA ±1.89°F 63°F 2020°F

E 32–600600–1600

±3°F±0.5%

——

±0.81°F 45°F 2100°F

J 32–530530–1400

±4°F0.75%

±2°F±0.375%

±0.81°F 45°F 2500°F

K 32–530530–2300

±4°F±0.75%

±2°F±0.375%

±0.81°F 45°F 2750°F

R 32–10001000–2700

±5°F±0.5%

±2.5°F±0.25%

±1.53°F 360°F 2950°F

S 32–10001000–2700

±5°F±0.5%

±2.5°F±0.25%

±1.53°F 360°F 2900°F

T −300 to −75−150 to −75−75−200200–700

—±2%

±1.5°F±0.75%

±1%±1%

±0.75°F±0.375%

±0.81°F 45°F 1025°F

N 32–530530–2300

±4°F±0.75%

±2°F±0.4%

NA NA NA

TABLE 4.13nResistance of Various Thermocouple Wire Sizes in Ohms per Double Foot of Wire Length at 20°C (68°F)

AWGNo.

DiameterInches

Thermocouple Type

K J T E R S G(W) C(W5) D(W3)

6 0.1620 0.23 0.014 0.012 0.027 0.007 0.007 0.008 0.009 0.009

8 0.1285 0.037 0.022 0.019 0.044 0.011 0.011 0.012 0.015 0.015

10 0.1019 0.058 0.034 0.029 0.069 0.018 0.018 0.020 0.023 0.022

12 0.0808 0.091 0.054 0.046 0.109 0.029 0.028 0.031 0.037 0.035

14 0.0641 0.146 0.087 0.074 0.175 0.047 0.045 0.049 0.058 0.055

16 0.0508 0.230 0.137 0.117 0.276 0.073 0.071 0.078 0.092 0.088

18 0.0403 0.374 0.222 0.190 0.448 0.119 0.116 0.126 0.148 0.138

20 0.0320 0.586 0.357 0.298 0.707 0.190 0.185 0.200 0.235 0.220

24 0.0201 1.490 0.878 0.753 1.780 0.478 0.464 0.560 0.594 0.560

26 0.0159 2.381 1.405 1.204 2.836 0.760 0.740 0.803 0.945 0.890

30 0.0100 5.984 3.551 3.043 7.169 1.910 1.850 2.030 2.380 2.260

32 0.0080 9.524 5.599 4.758 11.31 3.040 1.960 3.220 3.800 3.600

34 0.0063 15.17 8.946 7.660 18.09 4.820 4.660 5.100 6.040 5.700

36 0.0050 24.08 14.20 12.17 28.76 7.640 7.400 8.160 9.600 9.100

38 0.0040 38.20 23.35 19.99 45.41 11.95 11.60 12.90 15.30 15.30

40 0.0031 60.88 37.01 31.64 73.57 19.30 18.60 20.60 24.40 23.00

* CF 32

1.8° =

° −

© 2003 by Béla Lipták

682 Temperature Measurement

the more expensive noble metal TCs up to about 2200°F(1204°C), where type K starts to become unstable. The sta-bility of type N TCs is due to increased percentages of chro-mium, silicon, and magnesium.

THERMOCOUPLE CONSTRUCTION AND PROTECTION

There are some applications where a bare TC with an exposedjunction may be used either by itself or inserted into a pro-tective well. For most process applications, the TC is manu-factured with a protective outer sheath that uses an insulatingmaterial to electrically separate the TC from the sheath andprovide mechanical and environmental protection. In somecases the TC junction is placed in direct contact with the tipof the sheath to increase speed of response.

These sensors demand the use of an electrically isolatedmeasurement circuit. Even insulated TCs will eventuallysuffer from a breakdown of the insulation, and the TC tipwill contact the sheath and associated well. It is virtuallyassured that a ground loop will be present that will causemeasurement errors. These errors are usually insidious in thatthey usually vary over time and may go unnoticed. Recom-mended practice is to always use an instrument with fullisolation to eliminate this concern.

Measuring Junction Designs

A TC is only as accurate as the wire from which it is made.Therefore, it is common practice for best accuracy to makeall TCs from the same coil of wire. This assumes uniformityof the wire. Most manufacturers offer either standard or spe-cial calibrations, which imply more care in selection of wire,handling, and manufacturing. The careful selection of mate-rials, proper construction, installation, and handling alonewill not maintain highest accuracy; an adequate checkingprogram is also a must.

In order to protect the TC wire, it is usually covered bya thermal insulation and a sheath for mechanical protection.The purpose of this design is to expose only the measuringjunction of the TC to the temperature of the process. Thiscan be achieved in three different ways (see Figure 4.13o).The exposed thermocouple junction gives the best speed ofresponse; the time constant can be less than a 1 s with small(down to 0.01 mm diameter) TCs. Their main limitation isthat the process materials must not be corrosive to the TC wires.

In the ungrounded junction design, the TC wire is phys-ically insulated from the sheath by insulation material (usu-ally magnesium oxide powder). These designs can be usedin corrosive processes, but their speed of response is slow.The grounded junction design is also protected from thecorrosive process, but its thermal time constant is shorter (bya few seconds, depending on mass).

Extension Wires

The thermocouple extension wire is usually insulated withTeflon, polyvinyl chloride, nylon, rubber, asbestos, or fiber-glass. For higher temperatures refrasil or nextel are recom-mended. Teflon is used when the TC extension wire must besubmerged under water or if resistance to solvent, corrosion,flame, or humidity is critical. Individually insulated duplexwires are usually provided with a protective outer jacket,which can be wrapped, extruded, or stranded. The extensionwire to be used for types, E, J, K, and T TCs are designatedas EX, JX, KX, and TX extension wires and should extendall the way to the cold junction of the loop.

With connections correctly made, copper extension wirecan be used over long distances. However, it is recommendedthat iron-constantan and copper-constantan always be usedwith lead wire of the same material. To guard against mis-takes in connection, industry practice is to color-code thewires, with the negative lead always red. Smaller gauge wireprovides faster response, but heavier gauge wires last longerand resist contamination or deterioration at high temperatures.

FIG. 4.13o Thermocouple measuring junction designs. (Courtesy of ARI Industries Inc.)

D

D D

SheathSheath

Ungrounded No Seal

Exposed

UncompactedInsulation

ThermocoupleWires

ThermocoupleWires

Grounded

CompactedInsulationA

A SectionA-A

© 2003 by Béla Lipták

4.13 Thermocouples 683

Sheath Materials

The sheath material can be 304 stainless steel if the processtemperature is under 1650°F (900°C) and the process is nothighly corrosive. In furnaces that operate at up to 2100°F(1150°C), Inconel 600 sheathing is recommended if the atmo-sphere is oxidizing and there is no sulfur in the atmosphere.Platinum-rhodium alloy sheaths are used up to 3000°F(1650°C) in oxidizing furnaces if no silica or halogens arepresent. Molybdenum sheaths can be used up to 4000°F(2205°C) to detect molten metal or glass temperatures, butonly in oxygen-free vacuum or inert-gas-filled processes. Tan-talum sheaths can be used up to 4500°F (2482°C), but only inreducing or noble gas atmospheres where no oxygen is present.

The sheath is usually strong enough to stand up to highpressure (up to 50,000 PSIG, or 3,450 bars), but it is usuallynot used without a thermowell because the user wants to beable to take out the TC without opening up the process.Figure 4.13p illustrates some high-speed TC assemblies with-out thermowells.

Thermowells

Protecting tubes or wells are supplied (Figure 4.13q) to protectTCs from harmful atmospheres, corrosive fluids, or mechan-ical damage; to support the TC; or to permit TC entry into apressurized system. These tend to reduce the speed ofresponse of the TC, so small-mass, thin-wall, or needle-typeinstallations are supplied where feasible (Figure 4.13p).Disposable-tip thermocouples are supplied in furnace appli-cations (Figure 4.13r). They can also be peened or welded

into a tube or tank well. Their low cost makes it feasible toplace them in concrete beams while curing or to use them inother single-time operations.

When it is desirable to maximize the speed of responseof the measurement, but also necessary to periodicallyremove the sensor, the bare (sheathed) thermocouple can beremoved through a stuffing box and gate valve combination(Figure 4.13s). Most TCs are installed in a protecting well.In Figure 4.13t the R dimension is the immersion length,while the U dimension is the insertion length of the well, Rshould be at least 10 times the diameter of the protective tube(sheath) diameter of the thermocouple. The sheath diameterof different TCs can range from 0.04 to 0.84 in. (1 to 21 mm),while the TCs can range from gauge #36 to #8. The well can

FIG. 4.13p High-speed small O.D. thermocouple assemblies with stainless steelprotecting sheath.

(Max. Temp. 300°F)Open Head

Compression Fitting

1316

18 In. NPT

18 In. NPT

18 In. NPT

In.

1 In.

1 In.

1 In.

Specified Protecting Tube Lengthwith Open Head

Specified Protecting Tube Lengthwith Screw Cover Head

Specified Protecting Tube Length....with

Quick ConnectPlug Compression Fitting

Compression Fitting

Quick - Connect Plug

12 In. NPT

FIG. 4.13q Exploded view of thermocouple assembly and protecting tube (top);complete assembly with protecting tube (bottom).

FIG. 4.13r Molten steel expendable thermocouple.

FIG. 4.13s Installation of thermocouple without thermowells.2

Cover Gasket TerminalBlock

TerminalHead

Element andInsulators

ProtectingTube

Stuffing Box

Gate Valve

ThermocoupleMetal-Sheathed,Mineral-InsulatedThermocouple

© 2003 by Béla Lipták

684 Temperature Measurement

be inserted perpendicularly into the pipeline if R is not muchmore than the inside radius of the pipe. Otherwise, it shouldbe inserted at a 90-degree bend in the line.

The most often used well materials are 304 and 316stainless steel, which are usable up to 1200°F (649°C). Athigher temperatures ceramic thermowells are used becausemetallic ones start to “droop” (bend by gravity). High-purityalumina can be used up to 2200°F (1200°C); the same limitholds for mullite, but this material is not recommended foruse with platinum thermocouples as it contains impuritieswhich can contaminate platinum. The thermowell can havescrewed connections (Figure 4.13t) or, if frequent inspectionis required or if the well is glass-coated, it can be flanged.

Surface Temperature Detectors

When the surface temperature of tubes is to be measured, theTC must be shielded from furnace radiation. The TC can beattached to the heater tube surface by being furnished withstainless steel welding pads (Figure 4.13u, lower part) or bythe use of TC attachment blocks (Figure 4.13u, upper part).The multiple holes in these blocks allow for spare TC ele-ments for quick replacement.

Specialized Detectors

Needle Sensors The response time of the needle type sen-sors illustrated in Figure 4.13v is about 0.25 s. They are madeof hypodermic stainless steel in many lengths and diameters.They are available in blunt, center sharp, and hypodermic

FIG. 4.13t Screwed thermowell installation.2

Thermocouple Head

Union Optional

" IPS

Hex Head

" IPS

78

"

38

"

38

"

34

"

78

"1

78

"1U+

U

R1"=25.4 mm

FIG. 4.13u Tube surface temperature measured by thermocouple block (A) bywelded on stainless steel pad,2 or by directly welding the thermo-couple to the surface (B). A protective cover (C) gives the requiredmechanical protection.

FIG. 4.13vNeedle sensors detect the temperature of such penetrable solids asrubber and plastic melts, but can also be used in liquids. (Courtesyof Electronic Development Labs, Inc.)

Weld

ThermocoupleBlock

Peen Tight

HeaterTube

Metal-Sheathed, Mineral-Insulated Thermocouple3/16" Dia. Type 304 StainlessSteel-Sheath

2"

3"7/16"

Stainless Steel BandA

Detail ofThermocouple

Block

Thermocouple PadWeld

Metal-SheathedMineral-InsulatedThermocouple

StainlessSteel Band

HeaterTube

Thermocouple Welded to PadStainless Steel PadCurved to FitHeater Tube

Detail of ThermocouplePad Assembly

B 1"=25.4 mm

Solid Tip Welds to Surface of Tube or Vessel

Junction 316StainlessSteelProtectiveCover

CInsulatingMaterial

© 2003 by Béla Lipták

4.13 Thermocouples 685

sharp designs and can be made from hard or soft stainlesssteel. The latter allows for shaping and bending the needleto match the needs of the application.

Suction Pyrometers The suction pyrometer consists of asheathed TC (sheathed against chemical attack) located insidea radiation shield at the tip of a suction pipe (Figure 4.8d).The combustion gases are sucked through the shield and overthe TC at high velocity by aspirating equipment. The effi-ciency of this aspirating TC is a function of the quality ofthe radiation shield and of the suction flow rate. If, for exam-ple, a suction pyrometer has a 100°C error without suctionand a 10°C error when the suction flow is on, it is said tohave an efficiency of 90%.

The suction pyrometer probe is usually made of stainlesssteel and is water-cooled. These probes are designed for highgas velocities of 500 f/s (152 m/s). At such velocities, theradiation shields usually produce better than 90% pyrometerefficiencies. The furnace gases can be pulled out by fans orby air or steam ejectors. The main limitations of this designinclude plugging of the probe when the combustion productsare dusty (as in pulverized coal burners) and being unable tobe used in applications where the temperatures exceed2912°F (1600°C).

INSTALLATION AND PROTECTION

Multiple Thermocouples

The reason for inserting several TCs within the same thermo-well can be to obtain a temperature profile over some distance.In this case, each TC junction is located at a different distancefrom the tip. In order for such sensors to detect the temperatureoutside the well (and not the air temperature inside it), it isessential that good physical contact be made between the TCjunction and the metallic surface of the inside of the well.

Average Temperatures and Temperature Differences TCscan be connected in parallel to provide the average temperaturein a system (Figure 4.13w). They can also be used to measurethe difference between two temperatures (Figure 4.13x).

In the past, a single TC was often utilized by two separatemeasuring instruments (Figure 4.13y) because at balance, apotentiometer draws no current from the thermocouple circuit.

In case of burnout, a small current circulates through the ther-mocouple. Today, this kind of configuration is less likely andit is much more common to have transmitters installeddirectly in the TC head.

Thermopiles Thermopiles are TCs connected in series withelectrically insulated junctions (Figure 4.1p). Thermopilesgenerate large EMFs, reducing sensitivity requirements in thereadout instrument. To obtain the mean temperature at severalpoints being monitored by similar TCs in series, divide thetotal EMF by the number of sensing junctions and relate thisEMF value to a corresponding temperature reading in theEMF-temperature table for the type of TC being used. Aswas discussed in Section 4.1, thermopiles can be used toamplify the output signals in differential temperature mea-surements and to serve as heat-flow detectors.

The principal objections to the use of thermopiles are theneed for electrical isolation of individual TCs and the errorthat might go unnoticed when the output of one of the TCsis reduced by a short circuit. One satisfactory application forthermopiles is to use them as temperature differential detectors.

Thermocouple Burnout

When a TC detects the temperature in furnaces or superheat-ers (Figure 4.13z), the extension wire can pass throughflames. On high-temperature services, TCs are provided withplatinum, tantalum, or molybdenum sheath materials over theinsulation, which can be magnesium or beryllium oxide. Hotspots like a burning coal seam can eventually burn throughthe sheath and the insulation of the extension wire. When thewires melt, a short develops. This is called TC burnout. Underthese conditions the TC no longer indicates the temperature ofthe initial hot junction; instead, it measures the temperature

FIG. 4.13w Average temperature measurement.

MeasuringInstrument

+ − + − + − + −

T/C 1 T/C 2 T/C 3 T/C 4

FIG. 4.13x Temperature difference.

FIG. 4.13y Parallel operation from common thermocouple.

T/C1 T/C2

MeasuringInstrument

++ −−

Recorder

Controller

JunctionPoint

T/C

© 2003 by Béla Lipták

686 Temperature Measurement

at the hot spot. One cannot detect TC burnout by reading themillivolt signal. However, if one measures the TC resistance(see Table 4.13n), that will signal a change as a result of burn-out (Figure 4.13aa).

Under normal conditions, a running record of the thermo-couple resistance will show gradual changes with temperature.When the thermal insulation is beginning to fail and a short isbeginning to form, the TC resistance will rise first, and whenthe hot spot burns through and a short is formed, the TCresistance will drop abruptly. The method used to measure theTC resistance is called offset-compensated ohms measurement.

As shown on the top of Figure 4.13bb, normally themillivolts (V1) are measured across the TC. Then, a currentsource is connected periodically and the millivolts (V2) aremeasured again. The TC resistance is calculated by subtract-ing the thermocouple millivolts from the total and dividingit with the current flow in the loop: R = (V2 − V1)/I. Bycontinuously recording this resistance, one can detect whenan abrupt drop occurs, signaling TC burnout.

Protection Against Noise

The TC signal is very weak—a one degree change in tem-perature results in only a few millionths of a volt change inoutput. Because of this, precautions must be taken againsterrors due to stray currents resulting from the proximity ofelectrical wiring (common mode noise) or from capacitivesecondary grounds (normal mode interference). Commonmode noise (Figure 4.13cc) appears on both TC signal wiresand therefore can be filtered out as 60 Hz (or 50 Hz) har-monic noise. The filter does reduce the interference dramat-ically, but it also causes the voltmeter to be sluggish whenresponding to a step change. It is also possible to eliminatethe common mode interference by using twisted wire leadsbecause each time the wire is twisted, the flux-induced cur-rent is inverted.

Another recommended form of protection against any typeof common mode noise is guarding and shielding. If the shieldsurrounding the lead wires is connected to the guard surround-ing the voltmeter, the interfering current caused by AC inter-ference does not flow through the TC lead resistance butinstead is shunted. Naturally, when TCs are scanned, the scan-ner guard must be switched to the shield of the TC being readto eliminate ground loops. Harmonics can also be removed byintegrating the incoming signal over the power line cycle in

FIG. 4.13z The expansion loop allows for thermal expansion in furnace applications.

Reducing Bushing1" min. NPT

ExpansionLoop

WeldClamp

WeldPad

FurnaceWall

BoilerTube

FIG. 4.13aa Thermocouple burnout can be detected by measuring the resultingdrop in thermocouple loop resistance.1

T1

T1TS

t1 Time

Short

R

FIG. 4.13bb Offset-compensated ohms measurement allows detection of the ther-mocouple loop resistance.

R =V2−V1

I

T1

T1

V1

V2

+

−

+

− −

I

© 2003 by Béla Lipták

4.13 Thermocouples 687

an integrating analog-to-digital (A/D) converter or voltmeter.In short, common mode noise is relatively easy to remove.

Normal Mode Noise The same cannot be said about normalmode noise. An example of normal mode noise interferencecan occur in the measurement of the temperature in a moltenmetal bath, which is heated by electric current. In this case,the TC junction is in direct contact with a common modenoise source. In addition, the capacitive ground (C-stray)from the LO terminal of the TC to the chassis causes a currentflow in the low lead and an associated normal mode noisevoltage across the resistance RS (Figure 4.13dd).

If a guard lead wire is installed connected directly to theTC, the current flowing in the LO lead through the resistanceRS is drastically reduced. Therefore, the worst form of inter-ference is DC offset caused by a DC leakage current; what-ever normal mode noise remains in the system, it cannot bedistinguished from the measurement and, in case of weaksignals, even a small amount of noise can represent a largeamount of interference.

CALIBRATION, DIAGNOSTICS, AND TRANSMISSION

Calibration

Since all TCs are subject to drift, calibration checks are doneregularly in laboratories and industrial plants. For calibratingTCs, depending upon the application, various procedures areused. Primary standard thermocouples of platinum vs. plati-num plus 10% rhodium can be calibrated by the NationalBureau of Standards to fixed points on the International Prac-tical Temperature Scale. However, these TCs must be handledcarefully to retain their accuracy. Most major manufacturerscan supply TCs, which are against primary standard thermo-couples that are kept in their own metrology laboratories.Secondary reference TCs for in-plant use are usually madeof base metal. Comparison of these against the Primary Stan-dard TC is accomplished by placing them in close contact ina checking furnace. Users normally check their ordinary TCsagainst these secondary standards.

Diagnostics

TC diagnostics can be improved by the use of tip-branchedand leg-branched lead wires allowing redundant measure-ments, verification of system integrity, and other forms ofdiagnostics. These tools can be useful in detecting the failureof wire insulators, poor junction connections, wire degrada-tion due to overheating, or decalibration due to diffusion ofatmospheric particles into the metal. Added to the noise pro-tection and degradation problems are the intermediate wirejunctions, which if not at the same temperature (Figure 4.13e),also contribute errors.

Transmission

Because of the problems associated with long extensionwires, including noise interference (guarding, shielding,using twisted pairs, and integration), the best alternative isnot to send low-level TC signals over long distances at all,but to place the transmitter electronics directly on top of thethermowell (Figure 4.13ee).

In the past it was more economical to run the TC leadwires to the data acquisition systems, and this is still the case inthe laboratory and on various test stands. On the other hand, thecost of integral transmitters in most industrial applications hasbecome competitive with the cost of running the thermocouplelead wires to the control room. These integral transmitters are

FIG. 4.13cc Noise interference that enters only one of the lead wires (normalmode) is more difficult to remove than noise that acts on both leads(common mode).

FIG. 4.13dd The addition of a guard lead wire reduces the normal mode noise.1

Normal Mode

Common Mode

HI

LO

HI

LO

Rs+ −

240 VRMS

120 VRMS

NoiseCurrent

Guard

HI

LO

HI

LO

RS

RS

Noise Current

CStray

© 2003 by Béla Lipták

688 Temperature Measurement

also explosion-proof. Conventional transmitters are accurateto 0.15% of span, and intelligent units are accurate to 0.05%of span (Table 4.13m). It can be observed that while intelligenttransmitters give better performance than the standard ones,they too are limited to a minimum absolute error plus coldjunction error, which equals about 1°F (0.6°C).

Intelligent Transmitters During the last decade, micropro-cessor based temperature transmitters have continued toevolve in sophistication and capability. They usually includean input circuit referred to as an A/D converter that convertsthe sensor input signal from its analog form into a digitalrepresentation. The microprocessor performs the ranging, lin-earization, error checking, and conversion. The resulting dig-ital value is then converted back, usually to a 4–20 mA DCanalog signal. For some special applications, 0–1 V DC or0–10 V DC or digital signals using either an open or propri-etary protocol are also used.

Today, universal transmitters that accept inputs from anyTC, RTD, or other resistance and mV source are commonlyavailable. They make transmitters interchangeable andthereby reduce inventories. They check their own calibrationon every measurement cycle, minimize drift over a wideambient temperature range, incorporate self-diagnostics fea-tures, and can be configured by the use of simple push buttonsor personal computer software. Their reconfiguration processis quick and convenient.

In addition to improved performance, the intelligenttransmitters are capable of working with any one of eighttypes of TCs or two types of RTD elements. This increasestheir flexibility and reduces the need for spare parts. Theintelligent transmitters are also provided with continuous self-diagnostics and with automatic three-point self-calibration,which is performed every 5 s and does not interrupt the analogor digital output of the unit.

The intelligent transmitter can also be furnished with dualthermal elements that can be used to measure temperaturedifferentials, averages, and high/low sensors, or as redundantbackup elements. Another convenient feature of smart trans-mitters is their remote reconfiguration capability, which canchange their zero, span, or many other features withoutrequiring rewiring.

ADVANTAGES AND LIMITATIONS

The weakest link in virtually all measurements is the tem-perature sensor. For most industrial applications the thermo-couple (TC) has been popular, because it is relatively inex-pensive, can be produced in a variety of sizes, can be ofruggedized construction and covers a wide temperaturerange. Thermocouples are also small, convenient, and versa-tile (can be welded to a pipe), cover wide ranges, are reason-ably stable, reproducible, accurate, and fast. The EMF theygenerate is independent of wire length and diameter. WhileRTDs are more accurate and more stable and while ther-mistors are more sensitive, thermocouples are the most eco-nomical and the best to detect the highest temperatures.

The main disadvantage of the TC is its weak outputsignal. This makes it sensitive to electrical noise and limitsits use to relatively wide spans (usually the minimum trans-mitter span is 1.0 mV). It is nonlinear, and the conversion ofthe EMF generated into temperature is not as simple as indirect reading devices. TCs always require amplifiers, andthe calibration of the TC can change due to contamination orcomposition changes due to internal oxidation, cold-workingor temperature gradients. Another limitation is that bare TCscannot be used in conductive fluids, and if their wires are nothomogeneous, this can cause errors.

In general, one should use the largest size TC wire pos-sible, and avoid stress and vibration. Use of integral trans-mitters is also recommended whenever possible (and other-wise use twisted and shielded wires with the shield connectedto the guard of the integrating A/D converter). In addition,one should avoid steep temperature gradients, and be carefulin selecting the sheath and thermowell materials.

THERMOCOUPLE TABLES

Tables 4.13ff, 4.13gg, 4.13hh, 4.13ii, 4.13jj, and 4.13kk providetemperature vs. millivolts data for types J, K, R, S, T, and Ethermocouples. All thermocouple tables in this handbook are

FIG. 4.13ee Integral thermocouple transmitter mounted directly on top of thethermowell. (Courtesy of The Foxboro Co.)

Allow 102 mm (4 in.)for Cover Removal

Input and OutputScrew Terminals

TransmitterPackage

1/2 NPT for TerminalConnections. TwoHoles OppositeSides. Plug UnusedConnection Hole.

Nipple Coupler(3/4 NPT)Coupler withUnion

Plain Well

R3/4 or 3/4 NPTRL or L NPT

1113

0.438Dia.

122

48

274

108

117

46

178

20

51

20

64

23

"U"

3

45

+−

© 2003 by Béla Lipták

689Tem

perature Measurem

ent

TABLE 4.13ffType J—Iron-Constantan Thermocouple(Degrees Fahrenheit vs. Millivolts. Temperatures are based on the International Temperature Scale of 1948. EMF is expressed in absolute millivolts. Reference Junction 32°F (0°C).)

°F* −300 −200 −100 −0 +0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

MILLIVOLTS

05

1015

−7.52−7.59−7.66−7.73

−5.76−5.86−5.96−6.06

−3.49−3.61−3.73−3.85

−0.89−1.02−1.16−1.29

−0.89−0.75−0.61−0.48

1.942.092.232.38

4.915.065.215.36

7.948.108.258.40

11.0311.1811.3411.49

14.1214.2714.4214.58

17.1817.3417.4917.64

20.2620.4120.5620.72

23.3223.4723.6323.86

26.4026.5526.7026.78

29.5229.6829.8430.00

32.7232.8933.0533.21

36.0136.1836.3536.52

39.4339.6139.7839.96

42.9643.1443.3243.50

46.5346.1746.8947.07

20253035

−7.79 −6.16−6.25−6.35−6.44

−3.97−4.09−4.21−4.33

−1.43−1.56−1.70−1.83

−0.34−0.20−0.06+0.08

2.522.672.822.97

5.515.665.815.96

8.568.718.879.02

11.6511.8011.9612.11

14.7314.8815.0415.19

17.8017.9518.1118.26

20.8721.0221.1821.33

23.9324.0924.2424.39

27.0227.1727.3327.48

30.1630.3230.4830.64

33.3733.5433.7033.86

36.6936.8637.0237.20

40.1340.3140.4840.66

43.6843.8544.0344.21

47.2447.4247.6047.78

40455055

−6.53−6.62−6.71−6.80

−4.44−4.56−4.68−4.79

−1.96−2.09−2.22−2.35

+0.22+0.360.500.65

3.113.263.413.56

6.116.276.426.57

9.179.339.489.64

12.2612.4212.5712.73

15.3415.5015.6515.80

18.4118.5718.7218.87

21.4821.6421.7921.94

24.5524.7024.8525.01

27.6427.8027.9528.11

30.8030.9631.1231.28

34.0334.1934.3634.52

37.3637.5437.7137.88

40.8341.0141.1941.36

44.3944.5744.7544.93

47.9548.1348.3148.48

60657075

−6.89−6.97−7.06−7.14

−4.90−5.01−5.12−5.23

−2.48−2.61−2.74−2.86

0.790.931.071.22

3.713.864.014.16

6.726.877.037.18

9.799.95

10.1010.25

12.8813.0413.1913.34

15.9616.1116.2616.42

19.0319.1819.3419.49

22.1022.2522.4022.55

25.1625.3225.4725.62

28.2628.4228.5828.74

31.4431.6031.7631.92

34.6834.6035.0135.18

38.0538.2238.3938.57

41.5441.7241.9042.07

45.1045.2845.4645.64

48.6648.8349.0149.18

80859095

100

−7.22−7.30−7.38−7.45−7.52

−5.34−5.44−5.55−5.65−5.76

−2.99−3.12−3.24−3.36−3.49

1.361.511.651.801.94

4.314.464.614.764.91

7.337.487.647.797.94

10.4110.5610.7210.8711.03

13.5013.6513.8113.9614.12

16.5716.7216.8817.0317.18

19.6419.8019.9520.1020.26

22.7122.8623.0123.1723.32

25.7825.9326.0926.2426.40

28.8929.0529.2129.3729.52

32.0832.2432.4032.5632.72

35.3535.5135.6835.8436.01

38.7438.9139.0839.2639.43

42.2542.4342.6142.7842.96

45.8246.0046.1846.3546.53

49.3649.5349.7049.8850.05

Note: Instruments calibrated to this curve have scales identified as “type J” thermocouple.

* CF 32

1.8° =

° −

© 2003 by Béla Lipták

4.13T

hermocouples

690

TABLE 4.13ggType K—Chromel-Alumel Thermocouple(Degrees Fahrenheit vs. Millivolts. Temperatures are based on the International Temperature Scale of 1948. EMF is expressed in absolute millivolts. Reference Junction 32°F (0°C).)

°F* 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300

MILLIVOLTS

05

1015

−0.68−0.58−0.47−0.37

1.521.631.741.86

3.823.944.054.17

6.096.206.316.42

8.318.428.548.65

10.5710.6810.7910.91

12.8612.9713.0913.20

15.1815.3015.4115.53

17.5317.6417.7617.88

19.8920.0120.1320.24

22.2622.3722.4922.61

24.6324.7424.8624.98

26.9827.1027.2227.34

29.3229.4429.5629.67

31.6531.7631.8831.99

33.9334.0534.1634.28

36.1936.3136.4236.53

38.4338.5438.6538.76

40.6240.7340.8440.95

42.7842.8942.9943.10

44.9145.0145.1245.22

47.0047.1047.2147.31

49.0549.1549.2549.35

51.0551.1551.2551.35

20253035

−0.26−0.15−0.04+0.07

1.972.092.202.32

4.284.404.514.63

6.536.656.766.87

8.768.878.989.09

11.0211.1311.2511.36

13.3213.4413.5513.67

15.6515.7615.8816.00

18.0018.1118.2318.35

20.3620.4820.6020.72

22.7322.8522.9723.08

25.1025.2225.3425.46

27.4527.5727.6927.80

29.7929.9130.0230.14

32.1132.2232.3432.45

34.3934.5034.6234.73

36.6436.7636.8736.98

38.8738.9839.0939.20

41.0541.1641.2741.38

43.2143.3143.4243.53

45.3345.4345.5445.64

47.4147.5247.6247.72

49.4549.5549.6549.76

51.4551.5451.6451.74

40455055

+0.18+0.29

0.400.51

2.432.552.662.78

4.744.864.975.08

6.987.097.207.31

9.219.329.439.54

11.4811.5911.7111.82

13.7813.9014.0214.13

16.1216.2316.3516.47

18.4718.5818.7018.82

20.8420.9521.0721.19

23.2023.3223.4423.56

25.5725.6925.8125.93

27.9228.0428.1528.27

30.2530.3730.4930.60

32.5732.6832.8032.91

34.8434.9635.0735.18

37.0937.2037.3137.43

39.3139.4239.5339.64

41.4941.6041.7041.81

43.6343.7443.8543.95

45.7545.8545.9646.06

47.8247.9348.0348.13

49.8649.9650.0650.16

51.8451.9452.0352.13

60657075

0.620.730.840.95

2.893.013.123.24

5.205.315.425.53

7.427.537.647.75

9.669.779.88

10.00

11.9412.0512.1712.28

14.2514.3614.4814.60

16.5916.7016.8216.94

18.9419.0619.1819.29

21.3121.4321.5421.66

23.6823.8023.9124.03

26.0526.1626.2826.40

28.3928.5028.6228.74

30.7230.8330.9531.07

33.0233.1433.2533.37

35.2935.4135.5235.63

37.5437.6537.7637.87

39.7539.8639.9640.07

41.9242.0342.1442.24

44.0644.1744.2744.38

46.1746.2746.3846.48

48.2348.3448.4448.54

50.2650.3650.4650.56

52.2352.3352.4252.52

80859095

100

1.061.181.291.401.52

3.363.473.593.703.82

5.655.765.875.986.09

7.877.988.098.208.31

10.1110.2210.3410.4510.57

12.4012.5112.6312.7412.86

14.7114.8314.9515.0615.18

17.0617.1717.2917.4117.53

19.4119.5319.6519.7719.89

21.7821.9022.0222.1422.26

24.1524.2724.3924.5124.63

26.5226.6326.7526.8726.98

28.8628.9729.0929.2129.32

31.1831.3031.4231.5331.65

33.4833.5933.7133.8233.93

35.7535.8635.9736.0836.19

37.9838.0938.2038.3238.43

40.1840.2940.4040.5140.62

42.3542.4642.5742.6742.78

44.4944.5944.7044.8044.91

46.5846.6946.7946.9047.00

48.6448.7448.8548.9549.05

50.6550.7550.8550.9551.05

52.6252.7252.8152.9153.01

* CF 32

1.8° =

° −

© 2003 by Béla Lipták

4.13 Thermocouples 691

TABLE 4.13hhType R—Platinum vs. Platinum Plus 13% Rhodium ThermocoupleDegrees Fahrenheit vs. Millivolts. Temperatures are based on the International Temperature Scale of 1948. EMF is expressed in absolute millivolts. Reference Junction 32°F (0°C).

°F* 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

MILLIVOLTS

05

1015

−0.089−0.076−0.062−0.049

0.2200.2370.2550.272

0.5960.6160.6370.657

1.0301.0521.0751.098

1.5041.5291.5531.578

2.0122.0382.0652.091

2.5472.5752.6022.630

3.1033.1323.1603.188

3.6773.7063.7353.764

4.2644.2944.3244.354

4.8684.8994.9304.960

5.4885.5195.5515.582

6.1256.1566.1886.220

6.7736.8056.8386.871

7.4367.4707.5037.537

8.1168.1508.1848.218

20253035

−0.035−0.021−0.006+0.009

0.2910.3080.3270.345

0.6780.7000.7210.742

1.1211.1441.1671.191

1.6031.6281.6531.678

2.1172.1442.1702.197

2.6572.6852.7122.740

3.2173.2453.2733.302

3.7943.8233.8523.882

4.3844.4134.4434.473

4.9915.0225.0535.084

5.6145.6455.6775.709

6.2526.2856.3176.349

6.9046.9376.9707.003

7.5717.6057.6397.672

8.2538.2878.3228.356

40455055

+0.024+0.039

0.0550.071

0.3630.3810.4000.419

0.7630.7850.8070.828

1.2141.2381.2611.285

1.7031.7291.7541.779

2.2232.2502.2772.303

2.7682.7962.8232.851

3.3303.3593.3873.416

3.9113.9413.9703.999

4.5034.5334.5634.593

5.1155.1465.1765.208

5.7415.7735.8055.837

6.3816.4146.4466.479

7.0377.0697.1037.136

7.7067.7407.7747.808

8.3918.4268.4608.495

60657075

0.0860.1030.1190.135

0.4380.4570.4760.496

0.8500.8720.8940.917

1.3091.3331.3571.381

1.8051.8311.8561.882

2.3302.3572.3842.412

2.8792.9072.9352.963

3.4453.4733.5023.531

4.0294.0584.0874.116

4.6244.6544.6854.715

5.2385.2705.3015.332

5.8695.9015.9335.964

6.5116.5446.5776.609

7.1697.2027.2357.269

7.8427.8777.9117.945

8.5308.5658.5998.634

80859095

100

0.1520.1690.1860.2030.220

0.5160.5360.5560.5760.596

0.9390.9620.9841.0071.030

1.4061.4301.4551.4801.504

1.9081.9341.9601.9862.012

2.4382.4662.4932.5202.547

2.9913.0193.0473.0753.103

3.5603.5893.6183.6473.677

4.1464.1754.2054.2354.264

4.7464.7764.8074.8374.868

5.3635.3945.4265.4575.488

5.9966.0286.0606.0926.125

6.6426.6746.7076.7406.773

7.3027.3367.3697.4037.436

7.9798.0138.0478.0818.116

8.6698.7048.7398.7748.809

°F* 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000

MILLIVOLTS

05

1015

8.8098.8448.8798.914

9.5169.5529.5879.623

10.23710.27410.31010.347

10.97311.01111.04811.085

11.72611.76511.80211.840

12.48812.52612.56412.602

13.25513.29313.33213.371

14.02714.06514.10414.142

14.79814.83714.87514.914

15.56815.60715.64515.684

16.34016.37816.41716.455

17.11017.14817.18617.225

17.87517.91317.95117.989

18.63618.67418.71218.750

19.39419.43219.47019.508

20253035

8.9498.9849.0199.054

9.6599.6949.7309.766

10.38310.42010.45610.493

11.12211.16011.19711.235

11.87811.91611.95411.992

12.64112.67912.71812.756

13.40913.44813.48613.525

14.18114.21914.25814.296

14.95214.99115.02915.068

15.72215.76115.80015.838

16.49416.53216.57116.610

17.26317.30117.34017.378

18.02718.06518.10318.141

18.78818.82618.86418.902

19.54519.58319.58319.659

40455055

9.0909.1259.1619.196

9.8029.8389.8749.910

10.52910.56610.60310.639

11.27311.31011.34811.385

12.02912.06812.10512.144

12.79512.83312.87112.909

13.56413.60213.64113.679

14.33514.37414.41214.451

15.10715.14515.18415.222

15.87715.91515.95415.992

16.64816.68716.72516.764

17.41617.45517.49317.532

18.17918.21818.25518.294

18.94018.97819.01619.054

19.69719.73519.77319.811

60657075

9.2329.2679.3039.338

9.9469.98210.01910.056

10.67610.71210.74910.786

11.42411.46111.49911.537

12.18212.22012.25812.296

12.94812.98613.02513.063

13.71813.75613.79513.833

14.49014.52814.56714.606

15.26115.29915.33815.377

16.03116.07016.10816.147

16.80216.84216.88016.918

17.56917.60817.64617.685

18.33218.37018.40818.446

19.09219.12919.16819.205

19.84819.88619.92419.962

80859095

100

9.3749.4099.4459.4819.516

10.09210.12910.16410.20110.237

10.82310.86110.89810.93610.973

11.57511.61311.65111.68911.726

12.33512.37312.41112.45012.488

13.10213.14013.17813.21613.255

13.87213.91113.94913.98814.027

14.64414.68314.72114.76014.798

15.41515.45415.49215.53115.568

16.18516.22416.26316.30116.340

16.95716.99517.03317.07217.110

17.72317.76117.79917.83717.875

18.48418.52218.56018.59818.636

19.24319.28119.31819.35619.394

19.99920.03720.07520.11220.150

* CF 32

1.8° =

° −

© 2003 by Béla Lipták

692Tem

perature Measurem

ent

TABLE 4.13iiType S—Platinum vs. Platinum Plus 10% Rhodium Thermocouple(Degrees Fahrenheit vs. Millivolts. Temperatures are based on the International Temperature Scale of 1948. EMF is expressed in absolute millivolts. Reference

Junction 32°F (0°C).)

°F* 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

MILLIVOLTS

0 −0.092 0.221 0.595 1.017 1.474 1.956 2.458 2.977 3.506 4.046 4.596 5.156 5.726 6.307 6.897 7.498

5 −0.078 0.238 0.615 1.039 1.498 1.981 2.484 3.003 3.533 4.073 4.623 5.184 5.755 6.336 6.927 7.529

10 −0.064 0.256 0.635 1.061 1.521 2.005 2.510 3.029 3.560 4.100 4.651 5.212 5.784 6.635 6.957 7.559

15 −0.050 0.274 0.655 1.083 1.545 2.030 2.535 3.056 3.587 4.128 4.679 5.241 5.813 6.394 6.987 7.589

20 −0.035 0.291 0.676 1.106 1.569 2.055 2.561 3.082 3.614 4.155 4.707 5.269 5.842 6.424 7.017 7.620

25 −0.021 0.309 0.696 1.128 1.593 2.080 2.587 3.108 3.640 4.182 4.735 5.298 5.871 6.453 7.046 7.650

30 −0.006 0.327 0.717 1.151 1.616 2.105 2.613 3.135 3.667 4.210 4.763 5.326 5.899 6.483 7.076 7.681

35 +0.009 0.346 0.738 1.173 1.640 2.130 2.638 3.161 3.694 4.237 4.790 5.354 5.928 6.512 7.106 7.711

40 +0.024 0.364 0.758 1.196 1.664 2.155 2.664 3.188 3.721 4.264 4.818 5.383 5.957 6.542 7.136 7.742

45 +0.040 0.383 0.779 1.219 1.688 2.180 2.690 3.214 3.748 4.292 4.846 5.411 5.986 6.571 7.166 7.772

50 0.056 0.401 0.800 1.242 1.712 2.205 2.716 3.240 3.775 4.319 4.874 5.440 6.015 6.601 7.196 7.803

55 0.071 0.420 0.822 1.264 1.736 2.230 2.742 3.267 3.802 4.347 4.902 5.469 6.044 6.630 7.226 7.834

60 0.087 0.439 0.843 1.287 1.761 2.255 2.768 3.293 3.829 4.374 4.930 5.497 6.073 6.660 7.257 7.864

65 0.104 0.458 0.864 1.311 1.785 2.281 2.794 3.320 3.856 4.402 4.959 5.526 6.102 6.689 7.287 7.895

70 0.120 0.477 0.886 1.334 1.809 2.306 2.820 3.347 3.883 4.430 4.987 5.555 6.131 6.719 7.317 7.925

75 0.136 0.496 0.907 1.357 1.833 2.331 2.846 3.373 3.910 4.457 5.015 5.583 6.161 6.749 7.347 7.956

80 0.153 0.516 0.929 1.380 1.858 2.357 2.872 3.400 3.937 4.485 5.043 5.612 6.190 6.778 7.377 7.987

85 0.170 0.535 0.951 1.404 1.882 2.382 2.898 3.426 3.964 4.512 5.071 5.640 6.219 6.808 7.407 8.018

90 0.187 0.555 0.973 1.427 1.907 2.407 2.924 3.453 3.991 4.540 5.099 5.669 6.248 6.838 7.438 8.048

95 0.204 0.575 0.994 1.450 1.931 2.433 2.951 3.480 4.019 4.568 5.128 5.698 6.277 6.867 7.468 8.079

100 0.221 0.595 1.017 1.474 1.956 2.458 2.977 3.506 4.046 4.596 5.156 5.726 6.307 6.897 7.498 8.110

© 2003 by Béla Lipták

693Tem

perature Measurem

ent

0 8.110 8.732 9.365 10.009 10.662 11.323 11.989 12.657 13.325 13.991 14.656 15.319 15.979 16.637 17.292 17.943 18.590

5 8.141 8.764 9.397 10.041 10.695 11.356 12.022 12.690 13.358 14.024 14.689 15.352 16.012 16.670 17.324 17.975 18.622

10 8.172 8.795 9.429 10.074 10.728 11.389 12.055 12.724 13.391 14.058 14.722 15.385 16.045 16.702 17.357 18.008 18.655

15 8.203 8.827 9.461 10.106 10.761 11.423 12.089 12.757 13.425 14.091 14.755 15.418 16.078 16.735 17.389 18.040 18.687

20 8.234 8.858 9.493 10.139 10.794 11.456 12.122 12.790 13.458 14.124 14.789 15.451 16.111 16.768 17.422 18.073

25 8.265 8.890 9.525 10.171 10.827 11.489 12.155 12.824 13.491 14.157 14.822 15.484 16.144 16.801 17.455 18.105

30 8.296 8.921 9.557 10.204 10.860 11.522 12.189 12.857 13.525 14.191 14.855 15.517 16.177 16.834 17.487 18.137

35 8.327 8.953 9.589 10.237 10.893 11.556 12.222 12.891 13.558 14.224 14.888 15.550 16.210 16.866 17.520 18.170

40 8.358 8.984 9.621 10.269 10.926 11.589 12.256 12.924 13.591 14.257 14.921 15.583 16.243 16.899 17.552 18.202

45 8.389 9.016 9.654 10.302 10.959 11.622 12.289 12.957 13.625 14.290 14.954 15.616 16.275 16.932 17.585 18.235

50 8.420 9.048 9.686 10.334 10.992 11.655 12.322 12.991 13.658 14.324 14.988 15.649 16.308 16.965 17.618 18.267

55 8.451 9.079 9.718 10.367 11.025 11.689 12.356 13.024 13.691 14.357 15.021 15.682 16.341 16.997 17.650 18.299

60 8.482 9.111 9.750 10.400 11.058 11.722 12.389 13.058 13.725 14.390 15.054 15.715 16.374 17.030 17.683 18.332

65 8.513 9.143 9.782 10.433 11.091 11.755 12.423 13.091 13.758 14.423 15.087 15.748 16.407 17.063 17.715 18.364

70 8.545 9.174 9.815 10.465 11.124 11.789 12.456 13.124 13.791 14.457 15.120 15.781 16.440 17.095 17.748 18.396

75 8.576 9.206 9.847 10.498 11.157 11.822 12.490 13.158 13.825 14.490 15.153 15.814 16.473 17.128 17.780 18.429

80 8.607 9.238 9.879 10.531 11.190 11.855 12.523 13.191 13.858 14.523 15.186 15.847 16.506 17.161 17.813 18.461

85 8.638 9.270 9.912 10.564 11.224 11.888 12.556 13.224 13.891 14.556 15.219 15.880 16.538 17.194 17.845 18.493

90 8.670 9.302 9.944 10.597 11.257 11.922 12.590 13.258 13.924 14.589 15.253 15.913 16.571 17.226 17.878 18.526

95 8.701 9.333 9.976 10.629 11.290 11.955 12.623 13.291 13.958 14.623 15.286 15.946 16.604 17.259 17.910 18.558

100 8.732 9.365 10.009 10.662 11.323 11.989 12.657 13.325 13.991 14.656 15.319 15.979 16.637 17.292 17.943 18.590

* CF 32

1.8° =

° −

© 2003 by Béla Lipták

694 Temperature Measurement

based upon a reference junction temperature of 32°F (0°C);therefore, direct conversion from the tables can be made onlywhen an ice bath is used at the reference junction.

If it is not possible to maintain the reference junction tem-perature at 32°F (0°C), a correction factor must be applied tothe millivolt values shown in the TC tables. Note that the mil-livoltage produced by a given thermocouple is decreased whenthe temperature difference between the measuring junction andthe reference junction is decreased. Correcting for referencejunction temperatures other than 32°F (0°C) is described below.

Converting Millivoltage to Temperature

To apply the reference junction correction factor to a givenpotentiometer millivoltage reading, proceed as follows:

1. From the appropriate TC table, obtain the millivoltage(based upon a 32°F R/J) corresponding to the actualtemperature of the thermocouple reference junction.

2. Add the value obtained in step 1 to the millivoltageread on the potentiometer.

3. The corrected millivoltage may then be converted intoterms of temperature directly from the same table.

Example 1 A potentiometer indicates a 13.019 mV whenconnected to a type T thermocouple, and it is desired to

convert this value to its equivalent temperature. The actualTC reference junction temperature, as determined by an accu-rate mercury-in-glass thermometer, is 68°F (20°C). Interpo-lating* from the type T Table, 68°F = 0.787 mV, based upon

TABLE 4.13jjType T—Copper-Constantan ThermocoupleDegrees Fahrenheit vs. Millivolts. Temperatures are based on the International Temperature Scale of 1948. EMF is expressed in absolute millivolts. Reference Junction 32°F (0°C).

°F* −300 −200 −100 −0 +0 100 200 300 400 500 600 700

MILLIVOLTS

05

1015

−5.284−5.332−5.379

−4.111−4.179−4.246−4.312

−2.559−2.645−2.730−2.814

−0.670−0.771−0.872−0.973

−0.670−0.567−0.463−0.359

1.5171.6331.7511.869

3.9674.0964.2254.355

6.6476.7866.9267.066

9.5259.6749.8239.973

12.57512.73212.88813.046

15.77315.93716.10116.264

19.10019.26919.43919.608

20253035

−4.377−4.441−4.504−4.566

−2.897−2.980−3.062−3.143

−1.072−1.171−1.270−1.367

−0.254−0.149−0.042+0.064

1.9872.1072.2262.346

4.4864.6174.7494.880

7.2087.3497.4917.633

10.12310.27310.42310.574

13.20313.36213.52013.678

16.42916.59316.75816.924

19.77919.94920.12020.291

40455055

−4.627−4.688−4.747−4.805

−3.223−3.301−3.380−3.457

−1.463−1.559−1.654−1.748

+0.171+0.280

0.3890.499

2.4672.5892.7112.835

5.0145.1475.2805.415

7.7767.9208.0648.207

10.72610.87811.03011.183

13.83813.99714.15714.317

17.08917.25517.42117.588

20.46320.63420.805

60657075

−4.863−4.919−4.974−5.029

−3.533−3.609−3.684−3.757

−1.842−1.934−2.026−2.117

0.6090.7200.8320.944

2.9583.0823.2073.332

5.5505.6855.8215.957

8.3528.4978.6428.788

11.33611.49011.64311.797

14.47714.63714.79914.961

17.75417.92118.08918.257

80859095

100

−5.081−5.134−5.185−5.235−5.284

−3.829−3.901−3.972−4.042−4.111

−2.207−2.296−2.385−2.472−2.559

1.0571.1711.2861.4011.517

3.4583.5843.7123.8393.967

6.0946.2326.3706.5086.647

8.9359.0829.2299.3769.525

11.95312.10812.26312.41812.575

15.12215.28415.44715.61015.773

18.42518.59318.76118.93019.100

* CF 32

1.8° =

° −

* To interpolate between two printed values, add to the smaller value aproportionate part of the difference between the two printed values:Example 1a (positive temperature)

248°F = 245° + (250° − 245°).

In terms of millivoltage from the type T table,

248°F = 5.147 + (5.280 − 5.147)

= 5.147 + 0.0798 = 5.227 mV

Example 1b (negative temperature)

−248°F = −245° + (−250° − (−245 °))

In terms of millivoltage from the type T table,

−248°F = −4.688 + (−4.747 − (4.668))

= −4.688 + (−4.747 + 4.688)

= −4.688 − 0.0354 = −4.723 mV

3

5

3

5

3

5

3

5

3

5

© 2003 by Béla Lipták

4.13 Thermocouples 695

a 32°F reference junction. Adding this value to the potenti-ometer reading, 13.019 + 0.787 = 13.806 mV, which is thecorrected millivoltage based upon a 32°F reference junction.Interpolating from type T table, 13.806 mV = 539°F (282°C).

Example 2 A type T thermocouple under steady operatingconditions causes a potentiometer reading of −3.357 mv. Theactual TC reference junction temperature is 70°F (21°C).From the type T table, 70°F = 0.832 mV based upon areference junction of 32ºF. Adding these two millivoltagesalgebraically, −3.357 + 0.832 = −2.525 mV. Interpolating,*−2.525 mV = −98°F (–72°C).

Converting Temperature to Millivoltage

To determine the proper millivolt input required to check thecalibration of an instrument, proceed as follows:

1. From the appropriate table, obtain the millivoltagebased upon a 32°F reference junction correspondingto the actual temperature at the input terminals of theinstrument to be checked.

2. From the same table, obtain the millivoltage basedupon a 32°F reference junction for the temperature tobe checked.

3. Subtract the value obtained in step 1 from the valueobtained in step 2.

Example 1 It is desired to check the calibration of aninstrument at 300°F (149°C). The instrument has a scalegraduated in degrees Fahrenheit for a type T thermocou-ple. The actual temperature at the input terminals of theinstrument to be checked, as determined by an accuratemercury-in-glass thermometer, is 70°F (21°C). From thetype T table, 70°F = 0.832 mV and 300°F = 6.647 mVbased upon a reference junction temperature of 32°F. Sub-stracting these values, the corrected millivoltage inputrequired on the basis of a 70°F reference junction is 6.647 −0.832 = 5.815 mV.

Example 2 It is desired to determine the correct millivolt-age input required to check the calibration of an instrumentat −200°F (−129°C). The instrument scale is graduated in

TABLE 4.13kkType E—Chromel-Constantan Thermocouple

°F* −300 −200 −100 −0 +0 100 200 300 400 500 600 700

MILLIVOLTS

0102030

−8.30−8.45−8.60

−6.40−6.62−6.83−7.04

−3.94−4.21−4.47−4.73

−1.02−1.33−1.64−1.94

−1.02−0.71−0.39−0.07

2.272.622.973.32

5.876.256.627.00

9.7110.1110.5110.91

13.7514.1714.5915.00

17.9518.3818.8119.23

22.2522.6923.1323.57

26.6527.0927.5327.97

40506070

−7.24−7.44−7.62−7.80

−4.98−5.23−5.48−5.72

−2.24−2.54−2.83−3.11

0.260.590.921.26

3.684.044.404.77

7.387.768.158.54

11.3111.7112.1112.52

15.4215.8416.2616.68

19.6620.0920.5220.95

24.0024.4424.8825.32

28.4228.8629.3129.75

8090

100

−7.97−8.14−8.30

−5.95−6.18−6.40

−3.39−3.67−3.94

1.591.932.27

5.135.505.87

8.939.329.71

12.9313.3413.75

17.1017.5217.95

21.3921.8222.25

25.7626.2026.65

30.1930.6431.09

°F* 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

MILLIVOLTS

0102030

31.0931.5431.9832.43

35.5736.0236.4736.92

40.0640.5140.9641.41

44.5640.0145.4645.91

49.0449.4949.9350.37

53.5053.9454.3854.83

57.9258.3658.8059.24

62.3062.7463.1763.60

66.6367.0567.4867.91

70.9071.3271.7572.17

75.1275.5375.9576.37

40506070

32.8733.3233.7734.22

37.3737.8238.2638.71

41.8642.3142.7643.21

46.3646.8147.2647.71

50.8251.2751.7252.16

55.2755.7156.1556.59

59.6860.1160.5560.99

64.0464.4764.9065.34

68.3468.7669.1969.62

72.6073.0273.4473.86

8090

100

34.6735.1235.57

39.1639.6140.06

43.6644.1144.56

48.1548.6049.04

52.6153.0553.50

57.0357.4857.92

61.4361.8662.30

65.7766.2066.63

70.0570.4770.90

74.2874.7075.12

* CF 32

1.8° =

° −

© 2003 by Béla Lipták

696 Temperature Measurement

degrees Fahrenheit for a type T thermocouple. The actualtemperature at the input terminals of the instrument is 68°F(20°C). From the type T table, 68°F = 0.787 and −200°F =−4.111 mV based upon a 32°F reference junction. Subtract-ing these values, the corrected millivolt input on the basis ofa 68°F reference junction is −4.111 − 0.787 = –4.898 mV.

References

1. Hewlett-Packard Application Note 290, “Practical Temperature Mea-surement,” Palto Alto, CA.

2. American Petroleum Institute, Manual API 550, “Installation of Refin-ery Instruments and Control Systems,” 1965.

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