VLIMITATIONS CF THERMOCOUPLES IK TEMPERATURE MEASUREMENTS

R. L. Anderson, R. K. Adams, and B. C. Duggins

OAK RIDGE NATIONAL LABORATORYOak Ridge, Tennessee 37820

Operated.byUNION CARBIDE CORPORATION

for theDEPARTMENT CF EHERGr

ABSTRACT

Factors l im i t i ng the accuracy of temperaturemeasurements with type K and type S sheathedthermocouple assemblies are discussed. Theeffect of short-range ordering in Chromel isshown to l i m i t the accuracy of temperaturemeasurements niede with type K thermocouple toabout IS. Errors of as much as 150% have beenobserved in type K thermocouples in magneticf i e lds at temperatures below the Curietemperature of Alumel. Both pos i t ive andnegative errors were observed when theorientations of the applied magnetic f i e l d , thetemperature gradient and the axis of the AliKieluire were such as to produce an emf along thethermocouple wire due to the flernst-Et'cingshauseneffect. Dr i f t tests of type K thermocoupleassemblies sheathed in stainless steel showedchanges in indicated temperature of -13 C after50 hours at H00°C, while Inconel sheathedassemblies in the same test showed a change lessthan 1 C. Base metal sheaths caused largedecalibrations in type S thermocouples, whereaswith noble metal a l loy sheaths type Sthermocouples were stable to 1300 C. Errors dueto the decrease in el^ctr ic^l in resis-jrics withiiicreosinr; toTipsrature wens investigated. Ttieuncertainties associated with high speed dataacquisition systtrcs were analyzed and i t is shownthat the higher emf output of a type Kthermocouple does not result in an increase inaccuracy over type S thermocouples over a widerange of temperature.

Introduction

Thermocouples are easily the most widely usedtemperature sensors in process control systems.The advantages are obvious. Thermocouples areinexpensive, they can be remotely located and themany available types cover a temperature rangefrom about -269 to 3000°C. Because of theirvjide-spread use in c r i t i ca l control systems anain research at the Oak Ridge National Laboratory,the Instrumentation and Controls Division hasmaintained programs at ORNL to evaluate theperformance of thermocouple materials and sourcesof error in thermocouple measuring systems. Overthe years, since the establishment of the l&CDivision, we have accumulated an estimated 300man-years of experience in thermocouplethermoraetry, both d i rec t l y and through our

- NOTICE-

TTiU report >vas prepared at an account of workjpojisoied by the Ujiiled Stales Government. NeiUier theUnited States not the United States Department ofEnergy, nor any of their employees, not any of theircontractors, subcontractor!, or their r oloyees, maSesany warranty, express or Implied, or a^umes any legalliabillly or responsibility for the accuracy, completenessor usefulness of any Information, apparatus, product orprocess disclosed, or tepresents that its use would notinfringe privately owned righu.

function as a national laboratory by providingadvice and consultation to inquiries from insideand outside of the laboratory. • In this paper wewill discuss several common sources oftemperature measurement error and the steps whichcan be taken to minimize these errors. The sevensources of errors in thermocouple thenr.craetrywhich we will consider are given in Table 1. Wefrequently refer to this list as the "SevenDeadly Sins" in thermocouple therraometry.

I. Thermal Shunting

Thermal shunting errors occur because theemplacement or attachment of a thermocoupledisturbs the temperature distribution of anyobject to which it is immersed (See Figure 1).This is because the thermocouple has a finitesize and conducts heat away from (or to) thaobject. In addition the thermocouple loses orgains heat from the surroundings by conduction,convection and radiation. Thus the measuringjunction of the thermocouple may be at edifferent temperature, higher or lower, than theobject. In addition, if the object has a lowthc-raial conductivity, the thermocouple can changetho temperature of the object locally. These*t..;f.fcr

thermocouple enif. Such "electrical shunting" cancause large temperature measurement errors.Figure 3 i l lustrates three ways in which lowinsulation resistance at high temperatures cancause a thermocouple to read either too high ortoo low. In example 1, a thermocouple insertedinto the temperature profi le as i l lustrated wi l lindicate a temperature which is too low becauseof a loss of eraf by leakage through the loweredelectrical resistance of the insulator betweenthe thermocouple wires above approximately1000'x. In example 2, a thermocouple whichpasses through a zone that is hotter than that ofthe measuring junction wi l l tend to indicate atemperature that is too high because of thecreation of a "virtual junction" in the hottestportion* In example 3, a thermocouple in thepresence of a sr.all dc leakage current on thesheath can indicate either too high or too low atemperature, depending on the direction of thecurrent flow. The lowered insulation resistancein the high temperature region wi l l allow afract ion of the current on the sheath tocirculate in the thermocouple c i rcu i t . Since thetwo wires on a thermocouple generally havedifferent electrical resist iv i t ies (with Chromeland Alumel for example, the ra t io of thee lec t r ica l r es i s t i v i t i e s is about 2:1) , theresult is that a net emf is generated in thethermocouple c i rcu i t .

To recapi tulate, Figure 4 i l l us t ra tes howshunting and virtual junction errors combine as athermocouple is inserted through ahigh-temperature p r o f i l e . During i n i t i a linsertion, the temperatures are too low to causeelec t r ica l shunting, and the indicatedtemperatures sr& correct. Upon further insertioninto a region where the temperature is highenough (greater than ipproximately 1000 C) forshunting to occur, the thermocouple wi l l indicatetoo low. The thermocouple wi l l continue toindicate too low a temperature un t i l themeasuring junction reaches point a, where i t wi l layain read correctly because the virtual junctioneffect. Insertion beyond point "a" wi l l causethe thermocouple to indicate too high atemperature because the virtual junction effectwi l l predominate.

The errors resulting from electrical shunting ore lec t r ica l leakage can be estimated andinterpreted using an analytical model developedby M. J . Roberts, and T. G. Kollie of the ISCDivision at CRNL.

In this model, the thermocouple is divided intosmall sections as shown in Figure 5. Thesections are cascaded together and, together witha known temperature p r o f i l e , a solution isobtained which gives a good approximation to theerrors caused by either e lect r ica l leakage,electrical shunting, or both.

The necessary parameters (resistance per unitlength, the Seebeck coef f ic ients , end theconductances per uni,' length) for the model werea l l determined experimentally at severaltemperatures. Furthermore, tiie experiment wasconducted in such a way as to provide a check en

the model. In a comparison of the calculated andmeasured errors resulting from electrical leakagethe agreement, better than 10S, is excellentpar t icu lar ly considering that the inputparameters were observed to change with time athigher temperatures during the experiments.

3. Calibration Errors

Temperature is hotness, and valuts of temperatureare measures of hotness. To compare values oftemperature, i t is necessary to construct a scaleof temperature, and many different scales arepresently in use; i . e . , Fahrenheit, Rankine,Centigrade, Celsius, Kelvin, etc. The scale oftemperature which has widest usage in scientif icand engineering work is the InternationalPractical Temperature Scale of 1968 (IPTS-68)adopted by the International Conference onWeights and Measures. IPTS-68 not only definesvalues of temperature for selected reproduciblefixed points but prescribes standard instrumentsand methods for real iz ing the scale. A l lmeasurements of temperature should ultimately bereferable to the IPTS-68. The calibration of theworking thermometers is the means by which thisis accomplished.

Table 2 l i s t s the sources of errors inthermocouple calibrations. Most of these are, ofcourse, the same as those for any temperaturemeasurement in thermocouple thermoroetry. Theexception is No. 4. In 1974, the National Bureauof Standards issued Monograph 125, "Thermocouple.Reference Tables Based on IPTS-68"^, whichsuperceded the MBS Circular 561 Table. At thesame time a new Type S (Platinum-10% Rhodium vsPlatinum) was defined. The composition of thealloy leg as specified to have 10% +0.05 wt.Srhodium, and in addition the reference for theplatinum leg was changed from Pt-27 to Pt-67.*The net result is that there are now two Type Sthermocouples; a "nominal", based on the oldCircular 561 Tables and an "exact" based on theMonograph 125 Tables. Manufacturer's ofthermocouple wire w i l l supply e i ther , butexperience in the Katrology Laboratory at ORNLshows that occasionally the thermocouplemanufacturer's can get as confused as everyoneelse and wi l l supply exact for nominal or visaversa. As seen in Figure 6 the differencebetween the old tlSS 561 tables (marked N35) andthe Monograph 125 table can become significant athigher temperatures. I t is about 30 microvoltsat 1000 C or about 3 C. Many older processinstruments employing Type S thermocouples arebased on the N3S Circular 551 Tables, so that i fa therr,-xoupl-e on such an instrument wereinadvertently replaced by an "exact" thermocouple

*Pt- i : / and Pt-67 are NBS standard referencematerials for pure platinum. Pt-67 supercededPt-27 and can be used as a referencethermoelement for thermocouples. NBS Monograph125 has tables of posit ive end negativethermoelements vs. Pt-67 for Types S, R, B, J , Kand T thermocouples.

the temperatures would be in error by the amountsindicated in Figure 6. The only reliable way todetermine if a thermocouple is "nominal" or"exact" is by accurate calibration against a knowstandard.

4. Decaiibration Errors

Introduction - The calibration of a thermocoupleestablishes a functional relationship between thf>

• emf output of the thermocouple and thetemperature of the measuring junction. This willbe referred to as the "Teir.perature-EmfRelationship" (TER). D£calibration, results fromchanges of the TER with time at temperature andis usually a function of position. Ifhomogeneity of the thermocouple is destroyed,then the TER becomes dependent on the location ofthe decalibrated or inhomoseneous portion of thethermocouple with respect to temperature

_gradients. As a result, REcalibration is usuallynot possible.

Decal ibration can be caused by changes in themetallurgical state of the thermocouplematerials, or by changes in the composition 'ofthe therraoeleinents. The rate and extent of thesechanges depend on factors such as thetemperature, the composition of thethermoelements, the composition of thesurrounding materials (insulators, protectivesheaths, gases} and on the sizes of thethermocouple wires.

The^-Order-Disorder Effect in Type K - Kollie, etal. have reviewed the orcer-disorder phenomenonin Chromel and its effect on the emf output ofType K t! ennocouple. Some of the pertinentpoints are: between 200 and 600 C, the nickeland chromium atoms in Chromel tend to occupyspecific sites in the crystal lattice (the

• ordered state); above approximately 600 C, theatcms are distributed randomly among the latticesites (the disordered state); the change from theordered to the disordered stace or visa versa isccrapletely reversible; the raca and extent of theformation cf the ordered state is both time andtemperature dependent; and the temperaturemeasurement errors caused by the order-disordertransformation approaches 1% of the measuredtemperature between 0 and 6C0 C.

The net result of the order-disordertransformation on temperature measurements madewith Type K thermocouples is illustrated byFigure 7. The initial calibration of an annealedthermocouple, curve "A", lies within the 3/8% ISAtolerance limit for special grade Chromel-Alumel,but data taken on cooling lies well outside thislimit. The calibration curve for a thermocouplewhich was "pre-ordered" at 482 C is shown bycurve "B", and the hysteresis observed on coolingwas much less than that for the annealedthermocouple. '.Jhen the thermocouples are shiftedin the furnace and recalibrated, however, newcalibration curves result.

Decai ibration by Compositional Changes in Type K-. Numberous investigators have studied thedecalibration of Type K thermocouples in air.

Eurley , in one of the more recent studies,investigated the decalibration of bare wire TypeK thermocouples in air at temperatures to 1000 Cfor times up to 3000 h. For Chromel-Aluniel pairsfrom four different sources, the changes in theemf output of these thermocouples is caused bychanges in bcth the Chrcmel element and theAluniel element. At 600 and 800°C, the changes inChrcmel predominate, while at 1000 C, the changesin the Alumel element cause a major fraction ofthe change in the emf output of thesethermocouples.

The results of our investigation on smalldiameter (0.5 mm/0.020 in.) sheathedthermocouples have shown much more rapid andextensive decalibrations. This is mainly due todifferences in the sizes of the thermoelements(Burley used 3.3 mm dUmeter wires) and our useof sheathed thermocouples. The presence of isheath has been shown to contribute to thedecalibration both in being a source ofimpurities which contaminate the thermoelementsand in limiting the supply fo oxygen needed topassivate the surface of the thermoelement wires(See Figure 8).

One of the features of the Chrome'-Alumelthermocouple which has lead to its wide use inits excellent resistance to oxidation at hightemperatures in air. The partial pressure ofoxygen inside the sealed sheath is reduc -substantially when oxidation of the wires andsheath occurs at high temperatures. This beingthe case, the protective film of oxides an thesurfaces of the thermocouple wires cannot beformed and high temperature decalibrationsproceed rapidly, particularly in the small wiresizes.

The sorption of H20 due to hygroscopic tendenciesof MgO widely used as an insulatant in sheathedthermocouples can both lower the insulationresistance and provide a source of water

50 h period equivalent to about +1 C. similareffects might be expected to occur in largersizes of sheathed thermocouples, but over a muchlonger time, because of the larger wire sizes.

Decalibration of Noble-Metal Thermocouples - Thenoble-metal therniocouple-sheafi combinationsl isted in Table 3 were calibrated ô 1370 C. Onethermocouple of each type was cu at positionsselected to y ie ld samples which had receiveddif ferent, maximum temperature exposures. Thesamples were analyzed by an ion microprobe massanalyzer.

The 1MMA yielded a. host of data which has notbeen analyzed completely. Several facts areobvious, however, For example, noble-metalthermocouples and base metal sheaths areincompatible. As with the Type K materials, the"as-received" materials showed contamination from

- t h e sheath resu l t ing from the manufacturingprocess. A l , Kg, Cr, Ni , Hn, and Fe were foundin small quantities in the section which had notbeen heated in the calibration experiments. Theresults were in general similar to those whichhave been reported f o r a largec-diameter,Inconel-sheathed, Type S thermocouple .

As seen in Figure 11 the 90%Pt-102Rh sheathedType S thermocouples showed substantially lessdecal ibrat ion than any other type ofthermocouple-sheath combination, and the d r i f trote at 1305 C was approximately 1 nX/min. Thethermocouples showed deviations from the N8Sreference table;, for Type S thermocouples whichwere essent ia l ly the same as those of highqual i ty, "nominal", bare-wire, Type S standards.

In contrast, the decalibration of the stainlesssteel sheathed Type 5 thermocouple shown inFigure 12 was rauch more severe. Aboveapproximately 800-900 C, the reproducibility waspoor, and the total decalibration after heatingto icoG^C amounted to more than 20 C af tercooling tu approximately SCOT..

5. Extension Lead Wire Errsrs

In precision thermocouple thermometry, thethermocouples wires are brought out directly to areference junction whose temperature is preciselyknown and/or cont ro l led . That i s , thethermocouple wires extend from the measuringjunction to the reference junction in continuous,unbroken lengths without the intervent ion ofextension lead wire or connectors. On a largescale such practice is impractical because; (a)connections to the data acquisition system orprocass controller must be simple to fac i l i ta tethe exchange of equipment or thermocouples, (b)the lengths of the small diameter thermocouplesmust be minimized to reduce the total electricalresistance of the thermocouple c i rcu i ts ; (c) i fplatinum-10% rhodiuin versus platinumthermocouples are employed, long lengths of thesematerials for extension leads would beproh ib i t i ve l y expensive. For these reasons,extension leads are made of al loys which

.approximately match the thermoelectric propertiesof the materials from 0 to 200°C. The required

match of the thermoelectric properties of theextension lead wire materials to the standardthermocouple materials is given in ANSIHC96.1-1975S (see Table 4) in terns of themaximum allowable resultant error in the measuredtemperature; e.g. , Type K (Chrome! vs. Alumel) asa +2.2°C (0 to 200°C) maximum error end for TypeS ^ 0 % platinum-10% rhodium vs. platinum) as a±6.7 C (0 to 200 C) maximum error.

He have measured the deviations from 0 to 14O°Cof a random sampling of Typo S extension wireobtained from the ORNL Stores, from ourLaboratory, and from the !£C Field Shop. Theresul ts are shown in Figure 13. Thesemeasurements clearly show that the errors due toextension wire may be s ign i f i can t but alsocalibration of the extension wire can reducethose uncertainties to the order of +0.1 C orless. This is also true for Type K extensionHires; calibration can reduce the measurementuncertainty to a few tenths of a degree Celsius.

6. Reference Junction Errors

The output of a homogeneous thermocouple, whichfree from the other errors in Table 1, isdetermined solely by the difference intemperature between the measuring junction attemperature T and the reference junction attemperature T . An error in the referencejunction temperature will therefore, directlyresult in in equivalent error in the measuredtemperature. For large numbers of thermocouples,the use of a zone box to establish the referencetemperature, T , is standard practice* Theseboxes normally are thermostated at 65 C. Thistemperature should be monitored during anexperiment by a thermemeter, such as a resistancethermometer, v.hich cioes not require a referencejunciton. The uncertainty due to a lack ofuniformity of temperature within the zone box isabout +O.2°C.

7. D.ii.ri nccuisition Errors

Data acquisition errors or errors due to themeasuring instruments, have to be analyzed forthe particular instal lat ion involved. The errorsin measuring the thermal enf of a thermocouplevary from a few mil l idegrees when high-qualitypotentiometers are used, to tens of degrees whenhigh speed data loggers are employed. Because ofthe large number of data acquisition systemsavai lab le, these errors have not beencategorized. In general, data acqu is i t ionsystems designed for steady s ta te erafmeasurements, such as potentiometers, ars moreaccurate than data acquis i t ion systems fo rtransient measurements of emf. To paraphrase theHeisenberg uncertainty principle, the product ofthe speed of data acquisition and the accuracy ofdata acquisition is approximately constant.

Table 5 is a comparison of the temperaturemeasurement errors of a computer-operated dataacquisi t ion system and a high qua l i t ypotentiometer, both cofrcnercial u n i t s . Thecomputer system can record 20,000 thermocoupleoutput readings per second with an uncertainty of

+0.252 of f u l l scale (either 10, 20, 40, 80, 160. . . . mV). With a potentiometer, an experiencedope.-ator can read one thermocouple output inabout 1 min. with an uncertainty of +^0.01%, +0.C1 V). The computer system errors l isted inTable 5 are a factor of 20 to 40 greater thanthose of the potentiometer for a Type Kthermocouple and a factor of 30 to 300 greaterfor agType S thermocouple. Thus to gain a factorof* 10 in data acquisition speed,, a factor of atleast 20 must ba sacr i f i ced in temperaturemeasurement uncertainty, which is a jus t i f iab lecompromise for many applications-

In Table 5 the potent icnietric errors are less forthe Type S than for the Type K thermocouple, andthe computer system errors fo r the Type Sthermocouple are either equal to or less thanthose for the Type K thermocouple between 435 and1035 C. These data demonstrate the fallacy ofthe often stated super ior i ty of a Type Kthermocouple to a Type S thermocouple because theoutput of the Type K is four times greater thanthe output of the Type S. This is no longer truewith present instrumentation-One data acquisition error that is d i f f i cu l t toassess is due to nontherraal emfs, often called"noise" or "pick-up", which are either induced onthe thermoelements or added to the thermal emf bye lec t r i ca l leakage of the thermocoupleinsulation. Pick-up errors are coirmon in hightemperature thermocouple thermometry and maydistort the thermocouple signal so as to renderi t useless. Errors d'je to the DC component ofthe emf pick-up are very d i f f i cu l t to determine;.sometimes these errors can be elirainated' byturning o f f a l l e lec t r i ca l power tha t mightcontribute to DC pick-up while the thermocoupleemf is measured.

The AC component of the pick-up can be f i l te redout fo r steidy s t i t a and sorae t ransienttemperature :»easureir.ents. However, i f thetheraal emf changes rapidly, f i l te r ing, may causethe measured eaf to lag the true thermal emf ofthe thermocouple, resu l t ing in temperatureir.easunr.ent errors. For example, a single-polef i l t e r with a ro l l -o f f of 3 dB at 2 Hz in serieswith the thermocouple would cause thethermocouple output to lag -8 C while thethermocouple hot junction experienced a 100 C/sectemperature r i s e . This f i l t e r would alsoattenuate a 3-mV, peak-to-peak, 60-Hz induced, ACemf by a factor of 30. The result would be thatthe AC pick-up error would be reduced from +38°C,unf i l tered, to +9.3°C, f i l te red (j;1.3DC Tor aType K thannoccup~le due to AC emf passed throughthe f i l t e r and 3.0°C due to lag).

Numerical methods have been developed forcorrecting the transient emf of a thermocouplethat is distorted by f i l t e r i n g , by i t s timeresponse, or by thermal shunting. This techniqueis known as "deconvclution" of the distortedsignal to the true undistorted s igna l , andrequires a mathematical model of the system. Arcote direct technique, called "digi ta l signalaveraging", requires several repeatedmeasurements. Superposition and addition of the

repeated measurements allows the unwanted noiseto be averaged out. Data smoothing is alsoeffective in reducing errors due to noise orpick-up.

8. Errors Caused by Magnetic Fields

Recently, during at the start-up of a large scaleengineering experiment on heat transfer at ORKL,the Type K thermocouples were found ,to be inerror by as much as 1S05S at 100°C . Thebehavior shown in Figure 14 is typical of thatobserved in the heater sneath thermocouples. Theindicated temperatures were in error in bothpositive and negative directions when DC currentwas run through the electric heaters. When thecurrent was turned of f , however, the indicatedtemperature almost instant ly decayed to thecorrect value.

After considerable study, these large errors weredetermined to be due to the Ettingshausen-Nernst(EN) effect, which is the themiomagnetic analogof the Hall effect. The conditions necessary forthe EN effect are shown in Figure 15. I f aEagnetic f i e l d , B, is normal u> a temperaturegradient, VTj and the plane formed by these twovectors i s perpendicular to the axis of aconductor, then an emf is generated along theaxis of the conductor given by

where is Q is the EH coefficient and the lineintergral is taken along the length, 1 of thewire. The value of Q, the EH coeff icient, is aproperty of the material of the wire, and frompublished measurements on Chrcmel and Alurael, onewould predict an effect about 500 times smallerthan that observed.

An exploded view of the electric heaters used inthe fi:?at transfer experiment is shown in Figure16. At fu) l power, about 120 KW is generated inthe heater by a DC current of 550 A. Each of -19heaters in the experiment is about 1 cm indiameter and 4 m long with a 3 ra heated section.Small diameter Type K V "rmocouplas were imbeddedboth in the center of the heater as well asswaged in slots in the heater cladding. In thelower right hand corner the vector diagram ofFigure 15 is shown for reference.

Since the EN coef f ic ient is rclatet! to themagnetic propert ies of the mater ia l ,ferromagnetic materials such as Alumel havelarger values of Q then no '.-ferromagneticmaterials. Based on th is information, i t waspostulated that i f the Alumel was heated abovei t s Curie temperature (approximately 152 C), theerrors should be greatly reduced. This wasconfirmed by experiment, as shown in Figure 17.Other experimental data, confirmed that theerrors were caused by the Ettingshausen-fiernsteffect: the thermocouples located in the centerof the heater rods were unaffected by the neatercurrent, since they are in a f ie ld free region atthe center of the heater; a laboratory experiment(Figure 18) in which the temperature gradient v.as

?enerated by a bunsen burner and the magneticield by permanent magrets. produced changes inthe indicated temperature (Figure 19) similar tothose in Figure 17; and f ina l ly tha error inindicated temperature could be reversed in thebunien burner experiment by rotating the magnetsISO0.

To be sure, the errors found in this axperimentwere the resu l t of the par t i cu la r geometryadopted for the heaters and would not be found in.-nost experimental situations. Finally, since theerrors were shown to be negligible above about150 C, th is particular heat transfer experimentwas not affected, since the lowest temperaturecalled for in the experimental program was 300 C.

Having considered the sources of errors l isted inTable 1 , we are now in a position to estimate thebounds of the cumulative effects of these errorson" thermocouple thermcmetry. As an example, forone large scale engineering project at ORNL, thedesired temperature measurement uncertainty wasspecified as +8°C below 800°C and +.15°C above800°C / Sfnce many of these errors aretemperature dependent, they cannot easi ly bepresented in tabular form but are best visualizedgraphically. Figures 20 and 21 present theresults for Type K and Type S thermocouplesrespectively.

The cumulative e f fec t of the errors due toextension lead wires, the reference zone box andthe calibration of a single Type. K. lnconelsheathed thermocouple are shown as +2 C in themiddle of Figure 20. In practice, calibration ofeach thermocouple (100's) i s usually notpract ical , or in roost cases, even desirable. Forthis reason, thermocouples are normally purchasedunder specification which include a specificationof either "standard" qrade (3/4S) or "special"grade (2/8%). More spec i f i ca l l y the ISAtolerances for these grades are: 3 / « , +2.2°C 0to 277"'C and 0.752. of T from 277 to 12SITC; Z/S%,+2.2CC 0 to 277GC and 0.41 of T from 277 toi£5G°C. Figure 20 shows the result of adding theISA allowable tolerance for special grade (3/3%)Type K thermocouple materials to the cumulativeerror plot.

The ISA tolerance is normally considered a"batch" tolerance. That is the deviation of aparticular batch of thermocouple materials shouldbe within this tolerance with reference to theN3S thermocouple reference tables during theirheating cyc le. The var ia t ion betweenthermocouples made from a particular batch ofmaterials might reasonably be expected to beconsiderably smaller. Experience in KRDL and inthe Instrumentation and Cjntrols Field Shops withca l ibrat ions of larger diameter Type Kthermocouple assemblies (1 to 3 mm dia), showsthat the variations within a particular batch ofType K thermocouple assemblies is normally +5 Cor less at 1000 C. For reasons previously citedconcerning the greater var iabi l i ty of 0.5 mmdiameter thermocouple materials, the variationsobserved within single batches of 0.5 nin diameter

thermocouple assemblies is larger. To theseerrors is t.'ien added the contribution due touncertaint ies in the measuring system. Thethennostated, active f i l t e r s of the measuringsystem introduce an error estimated to be +1 C.

The data acquis i t ion system for the pro jec tcontains a high speed (20 kHz) analog-to-digital(A/D) converter fo r the measurement ofthermocouple emf's during fast t rans ien texperiments. This A/D converter has anuncertainty of 0.25% of range for ranges of 10,20, 40, and 80 mV. The steps shown in theCumulative error plot result from the changes inrange when the thermocouple output reaches 10 mVat 27O°C, 20 mV at 485°C and 40 mV at 867°C. -

Shown i n Figure " is a s im i la r p lo t ofcumulative errors for a Type S thermocouple. Theuncertaint ies due to extensions lead wires(calibrated), the reference box, and cal ibrat ioncontr ibute a t o ta l of t l .6°C to 1000°C andincreases to +2 C between 1000°C to 1400°C, dueto an increase in uncertainty in tha cal ibrat ion.

Adding the ISA tolerance for standard grade TypeS thermocouples (+_1.5°C or +0.255, whichever isgreater) resul t in the l ines marked +1/4%.Because of the lowr.r output of the type Sthermocouple, the A/D converter can remain on i t smost sensitive range t.o approximately 1035 C andconsequently tho resultant uncertainty inTemperature is equivalent to _+2.4 C between 350and 1035 C. I t is !;»lieved that even with thelower output of the Type S thermocouple, theaddition.-1 uncertainty in the DAS due to d r i f t inthe active f i l t e r amplifiers can be kept to +1 C.The increase in uncertainty below approximately300 C in Figure 21 is caused by the decrease insensit ivity of the Type S thermocouple, but th istemperature region is outside the range oftemperatures contemplated in these experimentsand i s , therefore, irrelevant to this discussion.

l i should be re-ctnphasized that the higher outputof. the Type K thermocouple over that of the TypeS, does not resul t in an improvement intemperature Beasuremsnt accuracy in a regionpar t i cu la r ly above 485 C, because the higheroutput of the Type K thermocouple in th is regionis offset by the greater uncertainties in the A/Dwhen i t is switched automatically from the 20 mVrange to the 40 mV range.

The uncertainties due to decal ibration cust beadded to the above uncer ta in t ies, which areinherent to the measuring system. Kith Type V.thermocouples, there is an added uncertaintybecause of the order-disorder transformation.This is indicated by the shaded area bordered bythe l i ne K d in Figure 20- Since theorder-disorder"error is always posit ive, the Kl ine only occurs in the upper half of the plot orcumulative uncer ta in t ies. n e compositionaldecalibration errors are indicated by K,,. Thecompositional changes w i l l a f fect thethermocouples only a f ter exposures totemperatures above approximately 800 C, however,so that uncertainties to the extent indicated byK,[ in th is diagram w i l l occur only a f t e r

approximately EC h at 1100 C. Since the hightemperature parts of the experimental programw i l l come only at the end of each tes t ,uncertainties of this magnitude w i l l not occurduring the major portion of the tests. In thiscase the error l imi t in the lower half of theplot w i l l be given by -DAS.

For Type S thermocouples, there i s roorder-disorder transformation in either Pt or

. 90£Pt-10SRh alloys, thus there is no additionaluncertainty over that of the (+) DAS curve in theuppe.l-ia/b,p.8, published by Instrument Society ofAmerica, Pittsburgh, PA, 1976.

10. Kol l ie , T. G., McElroy, D. L., and Brooks,C. R., "Convolute Method of Sraoothing orCalculating the Time Derivatives of a SignalRecorded i n D ig i ta l Format Equal TimeIntervals, ORHL-TM-2517. Apr i l , 1969.

11. Kol l ie , T. G. Anderson, R. L., Horton, J .L., and Roberts, H. J . , "Large thermocouplethermometry errors caused by magneticf ie lds" . Rev. Sci. Ins t . , 43. 501, (1977).

12. Hopkins, H. C , J r . , "Program Plan for GCFRCore Flew Test Loop", GA-AI3080. GeneralAtomic Company, San Diego, CA, 1974.

ORNL-DWG 78-7641

SEVEN SOURCES OF UNCERTAINTIES IN THERMOCOUPLETHERMOMETRY WHICH MUST BE CONSIDERED ARE:

1. THERMAL SHUNTING2. ELECTRICAL SHUNTING AND LEAKAGE3. CALIBRATION ERRORS4. DECALIBRATION ERRORS5. EXTENSION LEAD WIRE ERRORS6. REFERENCE JUNCTION ERRORS7. MEASUREMENT SYSTEM ERRORS

K)

(THE SEVEN DEADLY SINS)

Table 1

ORNL-DWG 78-7638

ERRORS IN CALIBRATION ! A Y BE CAUSED BYUNCERTAINTIES III:

1. REFERENCE TEMPERATURES2. THERMAL SHUNTING . .'3. MEASUREMENT OF EMF4. THE EH1F ¥8 TEMPERATURE TABLE OR5. INHOMOGEHEITiES OR QMPOSITION6. THE ̂ ETALLUIIOICAL STATE

ETC.)

Table 2

23

ICO

o

O

SS

CO

" o o

CD

t i . - - s»

d)

EH

LaJrz

-5 *^O^ L'-J

SH" T~ CO 2C

TABLE 4

TOLERANCES FOR EXTENSION WIRESa

ThermocoupleType

EJKTR,SB

aSee Reference 9.

ExtensionType

EXJXKXTXSXBX

Typ ica lAl 1oys

Ni -Cr /Cons tan tanFe/ConstantanNi-Cr/Ni AlloyCu/ConstantanCu/Cu-Ni Alloy BCu-Mn Alloy/Cu

TemperatureRange(°C)

0 to 2040 to 2040 to 204-59 to 930 to 2040 to 121

Tolerance

Standard Special

±1.7±2.2 ±1.1±2.2 ±1.1±0.8 ±0.4±6.7 . --±33

00

TABLE 5

ERRORS IN TEMPERATURE MEASUREMENT OF TYPE K AND TYPE S THERMOCOUPLES DUE

TO DATA ACQUISITION

TemperatureRange(°C)

0-250250-485485-965965-10351035-13721372-1768

EMFrange

Type K

1020408080__

(mV)

Type S

101010102020

Computererror

Type K

±0.6±1.2+2.4±5.0±5.0

Systema

(°C)

Type S

±3.2±2.7±2.4±2.2±4.2±4.2

Pot. systemerror

Type K

±0.02±0.03±0.07±0.11±0.12

A T

ORNL-DWG 78-7637

THERMAL SHUHTING RESULTS WHEftA THERMOCOUPLEIS ATTACHED TO AM OBJECT.

CONVECTION

UDUCTION

DIATION

DISTANCE X

WITH POOR THERMAL CONTACT, THE THERMOCOUPLE JUNCTIONWILL NOT ATTAIN THE TEMPERATURE OF THE OBJECT

Figure 1

ORNL-DWG 78-7642

BUT EVEN WITH GOOD THERMAL CONTACT, THEHEAT CONDUCTED AWAY BY THE THERMOCOUPLEWIRES LOWERS THE TEMPERATURE OF THEOBJECT LOCALLY (THE FIH EFFECT).

THERMAL SHUNTING IS AFFECTED BY THE THERMOCOUPLE WIRESIZE,THE THERMAL CONDUCTIVITIES OF THE OBJECT AND THETHERMOCOUPLE WIRES, THE TEMPERATURE OF THESURROUNDING MEDIUM AND THE HEAT TRANSFER COEFFICIENT OFTHE MEDIUM.

Figure 2

(1)

ORNL-DWG 77-14504

AT HIGH TEMPERATURES ELECTRICALCONDUCTION BETWEEN THE WIRES AND

BETWEEN THE WIRES AND SHEATH CAN CAUSE;

1. SHUNTING OF THE THERMOCOUPLESIGNAL (-ERRORS)

2. CREATION OF VIRTUAL JUNCTIONS(+ERRORS)

3. ALLOW ELECTRICAL CURRENTS ON THESHEATH TO LEAK INTO THE THERMO-COUPLE CIRCUITS (± ERRORS)

(2) (3)

Figure 3

tUJ

orUJa.

ORNL-DWG 73-7645

BOTH SHUTTING AND VIRTUAL JUNCTION ERRORS COMBINE AS ATHERMOCOUPLE IS yOVEdTHROUGH A HIGH-TEMPERATURE PROFILE:

-TRUE TEMPERATURE

^TEMPERATUREINDICATED BY

HERMOCOUPLE

DEPTH OF THERMOCOUPLE INSERTION

Figure 4

ORNL-DWG 77-14505

ERRORS CAUSED BY ELECTRICAL SHUNTINGAND ELECTRICAL LEAKAGE CAN BECORRECTED AND/OR INTERPRETED BY USINGAN ANALYTICAL MODEL DEVELOPED FORCFTL SMALL DIAMETER THERMOCOUPLES.

HO

Figure 5

ORNL-DWG 75-15494

80

> 60

Q_lO

iLJ

40

20

TYPE-S

J

jNBs/

/ (

f liBSl*

A=1 °C

0 400 800 1200TEMPERATURE (°C)

1600 2000

Figure 6. Differences between newly defined "exact" Type Sthermocouple defined by NBS Monograph 125 and the older"nominal" Type S thermocouples defined by NBS Circular 561and the British Standards Institute Tables: B. S. 1825:1952.

ORNL-DWG 78-7643

THE ORDER-DISORDER TRANSFORMATION \U CHROMELUU CAUSE UP TO 6-7°C ERROR WITHTYPE K THERMOCOUPLES AT 600°C

B AFTER 27 HR AT

6 -

ooS 4O

cm

A I BWITHDRAWN

5

A FIRST H E A T /

200 800TEMPERATURE °C

Figure 7

ORNL-DWG 78-7644

LARGE CHANGES WERE OBSERVED IN A STAINLESSSTEEL SHEATHED TYPE K THERMOCOUPLE DURINGA 50h EXPOSUIlE AT ?!50°G.

oo

CHROMELvs ALUMEL

I-13.5°C

ALUMECHROMEL

30 hr

too

Figure 9

ORNL-DWG 77-14509

CHANGESIMCONEL- A

TYPE KSMALLER

0 TIME (hr) 50

toI-

Figure 10

ORNL-DWG 77-14507

THE DRIFT OF THERMOCOUPLES IS STRQIMGLYDEPENDENT ON THE SHEATH MATERIAL:

TIME

TYPE S IN Pt-Rh

TYPE SIN INCONEL

TYPES IN S.S.

I I I

20 minFigure 11

ORNL-DWG 7 8 - 3 9 4 4

Oo

V)CD

o

- 2 0

- 3 0

- 4 0

- 5 0

O - 6 0

>o -70

- 8 0

- 9 0

-100

SEE ENLARGED SECTION

0

/~1hr

I I I I I I I I I300 600 900

TEMPERATURE (°C)

1200 1500

Figure 12. Data from the calibration of a Type 304 stainlesssteel sheathed,, 0.5 mm diameter, Type S thermocouple shows thesevere decalibration observed above 900 deg. C.

ORNL-DWG 78-3930

60

4 0

20

x x-

ISASTANDARDTOLERANCE

FOR SX WIRE

20

40 I—

60

x HIGH GROUP (6 OUT OF TEN)o LOW GROUP (4 OUT OF 10}

THREE SAMPLES MEASURED IN 1970

20 40 60 80 100 120 . 140 160 180TEMPERATURE (°C)

Figure 13. Experimental date from the calibration of varioussamples of. Type S extension lead wire.

ORNL-DWG 76-15493

A vr

CONDUCTOR

Figure 14, Vector relationship between the temperaturegradient, n , the magnetic induction, B, and the electricfield, E, of the tttingshausen-Nernst effect,

ORNL-DWG 76-15496

CENTRALTHERMOCOUPLE

HEATING ELEMENT

MgO INSULATION

HEATER SHEATHTHERMOCOUPLES

DOUBLE-CLADSTAINLESS STEELSHEATH

BN INSULATION

-VT

B

Figure 15. Heater used as a substitute for nuclear fuelrod in nuclear reactor simulation.

<

i 5 0

100

50

n

;

/

620

1

400-"

/30

1

1-Q50

\Q60

\ \

1

1

u

0 ~~—o_-_40 90^ 9 0

30o

ORNL-DWG

1

4,° 90o°50 |

76-19198

—

—

—

1200

50 100 150 200 250CALCULATED TEMPERATURE (°C)

300

Figure 16. Differences (AT) between the calculatedtemperature and the indicated temperature of one heater-sheath thermocouple of the 49-heater simulator versusthe calculated temperature (numbers at each point referto heater power in kilowatts).

ORNL-DWG 76-15498220

190

oo

LU

3

or

uj

160

100

70

CURRENT OFF

-THERMOCOUPLE 1

THERMOCOUPLE 2

4 6

TIME (sec)

8 10

Figure 17. Indicated temperature of two heater-sheaththermocouples as a funciton of time, showing rapid positiveand negative changes of indicated temperature at heater-current turnoff.

PERMANENTMAGNET

ORNL-OWG 76-15501

MICA INSULATION USEDBETWEEN WIRES,

ALUMINUMPLATE 0.5 mm DtAM. STAINLESS STEEL

SHEATH, CHROMEL vs ALUMEL THERMOCOUPLE

BUNSEN BURNER COPPER vs CONSTANTANINTRINSIC THERMOCOUPLE

(e)

Figure 18. Experimental apparatus used in Bunsen burner tests:(a) apparatus, (b) expanded view of thermocouple placement,and (c) further expanded view of intrinsic thermocouples weldedto the top and bottom of sheathed Chromel/Alumel thermocouplesto measure A T ,

ERROR PER UNIT TEMPERATURE DIFFERENCE

o

ORNL-OWG 78-3665

Cumulative Uncertainties In CFTL FRSthermocouples using Typa Kthermocouplas In Inconel shaatht

+16

+H

+12

+10

+8

+B

+4

uoCOui

-6

-8

-10

-12

-14

-16

-18

-20

-22

-24

L o-d

3/8%

Extension laads, Reference junctions, and Calibrations

r 3/8%

1370

II.

200 400 600 800 1000 1200 1400

TEMPERATURE (°C)

Figure 20.

ORNL-DWG 78-3662

Cumulitlve Uncertainties in CFTL FRS thermocoupletemperature meisurement* using a Type S thermocouplein a platlnum-10% rhodium theath

+16

+14

+12

+10

+8

+B

oo

Ui

o-*

- 6

- 8

-10

-12

-14

-16

Extension leads. Reference Junctions and Calibrations

1/4%

CFTL Limit of Error

I I I

1370

200 400 600 BOO 1000TEMPERATURE (°C)

1200 1400

Figure 21.