eoR isosi ISSN 1018-5593
Commission of the European Communities bcr information APPLIED METROLOGY
DEVELOPMENT OF NOBLE METAL THERMOCOUPLES
AN INVESTIGATION INTO THE REPRODUCIBILITY OF GOLD/PLATINUM THERMOCOUPLES
Blow-up from microfiche original
Commission of the European Communities bcr information APPLIED METROLOGY
DEVELOPMENT OF NOBLE METAL THERMOCOUPLES
AN INVESTIGATION INTO THE REPRODUCIBILITY OF GOLD/PLATINUM THERMOCOUPLES
M.J. DE GROOT", M.V. CHATTLE(2,, D.F. CARTER13, J.Y. LE POMMELLEC"31
(,) Nederlands Meetinstituut - Van Swinden Laboratory Postbus 654 2600 AR Delft The Netherlands
m National Physical Laboratory Queens Road, Teddington TW110LW United Kingdom
01 Instrtut National de Metrologie 292 Rue Saint Martin F-75141 Paris Cédex 03
Contract No 3235/1 /0/133/88/7-BCR-NL (30)
SYNTHESIS REPORT
Directorate-General Science, Research and Development
1994 EUR 15087 EN Published by the COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General XIII Telecommunications, Information Market and Exploitation of Research L-2920 LUXEMBOURG
LEGAL NOTICE Neither the Commission of the European Communities nor any person acting on behalf of the Commission is respons ble for the use which might be made of the following information
Catalogue number: CD-NA-15087-EN-C
© ECSC — EEC — EAEC, Brussels • Luxembourg, 1994 Ill
ABSTRACT
In an exercise in which three laboratories participated, six gold platinum thermocouples were evaluated in respect of their repeatabilities at the freezing point of silver. Short-term repeatabilities determined from measurement sequences at the laboratories were generally found to be better than 0,4 jiV ^ 0,02 "C. Reproducibilities between the laboratories varied between 2 |iV ± 0,1 *C for two thermocouples and 13 \i\f £ 0,5 °C to 21,7 jiV £ 0,8 *C for the remaining four thermocouples. The immersion characteristics were also checked and it was found that the effect of withdrawing a thermocouple from the freezing point cell generally caused a change in the emf output equivalent to about 1 nV/cm ^ 0,04 *C over the first few centimetres. Some measurements made at VSL at the freezing points of aluminium and tin showed short-term repeatabilities similar to the measurements at silver: 0,2 \iV ± 0,01 °C at aluminium and 0,2 |iV £ 0,02 *C at tin. The repeatability at the tin point for measurements alternated with high temperature heat treatment of the thermocouples was 0,4 |iV £ 0,04 *C. In general the results do not agree with some previous work on these thermocouples. A more recent publication does however seem to (partly) support the results obtained in this work.
Table of contents
1 Introduction 1 1.1 Identification 1 1.2 Organisation 2
2 Thermocouple construction and heat treatment 3 2.1 Construction of the cold junction 3 2.2 Heat treatment 4 2.3 Breakages 4
3 Equipment 6 3.1 Anneal furnaces 6 3.2 Thermal-EMF measurement 6 3.3 Fixed point realisations 6
4 Measurement procedures 8 4.1 NPL 8 4.2 INM 8 4.3 VSL 8 4.4 NPL repeat measurements 9 4.5 INM repeat measurements 9
5 Results 10
6 Uncertainties 12
6.1 Category A 12 6.2 Category B 12 6.3 Total uncertainty 12
7 Discussion 13 7.1 Oxidation 13 7.2 Strain 13 7.3 Gradient effects 14 7.4 Impurities 15 7.5 Calibration characteristic 16
8 Conclusions 18
References 19
1 Introduction
The introduction of the new temperature scale ITS-90 enabled very accurate temperature measurement from 630 *C to 961 °C as a consequence of the definition of high temperature standard platinum resistance thermometers (HTSPRT) for that range. Compared with the previous temperature scale, that utilised platinum rhodium thermocouples for that purpose, the accuracy of the scale realisation improved from 0,15 °C (IPTS-68) to 0,005 *C (fTS-90) near 960 *C.
HTSPRT's are difficult to handle and are used in combination with costly measuring devices. Thermocouples, on the other hand, are more straightforward in their use and can be measured with relatively cheap equipment. Therefore a thermocouple was sought with an accuracy better than that of platinum-rhodium versus platinum type thermocouples to serve as a standard for secondary calibration laboratories.
Work by McLaren et al [VIII, IX] suggested that gold versus platinum thermocouples would perform significantly better than platinum rhodium versus platinum type thermocouples. A thermocouple made of pure gold versus pure platinum wires would be more stable, homogeneous and sensitive. Platinum has a negative Seebeck coefficient, like most metals. The Seebeck coefficient of gold is positive, as are copper, lithium and silver. A gold platinum thermocouple adds the thermopowers that are generated along each wire, while in platinum rhodium type thermocouples the separate thermopowers are subtracted. As a result the combined Seebeck coefficient of a gold versus platinum thermocouple is larger and the thermocouple is more sensitive than for platinum rhodium type thermocouples. Accuracies were reported of the order of 0,01 °C from 0 °C to 1000 °C.
This work is to verify the results obtained by McLaren et al. Three laboratories were involved in an interlaboratory comparison to measure six Pt/Au thermocouples at the silver solidification point. The aim was to compare the repeatability at each laboratory with the reproducibilities between the laboratories. NPL and INM constructed the thermocouples and evaluated the homogeneity of the thermocouples by immersion measurements. VSL included measurements at the tin and the aluminium solidification point prior to measurements at the silver point. Subsequent measurements at tin were to allow evaluation of the repeatability at those points and to study anneal effects.
1.1 Identification
Six thermocouples were to be produced by two of the participating laboratories. INM produced four thermocouples with wire from three manufacturers, NPL made two thermocouples using wire from two sources. The following table relates thermocouple identification as used in this report to the supplier of the metal. The gold and platinum wires that were used for each thermocouple, were obtained from the same supplier. ID. supplier
NPL-1 Degussa Ltd
NPL-2 Johnson Matthey Metals Ltd
INM-1 Lyon Allemand
INM-2 Lyon Allemand
INM-3 Johnson Matthey Metals Ltd
INM-4 Degussa Ltd
1 2 Organisation
NPL produced its two thermocouples, anneaed them, and measured them at the silver freezing point. The thermocouples were brought to INM where by that time the other four thermocouples had been constructed. INM annealed the thermocouples and measured them at the silver freezing point. VSL then received the six thermocouples to anneal them and to measure them at the tin, aluminium and silver freezing points, in that order. The thermocouples were then anneaed again at 1000 °C, measured at the tin point, annealed at 450 "C and again measured at the tin point to evaluate the effect of this anneal procedure. All thermocouples were brought to NPL where they were again annealed and measured at the silver freezing point. Finally the four INM-made thermocouples were transported to INM where the thermocouples were again annealed and measured at the silver freezing point. INM and NPL have studied thermal EMF-profiles as a function of the immersion depth in the silver freezing point. This allows an uncertainty estimate of the immersion effect of these thermocouples.
The evaluation of uncertainties in this report is based upon the BIPM draft recommendation (Giacomo 1981), the terms that are in concordance with the vocabulary of legal metrology (OIML, 1978). Uncertainties are expressed in terms of the standard deviation s. 2 Thermocouple construction and heat treatment
The thermocouples were transported as pure metal wires of 0,5 mm diameter, partially insulated by alumina tubes. The remaining lengths of wire were insulated by PTFE sleeving. The cold junction had to be prepared by each participating laboratory upon receipt of the thermocouples. Hot junctions had been prepared by INM (thermocouples with identification INM-1, INM-2, INM-3 and INM-4) and NPL ( NPL-1 and NPL-2). To eliminate strain due to the rather large difference in thermal expansion between the platinum and gold wires, NPL had mounted a small strain relieving coil with 4 turns of some 0,5 mm to 1 mm in diameter, made of platinum wire of 0,15 mm diameter. This platinum coil was located outside the alumina, connecting the platinum and gold wires that extended to just beyond the ends of the bores of the alumina insulation. The wires were electrically insulated by a twin bore 99,7% purity alumina tube with 0,7 mm bores. INM did not use a coil, but used a 600 mm long twin bore, 99,7% purity alumina tube with larger bore diameter of 1,5 mm, the external diameter was 6 mm. This should relieve the strain by allowing the wires to spiral inside the larger bores. The remaining lengths of wires of all thermocouples were insulated with PTFE sleeving attached to the alumina with heat-shrink sleeving.
This chapter will describe the cold junction realisation that was applied at the laboratories. Then details of the relevant equipment for this exercise are given. Some breakages that occurred during the work will be discussed at the end of this chapter.
2.1 Construction of the cold junction.
In view of the expected accuracy for this type of thermocouple, extreme care had to be taken in the construction of the cold junction.
NPL twisted the reference end of each thermocouple wire tightly together with a 0,5 mm diameter tinned copper wire. These reference junctions were then inserted in a 300 mm close- fitting glass tube. The attributed uncertainty was 1 mK. Thermocouples assembled at NPL were referenced in a triple point of water cell with an immersion of 200 mm. The INM thermocouple wire was somewhat shorter, precluding this procedure for these thermocouples. Therefore an ice point made of crushed ice was used instead.
INM verified that their construction of the ice point resulted in an uncertainty better than 10 mK at an immersion of 100 mm. The cold junctions, gold - copper and platinum - copper, were fitted in separate glass tubes. INM used an ice point made of crushed ice.
At VSL the cold junction was made by twisting the ends of the platinum and gold wires tightly around the bare ends of a Guildline cable. The tip of each junction was fastened by heating some PTFE sleeving around it. Some immersion measurements were made to evaluate the minimum immersion depth of the cold junction. Narrow (internal diameter 5 mm, external 7 mm) glass tubes of 24 cm length proved long enough to assure the temperature of the cold junctions within a few millikervins of the ice temperature. Ice points were made of melting crushed ice.
2.2 Heat treatment
The thermocouple wires received a heat treatment prior to the construction of the thermocouples.
At NPL the platinum wire was annealed electrically at 1100 °C for 20 minutes. Then the wire was slowly cooled to room temperature during 10 minutes. The gold wire was heat treated thermally in the NPL stabilisation furnace (paragraph 3.1) at 1000 °C. To expose every part of the thermocouple wires to the uniform part of the furnace, ie the middle 30 cm of the furnace, the wires were passed through the furnace in several stages. Each stage took some 30 minutes.
INM thermally heat treated the wires at 1000 *C during 30 minutes.
After the construction of the thermocouples the alumina insulated parts were annealed thermally again to eliminate cold work effects.
NPL thermally annealed their mounted thermocouples at 1000 *C in the centre, homogeneous zone of the NPL stabilisation furnace. Each half of the alumina sheathed part of each thermocouple was left for 30 minutes in this zone. Then the PTFE sleeving was fitted. At INM the alumina insulated parts of the thermocouples received a second thermal heat treatment during 30 minutes at 1000 *C.
Upon receipt of the thermocouples the participating laboratories thermally heat treated the thermocouples prior to the measurements: NPL and INM used the same thermal heat treatment as already discussed; VSL heat treated the thermocouples for 30 minutes at 1000 *C and for three hours at 450 *C. The thermocouples were put in the furnace as deep as the PTFE sleeving would allow. NPL partially removed the PTFE sleeving to allow exposure of the entire alumina part of the thermocouples to the uniform zone of the furnace.
2 3 Breakages
On the receipt at INM one of the NPL thermocouples (NPL-1) had the alumina insulator broken some 525 mm from the hot junction. During the first stage at INM some heat-shrink sleeving kept the pieces together. In view of the heat treatment the thermocouples had to undergo at VSL, it was decided to remove the heat-shrink sleeving and to replace it during the high temperature treatments by some stainless steel wire to safeguard the wire from mechanical strain. During the heat treatment this part of the thermocouple would not rise in temperature above 500 *C. Any impurities that might be produced by this wire have a tendency to drift to the colder end of the thermocouple. Therefore no significant contribution to the thermal EMF was expected of the thermocouple during the fixed point measurements; either resulting from the stainless steel wire or from gold or platinum from one leg diffusing to, and thereby contaminating, the other leg. During the measurements at VSL and NPL the thermocouple was 'kept together' with heat-shrink sleeving.
The stress relieving coil of both thermocouples NPL-1 and NPL-2 had to be repaired after the high temperature anneal at VSL. A second repair of NPL-2 could not be made because the coil had become too short. It was decided that the thermocouple would continue in the measurements without the stress relieving coil. This affected the tin repeat measurements at VSL and the measurements at NPL. The cause of the breakage of the coils is likely to be related to the gradient of the VSL anneal furnace: during the high temperature anneal a relatively long part of the thermocouple is exposed to high temperature, causing an excessively large expansion of the thermocouple wires. This leads to a larger stress in the coil than what can be expected during fixed point measurements. The VSL anneal furnace had a zone of uniform temperature that extended nearer to the end of the furnace than its counterparts at the other laboratories, this probably led to the problems which occurred at VSL. 3 Equipment
3 1 Anneal furna es
The NPL anneal furnace was a 1120 mm long Carbolite furnace with a gradient better than 3 *C within 150 mm from the midpoint at 1000 *C. Beyond that range the gradient was significantly steeper.
INM used a specially built furnace of 1500 mm length. It's gradient was better then 25 #C, over the centre 1000 mm of the furnace.
VSL used a Carbolite three-zone furnace with a length of 950 mm. The furnace temperature was homogeneous to within 10 °C over the entire length of the furnace with the exception of the two 170 mm lengths from the extremes of the furnace.
3 2 Thermal EMF measurement
All uncertainties that are given in this chapter are one standard deviation.
INM measured thermal EMF with an uncertainty of 2 jiV (one standard deviation). Spurious EMF was estimated to be less then 0,1 jiV.
At NPL thermal EMF was measured using a Keithley 181 nanovoltmeter. Before and after the measurements this meter was calibrated by comparison with a potentiometer which, in its turn, had been standardized against a standard cell, traceable to the NPL 1990 reference standard of EMF. The uncertainties were 0,01 jiV for the standard cell, 0,19 |iV for the Potentiometer and 0,38 jiV for the K181. Spurious EMF was estimated to contribute 0,17 jiV to the uncertainty.
VSL also measured thermal EMF with a Keithley 181 nanovolt meter. The meter was calibrated every day with an uncertainty of 0,25 \iW by comparison with standard cells. These cells are zener reference eels caibrated to 0.5-10"6 by VSL Electrical Standards department every year and are therefore traceable to the VSL 1990 reference standard of EMF. Linearity checks of the meter had been carried out by VSL's Electrical Standards department.
3 3 Fixed point real sations
INM uses silver cells of the dosed type. During the assembly of the cells at INM the cells are purged with pure argon gas. The pressure is adjusted to 105 Pa when the cell is in its furnace at its freezing temperature. Then the cell is closed. The immersion depth of the thermocouples in this cell is 180 mm below the top of the ingot and some 480 mm below the top of the furnace. The metal purity was 5N. The furnaces that are used to realise the silver point are three zone furnaces and sodium heat pipe furnaces.
NPL made preliminary silver point measurements using the freezing point cells and furnaces usually used for calibrating noble metal thermocouples. These cells allow for an immersion of 97 mm in the metal and a total immersion of the thermocouple of 370 mm. The high-purity graphite crucibles containing the 6N purity metal are fitted in an alumina tube, closed at one end, and covered with alumina powder. Another alumina tube guides the thermometer in the graphite thermometer well inside the metal.
Further freezing points of silver were measured at NPL in the cells that were also used for the HTSPRT calibrations. Immersion depths of the thermocouples in these cells were at least 430 mm from the top of the furnace and at least 160 mm below the top of the ingot. The metal purity was 6N. The gas in the cell is purged using high purity argon preceding heating. The NPL furnaces are equipped with a sodium heat pipe for the higher temperature fixed points.
The solidification points of tin, aluminium and silver were performed, in this order, at VSL in the cells that were also used for the HTSPRT calibrations. Immersion depths of the thermocouples in these cells were 430 mm from the top of the furnace and 180 mm below the top of the ingot for the tin cells. These dimensions are 540 mm and 230 mm respectively for our aluminium and silver cells. The metal purity of the ingot was 6N for each cell. The gas in the cell is purged three times a week using high purity argon.
The furnaces at VSL are: a three zone furnace equipped with an equalizing aluminium block for tin; and a single zone heater around a closed (temperature controlled) sodium heat pipe for the aluminium and silver fixed points. 4 Measurement procedures
4 1 NPL
At NPL the thermocouples NPL-1 and NPL-2 were made and heat treated in order to assure a freshly annealed state. In the remainder of this report this state shall be referred to as the 'basic' state. Three separate realisations of the freezing point of silver were then performed in the cells with small ingots. Another three separate realisations were made in the cells with the large ingots. Measurements were made after the thermocouple output was stable enough at 1 minute intervals. Reversal of the connections of the thermocouples served to reduce stray EMFs. An immersion characteristic of the thermocouples was obtained by raising the thermocouples, in stages, by between 10 mm and 60 mm.
4 2INM
The thermocouples INM-1, INM-2, INM-3 and INM-4 were constructed and given their thermal anneal together with thermocouples NPL-1 and NPL-2. This ascertained that all thermocouples were in their basic state prior to the measurements. Four separate realisations of the freezing point of silver were performed for each thermocouple. The first plateau was used to assess the short time repeatability of the thermocouples by making four measurements of the thermocouples at the same plateau. Between two such measurements the thermocouples were quenched to room temperature. During the remaining three plateaus the thermocouples were monitored to allow averaging of the thermal emf over the entire plateau. Immersion characteristics were obtained of all thermocouples by raising the thermocouples 100 mm in total by 25 mm steps. Comparison of the thermal EMFs of the gold versus platinum thermocouples with those of a type S thermocouple allowed an estimate of the Seebeck coefficient near 960 "C.
4 3VSL
All thermocouples were given a thermal heat treatment to return them to their basic state again. Then the six thermocouples were measured at three separate realisations of the tin freezing point, again three realisations of the aluminium freezing point and three realisations of the silver freezing point. Reversal of the connections of the thermocouples during the measurements served to reduce stray emfs. To evaluate the effect of the anneal procedures that were used, another thermal anneal at 1000 "C was given to the thermocouples after which another three measurements were made at the tin freezing point. The influence of the 450 °C vacancy anneal was measured by another three measurements at tin. 4.4 NPL repeat measurements
These repeat measurements were again preceded by a thermal heat treatment of ail six thermocouples to obtain the basic state. The thermocouples were measured during three realisations of the silver freezing points with the large ingots. Again immersion measurements were made.
4.5 INM repeat measurements
INM received its four thermocouples back and annealed them again to return them to the basic state. Measurements were made at the freezing point of silver and included immersion measurements to evaluate the homogeneity of the thermocouples. 10
5 Results
In the following table the results of the measurements are summarized. The average thermopowers for the plateau measurements are printed with one standard deviation (1 -s) of the measurements in italics to ind cate the repeatabilities.
NPL-1 NPL-2 INM-1 INM-2 INM-3 INM-4
NPL short ingot 16068,7 16063,5 - - - - 0,4 0.8
(iV long ingot 16078,7 16074,5 - - - - 0,8 0,3
INM short time 16083,4 16074,3 16111,3 16110,3 16088,3 16100,4 0.3 0.4 0,76 0,09 0,2 0,4
nv long time 16083,3 16074,3 16111,6 16110,3 16088,5 16100,3 0,4 0.5 0.15 0,10 0,3 0,3
seebeck coeff. 25,9 25,8 26,2 26,4 25,8 26
VSL tin 2224,7 2223,8 2233,5 2233,4 2229,8 2234,5 0.01 0,03 0,1 0,1 0,015 0,3 nv aluminium 9286,8 9285.0 9311,8 9311,8 9302,9 9312,2 0.13 0.1 0,03 0,03 0,76 0,08
silver 16076,2 16072,4 16109,3 16109,3 16097,3 16111,0 0.8 0,16 0,3 0,1 0,04 0,36 tin after 1000 °C 2224,7 2222,9 2233,4 2233,4 2229,8 2234,0 0,02 0,16 0,03 0,07 0,01 0,1
tin after 450 "C 2224,6 2222,6 2233,3 2233,3 2229,8 2233,9 0.2 0,08 0,04 0,05 0,03 0,07
NPtepeat measurements 16079,3 16075,9 16084,0 16086,9 16069,4 16064,6 03 0.4 0,2 0.17 0,06 0,44
INM repeat - - 16089,2 16089,8 16057,5 16074,5 measurements 0.7 0,4 0,6 0,5
All units are jiV, except for the seebeck coefficients measured by INM, which are given in (iV/°C.
The results at silver are compared graphically in Figure 1. Figure 2 shows the repeatability of the tin measurements at VSL. Immersion measurements at NPL were only made in the long ingot: an attempt to measure immersion in the short ingot was halted because a 20 mm immersion change resulted in an thermopower change of some 4 jiV. The immersion 11
characteristics are plotted for each thermocouple separately in Figure 3 to Figure 8. In Figure 9 a comparison of the VSL measurements is made with the work of Ancsin (1991). 12
6 Uncertainties
6.1 Category A: the uncertainties that were evaluated statistically can be found in the previous chapter: they were printed in the table in italics.
6.2 Category B: All uncertainties are estimated in 2-s. INM NPL VSL
6.2.1 Keithley 181 nanovoltmeter 0,9 0,5
6.2.2 Tin point temperature. 0,03
Aluminium point temperature. 0,5
Silver point temperature. 0,02 0.03 0,25
6.2.3 Cold junction temperature.
TEMF equiva ent at tin 0,13
at aluminium 0,20
at silver 0,03 0,01 0,25
6.2.4 Spurious EMF 0,02 0,4 0.2
6.2.5 Immersion effects 0,3 0,4 1
62 6 total category B contrbution
at tin 1.14
at aluminium 1,22 at silver 1.0 1,2
6.3 Total uncertainty (2-s) for each thermocouple at the fixed points (in |iV)
NPL-1 NPL-2 INM-1 INM-2 INM-3 INM-4
NPL 1,9 1.1 - - - -
INM 4 4 4 4 4 4
VSL Sn 1.2 1.2 1.2 1,2 1.1 1.2
Al 1,2 1.2 1.2 1.3 1.2 1.2
Ag 1.2 1.5 1.2 1.2 1.2 1.3
NPL 1,2 1.3 1.1 1.1 1.0 1.3
INM . . 4 4 4 4 13
7 Discussion
In view of the high expectations of the gold versus platinum thermocouple raised by earlier work by McLaren (1987 I and 1987 II), the result of this work must be regarded as disappointing. Differences between the laboratories are generally of the order of 20 |iV (A 1 *C) for the INM made thermocouples and of the order of 2 \M (£ 0,1 °C) for those made at NPL This is not- much- better than can be obtained with S-type thermocouples for which the gold / platinum combination is supposed to be a superior alternative. An attempt is made to explain these results in terms of the physical structure of the thermocouples.
7.1 Oxidation
For platinum rhodium type thermocouples it is generally accepted that oxidation is a major cause of instability (CCT working group 2, 1990). Oxidation is not likely to have a significant influence on the performance of gold versus platinum thermocouples. This effect would be of the same order of magnitude for all thermocouples and all laboratories Gold and platinum are pure metals: For platinum rhodium thermocouples it is mainly the rhodium that oxidises, thereby effectively reducing the rhodium concentration. In gold / platinum thermocouples oxidation would lead to an effective reduction of the diameter of the thermocouple wires, which should have no influence on the Seebeck coefficient.
7.2 Strain
Mechanical strain is expected to be of less significance than the deviations which are now to be explained. McLaren (1987-1) exerted stress on the gold wires during measurements of the thermocouples in the indium freezing point and found a decrease of the TEMF of some 10 jiV. A residual increase of the TEMF of some 0,2 ^V was observed after removal of the stress inducing force. This is in agreement with an effect that was observed following the 450 °C anneal at VSL Generally the TEMF at the tin point decreased some 0,1 |iV as a result of that anneal. This is expected to be related to vacancies that were quenched in as the thermocouples were rapidly cooled down to room temperature after the 1000 °C anneal. These vacancies were removed during the 450 CC anneal. It was found that the lattice defects that were produced during the stress induction, cold work effects, have the same influence on the TEMF as quench induced vacancies. This leads us to think that strain can not be the cause of differences that are of the order of 1 °C.
NPL coils were not necessary in view of the uncertainty that was obtained. A positive effect is however clearly present when comparing the tin point stability with and without the coil. 14
7.3 Gradient effects
Another possible cause is discussed by Ancsin (1991). He performed an ageing study of several platinum rhodium type thermocouples and two gold / platinum thermocouples. Having initially annealed the thermocouples into their basic state, they were left for 200 hours in a silver cell at 960 *C to age. This gradient dependent ageing effect introduced some inhomogeneity in the thermocouple that could easily be measured by immersion measurements. Inserting the thermocouples more or less deeply in the furnace in which a silver freezing point was realised, Ancsin found the TEMF to vary some 5 |iV over 10 cm. Ancsin attributes this effect to local reordering of impurities in the thermocouple wires along the temperature gradient zone in the furnace. Changing the gradient results in a difference of some 1 |iV of the TEMF at the silver point. Ancsin associates this temperature gradient-related effect to an inhomogeneous impurity distribution. The temperature gradient, in his view, will cause some segregation of the impurities which will influence to the Seebeck coefficient along the wire.
The ingots that were used by INM, NPL and VSL had different immersion depths: INM 16 cm, NPL 18 cm and VSL 23 cm (section 3.3). This will inevitably result in different location of the maximum temperature gradient along the thermocouple. Also the thermal isolation of the furnace is of influence to this temperature grad ent.
In the following table the immersion gradients AE/fiz, that are given graphically in Figure 3 to Figure 8, have been summarised and a comparison is made with the reproducibilities dE between the laboratories. The reproducibilities dE are equal to one standard deviation of the measurements at silver that are found in Figure 1 for each thermocouple. In order to obtain some numeric representation of the gradient effect of these thermocouples, the differences between subsequent measurements were tabulated for each thermocouple immersion measurement sequence. These values were normalised to 1 cm immersion change. The immersion 6E/Az is represented below by the first number in each entry of the following table. It is the maximum gradient change observed between two subsequent measurements, the second number AE is equal to one standard deviation of the immersion change for each sequence. Thermo• dE Immersion measurements: gradient induced TEMF couple (|iV) fiE/fiz (iiV/cm) ± fiE (jiV) NPL INM NPL INM NPL-1 2,94 0,7 i t 1.6 0.7 ± 1,4 1,6 ± 2.2 0,6 ± 0,7 0,7 ± 0,8 - NPL-2 1,44 0.7 i t 2.0 0,8 ± 1,6 2,0 ± 3,7 0,9 ± 1,2 0,9 ± 0,7 - INM-1 13,96 - - 0,1 ± 0.2 0.1 ± 0.1 0,1 ± 0.1 1.1 ± 2.2 INM-2 12,45 - - 0,3 ± 0,3 0,3 ± 0.2 0.2 ± 0,2 1.0 ± 1,8 INM-3 18,04 - - 0.3 ± 0,3 1,2 ± 1.2 0,5 ± 0,6 1.0 t 2,1 INM-4 21,68 - - 0,3 ± 0.3 1.3 ± 1.5 1.0 ± 1.3 0,6 ± 1.0 15
From this table it can readily be seen that the differences between silver point measurements for thermocouples NFt-1 and NPL-2 can be explained by immersion effects: a 2 cm change of immersion (2 times AE/Az) generally suffices to get a change in thermal EMF that agrees with the observed differences dE. This seems to be in reasonably good agreement with the work of Ancsin, who found his thermocouples to vary with the same order of magnitude (maximum change some 0,7 jiV/cm) as they were immersed more or less deeply in the silver freezing point. Thermocouples INM-1 and INM-2 show a less significant immersion effect than NPL-1 and NPL-2 although the reproducibility is some ten times worse. INM-3 and INM-4 have similar immersion effects as NPL-1 and NPL-2 and the reproducibility is worse than that of INM-1 and INM-2. While the differences between the measurements of NPL-1 and NPL-2 can be related to inhomogeneity effects due to local reordering of the impurities, this effect can not explain the differences that were observed in the INM thermocouples. If contamination plays a role in these thermocouples it should have been more or less homogeneous over the entire length of the thermocouples that was subjected to different temperature gradients.
7.4 Impurities
Platinum has been studied extensively over a wide temperature range to evaluate the effect of impurities. Powell (1978) remarked that the major impurity responsible for thermopower differences in platinum is iron. While iron is often one of the major impurities in platinum, it also has a significant influence on the thermal EMF of the platinum wire: 0,658 (iV/ppm. Iron is also the major impurity in alumina: the 99,7% pure alumina, used in this work and commonly employed for standard thermocouple work, contains some 0,05% Fe203 according to manufacturer specifications. Other impurities that could result from the heaters of the stabilisation furnaces and possibly of importance to this problem are molybdenum that has a 1,8 (iV/ppm effect on the thermal EMF generated by the thermocouple and chromium that produces 1,1 (iV/ppm. Were the platinum wire solely responsible for the observed differences of some 25 \iV, then an impurity concentration of some 40 ppm iron, 6 ppm molybdenum or 15 ppm chromium would be present. The alumina contains 500 ppm iron and could therefore be a major source of this impurity if this impurity is not tightly bound in the ceramic: this might be related to the porosity of the alumina. Platinum of the purity that is used in this study usually contains less then 2 ppm of these contaminants according to manufacturer specifications.
Gold has mainly been studied at low, cryogenic temperatures (Kinzie 1973) where it is extremely sensitive to the doping level of iron. This sensitivity is related to the Kondo effect: the interaction between the conduction electrons and the local magnetic moment of iron causes the resistivity of some iron alloys to increase with decreasing temperature (Kittel 1986). Like the resistivity, the thermopower of such alloys is very sensitive to iron (Barnard 1972). This causes the "giant thermopowers" of dilute Au-0,1%Fe alloys. The effect is most eminent at temperatures near 8 K where the Seebeck coefficient can be as large as -10 jiV/K: note it to be negative. Between 8 K and 30 K the Seebeck coefficient increases with increasing 16
temperature to +1,3 jiV/K: then the main scattering mechanism is phonon drag. This positive thermopower due to phonon drag is not unusual, it is also observed in platinum. Between 50 K and 100 K the Seebeck coefficient decreases to 0,82 (iV/K. From 100 K a roughly linear increase with increasing temperature to a maximum of 3,88 |iV/K at 1100 K is mainly governed by diffusion thermopower. At 1200 *C and 1300 *C the Seebeck coefficient is 3,86 jiV and 3,78 \i\f respectively. Diffusion thermopower is explained from ordinary electron transport properties. The positive diffusion thermopower of gold, lithium, silver and copper is generally sought in terms of the shape of the Fermi surface and is related to the "Umklapp process", which works out inversely for these metals as compared to other metals.
The sensitivity of gold to iron at low temperatures can not simply be transferred to higher temperatures where the electron scattenng mechanism is dominated by different scattering processes. A comparison of the influence of iron on the Seebeck coefficients of platinum and gold is not credible since the metals have opposite diffusion thermopowers. Such a comparison is only feasible if the Fermi surfaces of the host metals are similar and apparently they are not.
As a concluding remark one could attribute the instability of the INM thermocouples to some impurity. It is however strange that thermocouples made at INM are more sensitive to this then those made at NPL while they were given the same treatment. There are only two relevant differences between the INM and NPL thermocouples. The INM thermocouples were mounted in alumina tubes from a different supplier than the alumina applied by NPL. The INM thermocouples have larger bore diameters and could therefore be more sensitive to convective transport of impurities from the stabilisation furnaces through the alumina tubes along, and to, the thermocouple wires. Since the cells that were used in this study are regularly applied for high temperature resistance thermometer studies, which are very sensitive to contamination, the thermocouples can not have been contaminated in these cells. Moreover the measurements at a laboratory show very good repeatabilitys. The thermocouples in this study were given more stabilisation treatments than in other studies. It would be interesting to continue this work with some study of impurity effects on the thermopower of gold wires at higher temperatures.
7 5 Calibration characterist c
To allow for a rudimentary evaluation of the calibration characteristic of the thermocouples, a comparison was made between the results obtained by Ancsin and the results obtained in this work. Ancsin caibrated two gold versus platinum thermocouples at the fixed points of gallium, indium, tin, zinc, antimony, a copper-silver eutectic and silver. The two 6th order polynomials that were obtained have been averaged to allow a comparison with the EMF of the thermocouples in this work. The difference of the EMF of these thermocouples and this average function at tin, aluminium and silver are given in Figure 9. A linear function seems to fit for most thermocouples within the estimated uncertainty limits that are based on the short time reproducibility only. This suggests that a very simple calibration would suffice for a large 17
temperature range if accuracies of 0,1 *C are requested: the deviation from a reference function will be simple, this will require only a few calibration temperatures. 18
8 Conclusions
Initial tests by NPL in short silver ingot assembly showed that the output EMF was significantly smaller than when the thermocouples were placed in the longer cells. The thermal conductivity of gold is too large to allow the use of shorter cells.
Strain-relief by the utilisation of a short coil of 0,15 mm diameter platinum wire welded between the main conductors of the NPL thermocouples improved the thermocouple performance. The improvement however was much less then the accuracies that were obtained with these thermocouples, while the construction was very fragile. Moreover the short term repeatabilities of the npl thermocouples were comparable with those obtained for the INM thermocouples where strain was relieved by larger bore diameters.
Gradient dependent thermal EMF was comparable to what was previously found by Ancsin (1991). The resulting differences between the laboratories could be related to this gradient- induced effect for the thermocouples constructed at NPL. Thermocouples assembled at INM showed much larger differences than could be explained by this gradient effect. Some homogeneous contamination might have caused the drift of these thermocouples. Whether this is related to the alumina of these thermocouples or to the larger bore diameter of the alumina tubes can not be concluded without further study in which more alumina tubes with different bore diameters from different suppliers are involved.
The reproducibilit es between the laboratories are disappointing when compared with repeatabilities that were found in previous studies in which only one laboratory was involved. Apparently the gradient effect is not negligible even for the platinum versus gold thermocouple. The ageing process is however still less than that of platinum rhodium type thermocouples. Platinum and gold are still expected to produce better long-term stability thermocouples than thermocouples based on platinum and platinum-rhodium alloys, that are known to show instabilities equivalent to 10 °C after several hundred hours at 1000 *C. Furthermore initial measurements suggest that calibration of these thermocouples can be simple and thereby relatively cheap, since only a few points are needed to assess the deviation of the EMF of an individual thermocouple from a reference function.
Even though this work does not support the high expectations that were set by earlier work, gold / platinum thermocouples seem to be more reliable for accurate temperature measurements than thermocouples presently used in this work. Some further study on long- term stability of this thermocouple is needed to gain some more confidence. 15
References
I. Ancsin J., A study of thermocouple stability, reproducibility and accuracy (Pt vs Pt-Rh and Pt vsAu), Metroiogia 28, 339..347, (1991).
II. ASTM, committee E-20 on temperature measurement, subcommittee IV, Manual on the use of thermocouples in temperature measurement, ASTM special technical publication 470, (1970).
III. Barnard R D, Thermoelectricity in metals and alloys, Taylor & Francis London, (1972).
IV. CCT working group 2, Techniques for approximating the international temperature scale of 1990, BIPM, (1990).
V. Giacomo P, BIPM draft recommendation on the Statement of Uncertainties, Metroiogia 17, 73 (1981).
VI. Kinzie P A, Thermocouple temperature measurement, John Wiley & Sons, New York, (1973).
VII. Kittel C, Introduction to solid state physics, John Wiley & Sons, New York, (1986).
VIII. McLaren E H, Murdock E G, The Pt I Au thermocouple: I essential performance, NRC monograph. NRCC 27703-I. (1987).
IX. McLaren E H, Murdock E G, The Pt I Au thermocouple: II Preparatory heat treatment wire comparisons and provisional scale. NRC monograph, NRCC 27703-II, (1987).
X. OIML, Vocabulary of legal metrology, fundamental terms, (1978).
XI. Powell R L, Sparks L L, Hust J G, Standard thermocouple material, Pt-67; SRM-1967, NBS Special Publication 260-56, (1978).
21
FIGURES
23
o o to
in
,3 o £ a> o -q m CO NPL INM VSL NPL INM measuring laboratory Figure 1 Comparison of the results obtained by the laboratories at silver. 24 bi b2 bj measurement Figure 2 Repeatability of tin point measurements performed by VSL. Three cycles (a,b,c) of three tin point measurements were performed following processes A, B and C. Process A consists of a 1000 °C followed by a 450 °C anneal. During process B the thermocouples were calibrated at the aluminium and silver points whereafter they were heat treated at 1000 °C again. Process C is the 450 *C vacancy anneal. 25 UJ 0) 4 6 8 decrease of immersion (cm) Figure 3 Immersion measurements by NPL and INM of thermocouple NPL-1. 26 Id 4-6 8 decrease of immersion (cm) Figure 4 Immersion measurements by NPL and INM of thermocouple NPL-2. 27 m i i \ c 4) E o £ Z. ^ I- o ■ repeat INM a) E ^ □ NPL ▲ NPL o first INM Ld (0 .C o E o ao 0) o i i *0 2 4 6 8 10 decrease of immersion (cm) Figure 5 Immersion measurements by NPL and INM of thermocouple INM-1. The first measurements made by INM refer to the right axis, as indicated by the arrows. All other measurements refer to the left axis. 28 tn T r- ID 3 •w acj E O 0) 3 V) V— ■ repeat INM aj a NPL E 1 --B-*--_72 ▲ NPL o first INM 2 in m (0 LJ 0o0 o (O E - UJ E o I. o00 4 6 8 10 decrease of immersion (cm) Figure 6 Immersion measurements by NPL and INM of thermocouple INM-2. The first measu ements made by INM refer to the right axis, as indicated by the arrows. All other measurements re er to the left axis. 29 m —i— o __ 5 4 \ C 2 C o 3 Figure 7 Immersion measurements by NPL and INM of thermocouple INM-3. The first measurements made by INM refer to the right axis, as indicated by the arrows. All other measurements refer to the left axis. 30 IT) O i 1— i i T 1 ~- ^-^ > *~ =1 41 v^ ■*J C a> 1 ■ E o \n (0 o L_ o **T o O ■ J ^ 0 4 6 8 10 decrease of immersion (cm) Figure 8 Immersion measurements by NPL and INM of thermocouple INM-4. The first measurements made by INM refer to the right axis, as indicated by the arrows. All other measurements refer to the left axis. 31 -10 - 5g -20 - -30 200 300 400 500 600 700 800 900 1000 t90 (°C) Figure 9 Deviation of thermocouples from the average of the polynomials found by Ancsin (1991). The uncertainty limits have been indicated for each measurement. —^ The Communities research and development information service CORDIS A vital part of your programme's dissemination strategy CORDIS is the information service set up under the VALUE programme to give quick and easy access to information on European Community research programmes. It is available free-of-charge online via the European Commission host organization (ECHO), and now also on a newly released CD-ROM. CORDIS offers the European R&D community: — a comprehensive up-to-date view of EC R&TD activities, through a set of databases and related services, — quick and easy access to information on EC research programmes and results, — a continuously evolving Commission service tailored to the needs of the research community and industry, — full user support, including documentation, training and the CORDIS help desk. The CORDIS Databases are: R&TD-programmes - R&TD-projects - R&TD-partners - R&TD-results R&TD-publications - R&TD-comdocuments - R&TD-acronyms - R&TD-news Make sure your programme gains the maximum benefit from CORDIS — Inform the CORDIS unit of your programme initiatives, — contribute information regularly to CORDIS databases such as R&TD-news, R&TD-pubhcations and R&TD-programmes, — use CORDIS databases, such as R&TD-partners, in the implementation of your programme, — consult CORDIS for up-to-date information on other programmes relevant to your activities, — inform your programme participants about CORDIS and the importance of their contribution to the service as well as the benefits which they will derive from it, — contribute to the evolution of CORDIS by sending your comments on the service to the CORDIS Unit. For more information about contributing to CORDIS, contact the DG XIII CORDIS Unit Brussels Luxembourg Ms I. Vounakis M. B. Niessen Tel. +(32) 2 299 0464 Tel. +(352) 4301 33638 Fax +(32) 2 299 0467 Fax +(352) 4301 34989 To register for online access to CORDIS, contact: ECHO Customer Service BP 2373 L-1023 Luxembourg Tel.+(352)3498 1240 Fax+(352) 3498 1248 If you are already an ECHO user, please mention your customer number. ^ ,990(H*i