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An Investigation Into the Reproducibility of Gold/Platinum Thermocouples

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An Investigation Into the Reproducibility of Gold/Platinum Thermocouples 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.
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