techniques for approximating the international temperature ... - BIPM

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100 inhomogeneities result from the unavoidable production of rhodium oxide on the alloy wires and from changes in vacancy concentrations in both wires in those portions that experience temperatures between 500 °C and 850 °C. Other mechanisms probably contribute also, such as order-disorder transitions in the structural lattices and differential thermal expansion between wires and the ceramics in which they are imbedded. McLaren and Murdock (1979a, 1979b, 1983) have shown in detail that the emf of a Type S thermocouple annealed according to the procedures given in Sec. 9.3 is not constant for different depths of immersion of the hot junction in a freezing metal ingot. In freezing copper, for example, the emfs vary somewhat irregularly but generally increase with a 10 cm increase in immersion, followed by a significantly higher decrease on withdrawal. The maximum variations in emf are typically a few microvolts. These variations are caused by inhomogeneities along the wires resulting from the processes mentioned above. A different type of anneal will produce homogeneous wires, but unfortunately they cannot be maintained in the homogeneous state in service. As a result, an initial insertion of a thermocouple with this type of anneal into a fixed-point ingot produces a constant emf if the temperature is not higher than 1000 °C, but subsequent withdrawals and insertions produce emfs that increase almost linearly (but moderately reproducibly) with immersion depth. A reanneal returns the wires to their original homogeneous state. Errors from the other sources can be reduced, with a little care, below ± 0.2 K. Apart from this, contamination is by far the most common cause of thermocouple error and often results in ultimate mechanical failure of the wires. Elements such as Si, AI, P, In, and Sn combine with platinum to form low-melting-point eutectics and cause rapid embrittlement, leading to failure of the thermocouple wires. Elements such as Ni, Fe, Co, Cr, and Mn all affect the emf output of the thermocouple to a greater or smaller degree, but contamination by these elements may not result in wire breakage and can only be detected by regular checking of the accuracy of the thermocouple. Contamination can be minimized by careful handling of the thermocouple materials during assembly and by using appropriate refractory sheathing. Care should be taken to prevent dirt, grease, oil, or soft solder from coming into contact with the thermocouple wires. If the atmosphere surrounding the thermocouple sheath contains any metal vapor, the sheath must be impervious to such vapors. Contamination of the pure-platinum arm by transfer of rhodium (probably through the oxide phase) from the alloy arm can be suppressed by using a continuous twin-bore alumina insulator to support and protect the wire over its whole length in the temperature gradient. For the higher temperatures where this is likely to occur, it is preferable to use Pt30Rh/Pt6Rh or other double-alloy thermocouples because these are much less affected by contamination or rhodium transfer.
101 Contamination by reduction of the alumina can be avoided by ensuring that the furnace within which the thermocouple is used allows free access of air to the thermocouple. If the furnace itself operates under an inert or reducing atmosphere the thermocouple must be protected by a closed-end alumina or silica sheath, one end of which is open to the air. Errors can also result from loss of homogeneity due to strain in one or both wires. The effect of strain is to reduce the emf and so give a temperature reading that is too low. The strain can be removed by reannealing the whole thermocouple. Thermocouple installations should be designed in such a way that there is no strain on the wire while in service; for example, use in a horizontal position is preferable to use in a vertical position. Thermocouples must be installed in such a way that they have adequate immersion and the reference junction is at a uniform and known temperature (preferably 0 °C, see below). Immersion is adequate when the heat transfer to the thermocouple is such that the difference between the measuring-junction temperature and the temperature to be measured is smaller than a given tolerance. On the other hand, the means of ensuring adequate immersion must not invalidate the measurement sought. It is common that the design of the experiment is substantially affected by balancing these requirements. Thus the installing of the thermocouple requires considerations regarding annealing, calibration, tempering, insulation of the thermoelements from each other and from the surroundings, and protection from chemical or physical deterioration. The installation should also provide for means of occasional recalibration. Very important to the correct use of a thermocouple is the realizing of a stable reference junction. It is easy to make a reference junction at 0 °C reliable to ± 0.01 °C by using a vacuum flask into which is placed a mixture of shaved ice and distilled water. This will enable the user to refer this thermocouple cold junction directly to 0 °C, which is advantageous because most reference tables are so referred. The thermocouple wires are connected to pure copper wires, by suitably twisting them together or by soldering without flux, and inserted into separate closed-end glass tubes that are in turn inserted into the ice mixture. The thermocouple-to-copper junction is placed at a sufficient depth (> 20 cm) in the reference unit that a stable and accurate reference temperature is ensured [Sutton (1975)]. Oil should not be placed in the glass tubes (in an attempt to improve thermal contact) as this can do more harm than good; the oil tends to migrate up the insulation towards the hot zone and, in any case, thermal contact is adequate without it. Automatically-operating ice-point devices with sufficient reproducibility are also available. It is not recommended to use extension wires for accurate temperature measurements as they cause additional errors.
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BUREAU INTERNATIONAL DES POIDS ET M
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TECHNIQUES FOR APPROXIMATING THE IN
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iv W(100 °C) = 1.385 (exact value
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Centre for Quantum Metrology Nation
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2. Type J viii a) temperature range
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c) temperature range from 1664.5 °
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xii
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xiv Acknowledgments This monograph
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xvi 3.3.2 Melting Points of Gold (1
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xviii 11.3 Thermal Contact 111 11.4
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1 1. Introduction The Comité Consu
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3 thermometers except in special ex
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5 The accuracies with which tempera
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Table 1.1: Summary of Some Properti
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PART 1: TECHNIQUES AND THERMOMETERS
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11 Fig. 2.1: One form of apparatus
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13 Fig. 2.3: Flow cryostat, shown w
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15 Fig. 2.4: Stirred liquid bath fo
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17 contained within a cylindrical c
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19 Fig. 2.5: Schematic drawing of a
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21 device SRM 767 [Schooley et al.
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23 Table 3.1 : Current Best Estimat
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25 The widely-used, but not very re
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28 fraction of sample melted) can g
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30 Fig. 3.2a: Apparatus for the cal
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32 Fig. 3.3: Sealed cell for realiz
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34 Final readings of the thermomete
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36 temperatures differ (usually) sy
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38 Fig. 3.4: Cross sectional drawin
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40 Fig. 3.5: Miniature graphite bla
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42 4. Germanium Resistance Thermome
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44 Fig. 4.3: Example of the Π-type
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46 Fig. 4.4: Differences between dc
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48 - conversely, p-doped thermomete
Page 70 and 71: 50 Fig. 4.6: Effect of a radio-freq
Page 72 and 73: 52 that it will not be subject to m
Page 74 and 75: ln R n = ∑ i= 0 54 ⎛ ln T - P
Page 76 and 77: 56 Fig. 5.1: Resistance (Ω) and s
Page 78 and 79: 58 thermometer wires) caused a more
Page 80 and 81: 60 6. Vapour Pressure Thermometry*
Page 82 and 83: 62 transitions, but it could be app
Page 84 and 85: n ∑ i= 2 64 L x [ Π − k ] P =
Page 86 and 87: 66 Fig. 6.3: Diagram at constant pr
Page 88 and 89: 68 Fig. 6.4: Schematic construction
Page 90 and 91: 70 Fig. 6.6: Use of an evacuated ja
Page 92 and 93: 72 Following this, the connecting t
Page 94 and 95: 74 Fig. 6.9: (c) N2, CO, Ar, O2, CH
Page 96 and 97: 76 the Weber-Schmidt equation [Webe
Page 98 and 99: 78 Table 6.1: Temperature values (K
Page 100 and 101: 80 into the bulb (hydrous ferric ox
Page 102 and 103: 82 Fig. 6.12: Effect on the vapour
Page 104 and 105: 84 the remaining liquid increases.
Page 106 and 107: 86 Curie constant (C⊥ is about 0.
Page 108 and 109: 8.1 General Remarks 88 8. Platinum
Page 110 and 111: 90 For a group of 45 thermometers h
Page 112 and 113: 92 More recently, Seifert [(1980),
Page 114 and 115: 94 For thermometers with W(4.2 K) <
Page 116 and 117: 96 after 500 h at 1700 °C in air,
Page 118 and 119: 98 normally extends about 50 cm bac
Page 122 and 123: 9.5 Approximations to the ITS-90 10
Page 124 and 125: 104 10. Infrared Radiation Thermome
Page 126 and 127: 106 almost negligible. The final ac
Page 128 and 129: 108 11. Carbon Resistance Thermomet
Page 130 and 131: 110 Fig. 11.1: Resistance-temperatu
Page 132 and 133: 112 Table 11.1: Typical Time Consta
Page 134 and 135: 114 calibration after each cool-dow
Page 136 and 137: 116 12. Carbon-Glass Resistance The
Page 138 and 139: 118 Fig. 12.1: Resistance-temperatu
Page 140 and 141: 120 14. Diode Thermometers Diodes c
Page 142 and 143: 122 2 BT V = E0 − − CT ln ( DT)
Page 144 and 145: 124 Fig. 15.1: Principal features o
Page 146 and 147: 126 For a thermometer graduated abo
Page 148 and 149: 128 rise decreases with time but in
Page 150 and 151: 130 Fig. 16.1: (a) Fabrication of I
Page 152 and 153: 132 capillaries of a twin- (or four
Page 154 and 155: 134 advised to choose the thermomet
Page 156 and 157: 136 Fig. 16.6: Distribution of the
Page 158 and 159: 138 and between different thermomet
Page 160 and 161: 140 therefrom) can then be used wit
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Page 164 and 165: 144 Fig. 16.8: Tolerances for indus
Page 166 and 167: 146 Stability on thermal cycling is
Page 168 and 169: 18.2.1 Type T Thermocouple 148 With
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150 within ± 0.5 to 0.7 percent. T
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152 insulation even though it be af
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154 Table 18.4: Recommended Sheath
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156 In the case of the Types K and
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158 temperature is measured with th
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160 19. Thermometry in Magnetic Fie
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162 Type of Sensor T(K) Magnetic Fl
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164 be noted that these lower tempe
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166 Fig. 19.2: The change in calibr
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168 Carbon-glass thermometers (Chap
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Ancsin, J. and Phillips, M.J. (1984
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172 Berry, R.J. (1972): The Influen
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174 Brodskii, A.D. (1968): Simplifi
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176 Crovini, L., Perissi, R., Andre
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Hudson, R.P. (1972): Principles and
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180 Lengerer, B. (1974): Semiconduc
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182 OIML (1985): International Reco
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184 Rusby, R.L. (1975): Resistance
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186 Seifert, P. and Fellmuth, B. (1
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188 Ween, S. (1968): Care and Use o
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190 Fig. A.1: Differences between t
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National Institute of Metrology P.O
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194 Appendix C Some Suppliers of Va
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196 Appendix D Calculations Relativ
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198 4. The calculation of the numbe
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200 Appendix F Interpolation Polyno
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- temperature range from 0 °C to 1
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204 where: d0 = 0; d5 = -3.17578007
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