techniques for approximating the international temperature ... - BIPM

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34 Final readings of the thermometer should not be taken until temperature equilibrium has been achieved as indicated by a constant reading over several minutes. A useful check against possible contamination introduced with the thermometer is to withdraw the thermometer and reinsert it immediately in a different location - then go through the reading procedure a second time. In very precise work, or when immersion is limited, a clean aluminium foil over the top of the ice should be used to prevent transmitted radiation from affecting the temperature of the sensing element. If all of the precautions described above are taken, an ice bath should be capable of providing the conditions for the realization of the ice point to an accuracy within about 0.3 mK. However, at this level of accuracy the triple point of water is preferable: the triple-point cell is simpler to use, less prone to errors, and may be maintained for a longer time without attention. 3.3 Fixed Points above 630 °C Except for the very highest temperatures, all of the fixed points in this region are metal freezing or melting points. For calibrations to very high accuracy, the metal is contained in a (usually) graphite crucible in such a way that a continuous liquid/solid interface encloses (as nearly as is practical) the sensing element of the thermometer. For resistance-thermometer calibrations, two such interfaces are generally induced. The outer one forms a solid shell that surrounds the liquid phase and the inner one, whose temperature is measured by the thermometer, surrounds the thermometer well. The techniques required in the generation of these interfaces, the crucible assemblies, suitable furnaces, and methods for checking the quality of the results are described in detail in "Supplementary Information for the ITS-90" [CCT (1990)]. Variations of the techniques suitable for resistance thermometer, thermocouple, or optical pyrometer calibrations are included there. Above 630 °C the metals to which the methods apply are Sb (630.633 °C), AI, Ag, Au, and Cu. The methods can also be used for the melting point of the eutectic alloy Cu 71.9% Ag 28.1% (779.91 °C), but because of the nature of eutectic phase transitions the accuracy is limited to about ± 0.03 °C, even with resistance thermometers. Remarks on the eutectic point are given in Sec. 3.3.1. Sealed cells (Sec. 3.1.4) can often be used for the realization of these metal freezing or melting points. At higher temperatures the most-used fixed points are the melting or freezing points of Pd and Pt, to which the above methods are applicable only with considerable difficulty because the extreme temperatures require different and more complicated furnace construction, different crucible materials, and so on. In any case, above 1100 °C a much lower accuracy is tolerable with fixed-point calibrations and so specialized techniques have
35 been developed. Also, below 1100 °C, much simpler and less costly but less accurate techniques are possible. These various special procedures are discussed in Secs. 3.3.2 - 3.3.3. 3.3.1 Copper 28.1% Silver 71.9% Eutectic Alloy It is unfortunate that no pure metal has a freezing temperature in the approximately 300 kelvin interval between the freezing points of aluminium and silver - a strategic interval for both thermocouple and resistance thermometry. The melting point of the eutectic alloy Cu 28.1% Ag 79.1 % (Cu/Ag) at 779.91 °C is ideally placed but, even when realized according to the same stringent procedures as for the pure metals, it is reliable to at best ± 30 mK in contrast to ± 1 mK for the pure metals. Nevertheless, the point can be useful in connection with thermocouple calibrations. Studies of it with resistance thermometers are described by Bongiovanni et al. (1972) and McAllan (1982), with thermocouples by Itoh (1983), and with optical pyrometers and thermocouples by Bedford and Ma (1982), all with techniques comparable to the foregoing. The eutectic freezing point is not a good reference point because its temperature value is freezing-rate dependent. This is so because, during freezing, the components in the liquid have to separate by diffusion to form the two different solid eutectic phases, and because the temperature also depends upon the details of nucleation. Consequently, the eutectic melting point is recommended as the reference. It is much more reproducible, and is not strongly dependent on the overall composition. Itoh (1983) found that changing the relative compositions by ± 2% from the exact eutectic composition had no significant effect on the observed melting temperature. The eutectic freezing temperature is always lower than the melting temperature and can vary over several tenths of a kelvin depending largely upon the rate of freezing. Because the rate of freezing also affects the slope of the subsequent melting curve, it is recommended that each melting point determination be preceded by a very slow (several hours) freeze. Also, since even under optimum conditions the melting curve has a more pronounced slope than for a pure metal, some consistent criterion for choosing the melting temperature is required. McAllan (1982) suggests that the most reliable estimate for the equilibrium eutectic temperature is given by the intersection of the extrapolation of the region of the melting curve just before the commencement of the rapid rise with the 100%-melted axis. Alternatively, the maximum in a histogram that shows percentage of time spent in consecutive temperature intervals can be taken as the melting temperature. An extension of this latter method when the histogram has several peaks indicating segregation of impurities is to use the centroid rather than the maximum. These three
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BUREAU INTERNATIONAL DES POIDS ET M
Page 3 and 4: TECHNIQUES FOR APPROXIMATING THE IN
Page 5 and 6: iv W(100 °C) = 1.385 (exact value
Page 7 and 8: Centre for Quantum Metrology Nation
Page 9 and 10: 2. Type J viii a) temperature range
Page 11 and 12: c) temperature range from 1664.5 °
Page 13 and 14: xii
Page 15 and 16: xiv Acknowledgments This monograph
Page 17 and 18: xvi 3.3.2 Melting Points of Gold (1
Page 19 and 20: xviii 11.3 Thermal Contact 111 11.4
Page 21 and 22: 1 1. Introduction The Comité Consu
Page 23 and 24: 3 thermometers except in special ex
Page 25 and 26: 5 The accuracies with which tempera
Page 27 and 28: Table 1.1: Summary of Some Properti
Page 29 and 30: PART 1: TECHNIQUES AND THERMOMETERS
Page 31 and 32: 11 Fig. 2.1: One form of apparatus
Page 33 and 34: 13 Fig. 2.3: Flow cryostat, shown w
Page 35 and 36: 15 Fig. 2.4: Stirred liquid bath fo
Page 37 and 38: 17 contained within a cylindrical c
Page 39 and 40: 19 Fig. 2.5: Schematic drawing of a
Page 41 and 42: 21 device SRM 767 [Schooley et al.
Page 43 and 44: 23 Table 3.1 : Current Best Estimat
Page 45: 25 The widely-used, but not very re
Page 48 and 49: 28 fraction of sample melted) can g
Page 50 and 51: 30 Fig. 3.2a: Apparatus for the cal
Page 52 and 53: 32 Fig. 3.3: Sealed cell for realiz
Page 56 and 57: 36 temperatures differ (usually) sy
Page 58 and 59: 38 Fig. 3.4: Cross sectional drawin
Page 60 and 61: 40 Fig. 3.5: Miniature graphite bla
Page 62 and 63: 42 4. Germanium Resistance Thermome
Page 64 and 65: 44 Fig. 4.3: Example of the Π-type
Page 66 and 67: 46 Fig. 4.4: Differences between dc
Page 68 and 69: 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
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84 the remaining liquid increases.
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86 Curie constant (C⊥ is about 0.
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8.1 General Remarks 88 8. Platinum
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90 For a group of 45 thermometers h
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92 More recently, Seifert [(1980),
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94 For thermometers with W(4.2 K) <
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96 after 500 h at 1700 °C in air,
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98 normally extends about 50 cm bac
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100 inhomogeneities result from the
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9.5 Approximations to the ITS-90 10
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104 10. Infrared Radiation Thermome
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106 almost negligible. The final ac
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108 11. Carbon Resistance Thermomet
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110 Fig. 11.1: Resistance-temperatu
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112 Table 11.1: Typical Time Consta
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114 calibration after each cool-dow
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116 12. Carbon-Glass Resistance The
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118 Fig. 12.1: Resistance-temperatu
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120 14. Diode Thermometers Diodes c
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122 2 BT V = E0 − − CT ln ( DT)
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124 Fig. 15.1: Principal features o
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126 For a thermometer graduated abo
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128 rise decreases with time but in
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130 Fig. 16.1: (a) Fabrication of I
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132 capillaries of a twin- (or four
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134 advised to choose the thermomet
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136 Fig. 16.6: Distribution of the
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138 and between different thermomet
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140 therefrom) can then be used wit
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142
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144 Fig. 16.8: Tolerances for indus
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146 Stability on thermal cycling is
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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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