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20 3. Specialized Fixed Points A temperature fixed point is a calibration point for thermometers that differs from a comparison calibration point in that its temperature value is not measured, during the calibration process, by another already calibrated thermometer, but has been previously established either by definition (if it is a defining point of the ITS-90) or by previous experiments (if it is a secondary reference point) [Bedford et at. (1984)]. Whether measured or assigned, the temperature value is associated with a thermodynamic state of a substance; how closely that state is approximated during the calibration depends upon the stringency of the specifications on it (e.g. requirements on purity, isotopic compositions, annealing, ...) and upon the experimental uncertainty of the measurements. The ITS-90 can be approximated not only by simplified interpolation with primary or nonprimary thermometers (as will be discussed in the next sections of this monograph), but also by relaxing the fixed point specifications or by using the fixed points in different ways. In this chapter we describe a number of specialized fixed points and unconventional ways of realizing conventional fixed points, with indications of the accuracies that may be attained. Methods for highly accurate fixed point realizations are described in detail in "Supplementary Information for the ITS-90" [CCT (1990)] and will not be repeated here. 3.1 Fixed Points Below 0 °C 3.1.1 Superconductive Fixed Points 3.1.1.1 General Remarks Superconductive transitions are solid state phase changes of second order in contrast to most fixed points used in thermometry which are first-order phase changes. Second order phase changes are distinguished by the absence of an associated latent heat. Essential peculiarities of the superconductive transitions are the larger influence on them of impurities, crystal defects, and physical strain which affect both the transition widths and temperatures (T C ). This does not prevent their application as thermometric fixed points, however, if special sample preparation techniques and special methods for the detection of the superconductive transitions are used. With the promulgation of the EPT-76 [CCT (1979)], superconductive transition temperatures (ST -temperatures) were officially used as fixed points for the realization of an international temperature scale. However, connected with the peculiarities mentioned above, the definition of the EPT -76 was based only upon the superconductive fixed-point
21 device SRM 767 [Schooley et al. (1980) and Schooley and Soulen (1982)], so that the ST - temperatures were only used officially in the sense of a reference material (see Sec. 3.1.2). Furthermore, ST-temperatures are not used as fixed points in the definition of the ITS-90. Metrological investigations of SRM 767 fixed-point samples [EI Samahy et al. (1982)] verified that T C values are reproducible within about 1 mK. Efforts towards improvement have shown that a reduction of the non-uniformity of the TC values well below 1 mK is possible by carefully annealing all samples and by selecting them on the basis of maximum allowed transition widths [Schooley (1984)]. 3.1.1.2 Requirements for Superconductive Fixed Points Problems in, and possibilities for, the realization of superconductive transitions as true thermometric reference points are reviewed for the metals Cd, In, AI, In, Pb, and Nb by Fellmuth and Maas (1987). This review shows that modern sample preparation and handling techniques, in conjunction with convenient material characterization methods, are sufficient to guarantee an accuracy and stability of the ST-temperatures of these elements within about 1 mK if definite specifications concerning sample parameters are fulfilled. The main difficulty in realizing a superconductive fixed point is the influence of impurities and crystal defects on the ST-temperature. The influence of impurities is of the same order of magnitude as for metal freezing points: typically (|dT C /dc| ≈ 1 mK per ppma*, where c is the impurity content). The residual resistance is an excellent indicator of this influence except in the few cases where localized magnetic moments exist; in this latter case dTC/dc can be much larger so the concentration of the magnetic impurities must be determined separately. The effect of crystal defects can be reduced to a few tenths of a millikelvin by using suitable preparation techniques, which may differ for different elements. For checking the magnitude of this effect it is important that the transition width be always smaller than the change in the ST-temperature due to the defects. Hence, for the realization of superconductive reference points, detailed information concerning both the physical properties of the materials and the preparation and characterization methods is necessary. Such information is given for the six metals listed above in the review of Fellmuth and Maas (1987) and, for niobium, by Fellmuth et al. (1985), (1987). _________________________________________________________________________ * The abbreviation ppma is used to mean an impurity content of one solute atom per 10 6 solvent atoms.
Page 1 and 2: 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: 19 Fig. 2.5: Schematic drawing of a
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 54 and 55: 34 Final readings of the thermomete
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
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70 Fig. 6.6: Use of an evacuated ja
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72 Following this, the connecting t
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74 Fig. 6.9: (c) N2, CO, Ar, O2, CH
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76 the Weber-Schmidt equation [Webe
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78 Table 6.1: Temperature values (K
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80 into the bulb (hydrous ferric ox
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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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