techniques for approximating the international temperature ... - BIPM

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24 For these, there exist two SRMs issued by the National Institute of Standards and Technology: SRM 767 [Schooley and Soulen (1972), EI Samahy et al. (1982)] and SRM 768* [Schooley and Soulen (1982)], each of which includes five metals with specified superconductive transition temperatures (Table 3.2). Within 1 mK, the devices can be considered to reproduce the temperature values of the physical states, though they are supplied calibrated. For the niobium transition near 9.3 K, another calibrated device is now available [Fellmuth et al. (1985)]. Above 14 K, the fixed points are triple and boiling points obtained from condensing gases; the reliability of bottled gases as SRMs is questionable as their use involves manipulation and consequent risk of contamination. However, the technique of sealing gaseous samples in metal cells makes it possible to obtain both reference fixed points [CCT (1983), Pavese et al. (1984)] and reference materials [Pavese (1980)]. Examples of sealed cells as SRMs as opposed to fixed points are the deuterium triple point when using gas obtained by hydrolysis of heavy water because of the high HD content, and the oxygen triple point when using gas distilled from air because of the unknown remaining contamination by argon. Most of the fixed points listed as second- or third-quality by Bedford et al. (1984) could still be used as SRMs in sealed cells. 3.1.3 Vapour Pressure Thermometers Vapour pressure scales may be considered as giving a continuous set of fixed points, some of which are used also as defining points of the ITS-90 (the hydrogen boiling points and the helium vapour pressure scales). A realization of the ITS-90 that involves vapour pressure points, however, is complex and must include purity-effect analysis. Even without such analysis, any point of the vapour pressure curve can be used to obtain in a simple way and with an accuracy that depends upon the gas purity (see Chapter 6), a temperature value from the relationship to pressure and so to calibrate thermometers fitted into the same bulb. The use of vapour pressure thermometers is fully described in Chapter 6. The requirements for the cryostat are essentially the same as for thermometer calibration in a comparison copper block, except that a bulb and sensing tube must be provided. Often the regulation is by control of the pressure rather than of the temperature. At the higher temperatures (for example, in a hypsometer for the steam point or for a sulphur boiling point) the boiler and comparison chamber are connected to a large ballast volume to ensure good stability. _________________________________________________________________________ * SRM 768 provides fixed points below the lower limit of the ITS-90.
25 The widely-used, but not very reliable, method of taking advantage of an open bath of a boiling cryogen (oxygen, nitrogen, helium, rarely hydrogen or argon) is a very simplified way to do vapour pressure thermometry at a single point at atmospheric pressure. The bath should be stirred or other precautions taken to reduce temperature gradients, for example by lagging the comparison block. The sensitivities of some fluids commonly used in vapour pressure thermometers are given in Table 3.3. 3.1.4 Sealed Cells Though possible [Pavese (1981), Bonhoure and Pello (1983)], pressure measurement is generally not convenient with sealed samples, so the sealed-cell method is best used only for triple points. A general discussion is given elsewhere [CCT (1990), Pavese et al. (1984)]. Some thermal parameters of thermometric substances that are relevant to the design of sealed cells are given in Table 3.4; a detailed tabulation of all parameters is available in, for example, L'Air Liquide (1976). To approximate the ITS-90, solid-solid transitions can also be measured although, in general, they are of lower precision. Compared with triple or boiling points, they are at a disadvantage in that there is less selfstabilization of the cell because of the lower heat of transition and the less effective thermal coupling with the condensed sample. Such a self-stabilization is largely due to the presence of the liquid phase, especially when it is kept in thin layers. In solid-solid transitions, on the contrary, the solid has poor thermal contact with the cell and also a much lower vapour pressure. Additionally, the diffusion of the transition is generally a slow process. The self-stabilization effect can be better understood from Table 3.4 where the enthalpy change in melting of the sample is compared with the heat capacity of the cell. Any small perturbation of the temperature due to stray heat exchanges with the cryostat can be compensated by melting or freezing of a small fraction of the sample. For this reason, any of the cryostats shown in Chapter 2 are satisfactory, and their performances are much less critical than in comparison, or vapour pressure, measurements. Values for temperature error versus heat leak are as low as 10 µK/mW above 20 K, increasing to 500 µK/mW at 14 K. Examples of thermal analyses are given by Seifert and Fellmuth (1989), Seifert (1982), and Bonnier and Hermier (1982). This self-stabilization is also beneficial for simplifying the use of sealed cells. Firstly, the cell can become the reference for an inexpensive differential temperature regulation of the isothermal shield of the cryostat, thus eliminating costly and critical absolute-temperature controllers. Secondly, a cell acts as an almost ideal temperature-generator, able to drive a large thermal load. It may be
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 and 40: 19 Fig. 2.5: Schematic drawing of a
Page 41 and 42: 21 device SRM 767 [Schooley et al.
Page 43: 23 Table 3.1 : Current Best Estimat
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
Page 90 and 91: 70 Fig. 6.6: Use of an evacuated ja
Page 92 and 93: 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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