techniques for approximating the international temperature ... - BIPM

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38 Fig. 3.4: Cross sectional drawing of a miniature graphite crucible for thermocouple calibrations (dimensions are in millimetres) [Tischler and Koremblit (1982)]. The tightly-fitting crucible lid has a small perforation to vent occluded gases that are released as the temperature is raised. For calibration, the thermocouple with the crucible attached is placed in a small- diameter, fused-silica tube through which a slow flow of nitrogen is maintained to protect against oxidation of the graphite. The silica tube in turn is inserted into a conventional furnace at the appropriate temperature with the silica tube extending to room temperature. The furnace power is then increased so that the furnace equilibrium temperature will be slightly (typically 1 K to 5 K) higher than the melting temperature of the ingot and the melting transition is recorded. Following this, the furnace power is reduced so that the temperature will equilibrate at an equal amount below the transition temperature and the freezing transition is recorded. A typical transition plateau lasts about 20 min. Depending upon the rate of change of furnace temperature, and upon the difference between furnace temperature and transition temperature, the temperature of the freezing plateau will be significantly lower than that of the melting plateau because of the relatively small latent heats, large heat transfer, and effect of pressure on the ingot due to surface tension. Let VP be the thermocouple emf at the temperature of the plateau, VF the emf when the thermocouple has equilibrated at the furnace temperature following the transition, and VT the emf at the true transition temperature. On a plot of VF vs VP, the intersection of a line joining the points corresponding to melting (VPm ) and freezing (VPf ) with the line VF = VP gives the desired emf VT, which is usually reproducible to about ± 1 µV and accurate to ± 0.3 K. It is likely almost as accurate to take for VT the mean of the melt and freeze emfs.
39 With some deterioration in accuracy (± 0.5 K), a simpler method is not to stabilize the furnace temperature close to the transition temperature, but to let it pass through at a relatively high rate. For Cu, Sb, Pb, Sn, In take VT = VPm from the melt; the large amount of undercooling makes VP from the freeze unreliable. For Au, Ag, In, and Cd take VT = (V Pm + VPf)/2, or VT = VPm, which will be almost the same. For AI it is necessary to take VT = (VPm + VPf)/2. Variations of this technique are described by Tischler and Anteneodo (1986) where the freezing point is a permanent part of the thermocouple. Other workers are applying miniature fixed points for the calibration of standard-welded and metal-sheathed thermocouples. 3.4 Fixed Points for Pyrometry 3.4.1 Blackbody Furnaces Infrared pyrometers (monochromatic or broad-band) must be calibrated against blackbodies. If modest accuracy is sufficient, it is possible to use semi-miniature furnaces as opposed to those recommended for highest accuracy [CCT (1990)]. Sakuma and Hattori (1982a) have designed furnaces of this sort that are in moderately wide use. A graphite blackbody crucible 4 cm diameter, 6.5 cm long, with a 1 cm aperture (Fig. 3.5) is encapsulated in a stainless-steel cylinder which carries the furnace heater, either embedded in grooves parallel to the axis or wound around the circumference. The steel cylinder extends beyond the blackbody aperture for better temperature uniformity, and includes a stainless-steel disk with a 6 mm aperture covering the front of the blackbody. Argon flows through the steel capsule to prevent oxidation of the graphite. This steel cylinder containing the crucible and furnace heater is mounted in an alumina tube that is surrounded by suitable radiation shields and thermal insulation (Fig. 3.6). The whole furnace is 10 cm diameter by 25 cm long, and is attached to a temperature-control unit consisting of a SCR power regulator, an on-off temperature controller, and indicators. A monitoring thermocouple is attached to the steel capsule. With the main power switch on and the regulator switch off, the furnace is maintained about 10°C above the freezing point of the metal in the blackbody. Then, with the regulator switch on, the crucible cools at a pre-determined rate through the freezing transition. The freezing plateau lasts about 10 min and is accurate to within about - 0.1 ± 0.1 K. Such a furnace is available for each of the metals Cu, Ag, AI, Sb, and Zn.
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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
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 54 and 55: 34 Final readings of the thermomete
Page 56 and 57: 36 temperatures differ (usually) sy
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
Page 104 and 105: 84 the remaining liquid increases.
Page 106 and 107: 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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