techniques for approximating the international temperature ... - BIPM

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2.3 300 °C to 1100 °C 16 For temperatures above the range of stirred oil, no completely satisfactory bath is available. A salt bath (containing a mixture of equal parts of potassium nitrate and sodium nitrite) will provide an extremely uniform temperature from about 250 °C to 550 °C, but has substantial drawbacks. These include: a highly corrosive action on the bath container and on any devices inserted into the salt (a quartz-sheathed thermometer must be protected from the salt by an auxiliary Pyrex protection tube); a salt residue left on a device after calibration which is often difficult to remove; a potentially dangerous condition during heat-up if expansion of liquid is prevented by entrapment in solid salt; an extremely long heat-up time for the solid phase; possible violent splatter when foreign (organic) materials come in contact with the salt; requirement for venting fumes; and a generally messy housekeeping problem. Industrial safety codes for the operation of salt baths should be consulted [for example, HMSO (1964)]. An alternative to the salt bath is a fluidized bed, or fluidized powder bath. This can be capable of operation over an extremely wide range (from below ambient to 1100 °C) but it is difficult to obtain within the working volume a temperature that is uniform enough. The fluidizing medium is usually very finely powdered aluminium oxide through which dry air, nitrogen, or argon is allowed to flow slowly upward from an underlying porous diffuser of metal or ceramic. When fluidization begins, the individual AI2O3 particles circulate freely, and the powder takes on many of the properties of a liquid. The appearance is much like that of swirling cream with gentle surface bubbling. The powder tends to rise and migrate slowly from the centre to the walls. Heat is supplied in much the same way as in a liquid bath. Frequently, in the higher temperature units, a layer of zirconium oxide that does not fluidize but assists in the diffusion and inhibits the burning of the porous plate lies between the AI2O3 and the solid diffuser plates. Typical temperature differences in the fluidized bath range from tenths of kelvins to as high as a few kelvins. If a metal block is used within the fluidizing medium, better uniformity is possible. In addition to a frequently inadequate temperature uniformity, another disadvantage is the deposition of AI2O3 dust on everything near the bath. Care must be taken to avoid mechanical vibrations arising from the circulation of the fluid powders. Wire-wound resistively-heated electric furnaces are the most common uniformtemperature enclosure above the range of liquid baths. These are essentially the same as those described in "Supplementary Information for the ITS-90" [CCT (1990)] for the realization of metal freezing points. A metal block (aluminium, copper, Inconel) is
17 contained within a cylindrical ceramic muffle on which the heaters are wound. The main heater runs the whole length of the muffle, while smaller ones are placed either over it or within the muffle near each end, compensating for heat losses and thereby lengthening the uniform-temperature zone. The whole is suitably insulated. With care, for a 60 to 90 cm furnace, it is possible to adjust the heater powers so as to obtain a central 20 to 30 cm zone that is uniform in temperature to some tenths of a kelvin. The convenient upper temperature limit for such furnaces is 1100 °C, up to which Nichrome or Kanthal heater windings have long lifetimes. For higher temperatures one must resort either to expensive platinum-alloy heaters or to refractory-metal heaters that require protection from air or oxygen. A simpler arrangement with even better temperature uniformity uses a cylindrical heat-pipe liner within the furnace muffle. In this case a single heater will suffice. A heat pipe is essentially a closed tube or chamber whose inner surfaces are lined with a porouscapillary wick. The wick is saturated with the liquid phase of a working fluid and the remaining volume of the tube contains the vapour phase. Applied heat vaporizes the working fluid in some sections, resulting in a pressure difference that drives vapour to cooler regions where it will condense, releasing the heat of vaporization in that section of the pipe. Depletion of liquid by evaporation causes the liquid-vapour interface to enter into the wick surface. The capillary pressure developed there pumps the condensed liquid back for reevaporation. That is, the heat pipe can continuously transport the heat of vaporization from the evaporator to the condenser section. The amount of heat that can be transported in this way is several orders of magnitude larger than that which can be transported in a conventional system; the heat pipe acts as a tube of extremely high thermal conductivity. Hence, when it is placed within an already-heated furnace it increases the uniformity immensely. Quinn (1983) gives a useful, more detailed discussion. The working fluid is usually sodium or potassium for heat pipes of interest here. A metal block may be placed within the heat pipe, but in many cases may be unnecessary. Care must be taken in the use of metal-enclosed sodium or potassium in heat pipes; the usual precautions for use of these metals should be taken. A more sophisticated use of the heat pipe is in the pressure-controlled or gascontrolled heat pipe [see, for example, Bassani et al. (1980)]. The cylindrical heat pipe is installed in a conventional electric furnace as above, but in this case most of the applied heat is concentrated near one end (the concentrator); a vertical (or tilted, for vertical furnace operation) water-cooled chimney (the condenser) is attached at the other
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: 15 Fig. 2.4: Stirred liquid bath fo
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 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
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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
Page 142 and 143:
122 2 BT V = E0 − − CT ln ( DT)
Page 144 and 145:
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
Page 170 and 171:
150 within ± 0.5 to 0.7 percent. T
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152 insulation even though it be af
Page 174 and 175:
154 Table 18.4: Recommended Sheath
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156 In the case of the Types K and
Page 178 and 179:
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
Page 184 and 185:
164 be noted that these lower tempe
Page 186 and 187:
166 Fig. 19.2: The change in calibr
Page 188 and 189:
168 Carbon-glass thermometers (Chap
Page 190 and 191:
Ancsin, J. and Phillips, M.J. (1984
Page 192 and 193:
172 Berry, R.J. (1972): The Influen
Page 194 and 195:
174 Brodskii, A.D. (1968): Simplifi
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176 Crovini, L., Perissi, R., Andre
Page 198 and 199:
Hudson, R.P. (1972): Principles and
Page 200 and 201:
180 Lengerer, B. (1974): Semiconduc
Page 202 and 203:
182 OIML (1985): International Reco
Page 204 and 205:
184 Rusby, R.L. (1975): Resistance
Page 206 and 207:
186 Seifert, P. and Fellmuth, B. (1
Page 208 and 209:
188 Ween, S. (1968): Care and Use o
Page 210 and 211:
190 Fig. A.1: Differences between t
Page 212 and 213:
National Institute of Metrology P.O
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194 Appendix C Some Suppliers of Va
Page 216 and 217:
196 Appendix D Calculations Relativ
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198 4. The calculation of the numbe
Page 220 and 221:
200 Appendix F Interpolation Polyno
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- temperature range from 0 °C to 1
Page 224 and 225:
204 where: d0 = 0; d5 = -3.17578007
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