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112 Table 11.1: Typical Time Constants, τ, for a Carbon Resistance Thermometer [Linenberger et al. (1982)]. Temperature 4.2 K 77 K _________________________________________________________________________ τ(ms) τ(ms) _________________________________________________________________________ Environment Liquid Vapour Liquid Vapour _________________________________________________________________________ Type of Sensor _________________________________________________________________________ AB 1/8 W 220 Ω 6 7 113 7276 AB 1/2 W 220 Ω 30 31 426 3290 _________________________________________________________________________ leads are not thermally grounded. Grinding the resistor clearly shortens the thermal time constant. Also, to improve time response, attempts have been made to use carbon thin films deposited on different substrates. Time constants approaching 0.42 ms at 4.2 K have been achieved [Bloem (1984)]. : 11.5 Influence of External Factors a. Pressure CRTs are only slightly dependent upon pressure [Dean and Richards (1968) and Dean et al. (1969)]; a typical value for the pressure dependence is ∆R/R ~ -2 x 10 -9 Pa -1 . b. Radiation CRTs are relatively insensitive to nuclear irradiations. After long exposure to gamma rays and fast neutrons, changes observed in resistance were less than 1 % at 20 K [NASA (1965)]. c. Humidity CRTs are very sensitive to humidity. The nominal resistance value R(300 K) increases by 5 to 10% after the resistor has been soaked for 240 h at 95% humidity [Ricketson (1975)]. Since ∆R/R is about the same at 77 K and at 300 K, the indication is that the absorbed water introduces, a temperature dependent resistance, i.e. changes in calibration come from changes in the exponential coefficient. The process is not reversible, but the resistance can be reduced by heating the unit. d. Magnetic Field The effects of magnetic fields on CRTs is discussed in Chapter 19.
11.6 Stability and Reproducibility 113 It is difficult to determine the extent of the repeatability of CRTs. Reports on repeatability vary widely, from ± 1 mK up to 2% change in resistance at 4.2 K. By cycling resistors between room temperature and the temperature of use, it is possible to select units that are stable to within 10 mK. Nevertheless, one must be alert for changes in the R-T characteristic. Provided that they are not mistreated between runs, both Allen-Bradley and Speer units will retain their calibration within about 1 % over a two year span and 30 or 40 thermal cyclings. When the resistance is monitored at a fixed temperature, a drift with time is observed which is mostly towards higher resistance, giving an apparent reduced temperature. It occurs in all units tested [Ricketson (1975), Johnson and Anderson (1971), Forgan and Nedjat (1981 )]. For example, at 77 K, drift rates have been as high as 50 mK/h just after immersion, decreasing to 25 mK/day after a two-day soak. The higher the temperature sensitivity of the unit, the greater the drift in terms of ∆T/T. This means that the problem becomes more serious for a given thermometer as the temperature is reduced. Resistors of the same nominal value and brand exhibited the same drift rates to within about 15%. Therefore, the drift is intrinsic to the resistance material itself and is probably associated with a decrease in carrier concentration. An increase in the resistance at liquid helium temperature is always observed after the first few thermal cycles (2% for a 1000 Ω Allen-Bradley unit). The phenomenon is associated with carbon granule rearrangement due to thermal shocks. Typically, maximum changes observed for an Allen-Bradley unit of 220 Ω are 13 mK at 4.2 K, 337 mK at 20 K and 2 K at 60 K [Ricketson (1975)]. After nearly twenty thermal cyclings between low temperature and 300 K during two years, the change in resistance of Matsushita units have remained within 1.5% [Kobayasi et al. (1976)]. The observed instabilities can be roughly explained by the thermal history of the unit, remembering the extreme sensitiveness to annealing and thermal heating of a CRT. Broadly speaking, CRTs are less stable by more than an order of magnitude than germanium thermometers. Considerable improvement might be achieved if the low-cost CRTs were treated with as much care as encapsulated germanium sensors. Nevertheless, it is always wise to check that the resistance has not drifted nor undergone a step change during measurements. Whether a carbon resistance thermometer requires a new
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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
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Centre for Quantum Metrology Nation
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2. Type J viii a) temperature range
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c) temperature range from 1664.5 °
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xii
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xiv Acknowledgments This monograph
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xvi 3.3.2 Melting Points of Gold (1
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xviii 11.3 Thermal Contact 111 11.4
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1 1. Introduction The Comité Consu
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3 thermometers except in special ex
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5 The accuracies with which tempera
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Table 1.1: Summary of Some Properti
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PART 1: TECHNIQUES AND THERMOMETERS
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11 Fig. 2.1: One form of apparatus
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13 Fig. 2.3: Flow cryostat, shown w
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15 Fig. 2.4: Stirred liquid bath fo
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17 contained within a cylindrical c
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19 Fig. 2.5: Schematic drawing of a
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21 device SRM 767 [Schooley et al.
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23 Table 3.1 : Current Best Estimat
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25 The widely-used, but not very re
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28 fraction of sample melted) can g
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30 Fig. 3.2a: Apparatus for the cal
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32 Fig. 3.3: Sealed cell for realiz
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34 Final readings of the thermomete
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36 temperatures differ (usually) sy
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38 Fig. 3.4: Cross sectional drawin
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40 Fig. 3.5: Miniature graphite bla
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42 4. Germanium Resistance Thermome
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44 Fig. 4.3: Example of the Π-type
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46 Fig. 4.4: Differences between dc
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48 - conversely, p-doped thermomete
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50 Fig. 4.6: Effect of a radio-freq
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52 that it will not be subject to m
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ln R n = ∑ i= 0 54 ⎛ ln T - P
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56 Fig. 5.1: Resistance (Ω) and s
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58 thermometer wires) caused a more
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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.
Page 108 and 109: 8.1 General Remarks 88 8. Platinum
Page 110 and 111: 90 For a group of 45 thermometers h
Page 112 and 113: 92 More recently, Seifert [(1980),
Page 114 and 115: 94 For thermometers with W(4.2 K) <
Page 116 and 117: 96 after 500 h at 1700 °C in air,
Page 118 and 119: 98 normally extends about 50 cm bac
Page 120 and 121: 100 inhomogeneities result from the
Page 122 and 123: 9.5 Approximations to the ITS-90 10
Page 124 and 125: 104 10. Infrared Radiation Thermome
Page 126 and 127: 106 almost negligible. The final ac
Page 128 and 129: 108 11. Carbon Resistance Thermomet
Page 130 and 131: 110 Fig. 11.1: Resistance-temperatu
Page 134 and 135: 114 calibration after each cool-dow
Page 136 and 137: 116 12. Carbon-Glass Resistance The
Page 138 and 139: 118 Fig. 12.1: Resistance-temperatu
Page 140 and 141: 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
Page 146 and 147: 126 For a thermometer graduated abo
Page 148 and 149: 128 rise decreases with time but in
Page 150 and 151: 130 Fig. 16.1: (a) Fabrication of I
Page 152 and 153: 132 capillaries of a twin- (or four
Page 154 and 155: 134 advised to choose the thermomet
Page 156 and 157: 136 Fig. 16.6: Distribution of the
Page 158 and 159: 138 and between different thermomet
Page 160 and 161: 140 therefrom) can then be used wit
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Page 164 and 165: 144 Fig. 16.8: Tolerances for indus
Page 166 and 167: 146 Stability on thermal cycling is
Page 168 and 169: 18.2.1 Type T Thermocouple 148 With
Page 170 and 171: 150 within ± 0.5 to 0.7 percent. T
Page 172 and 173: 152 insulation even though it be af
Page 174 and 175: 154 Table 18.4: Recommended Sheath
Page 176 and 177: 156 In the case of the Types K and
Page 178 and 179: 158 temperature is measured with th
Page 180 and 181: 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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