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18.2.1 Type T Thermocouple 148 Within its range ( -200 to 350 °C) this thermocouple is the most accurate base-metal thermocouple, due in large part to the ready availability of high-purity, strain-free copper. Above 370 °C it is limited by oxidation of the copper. The accuracy usually attainable is 0.1 K, but with special calibration it may approach 0.01 K from 0 °C up to 200 °C. Because copper is universally used for electrical conductors, one may easily obtain thermal-free connections to the measuring instruments. On the other hand the high thermal conductivity of the copper thermoelement may be disadvantageous in some applications. The type T thermocouple can be used in a vacuum and in oxidizing, reducing, or inert atmospheres. 18.2.2 Type J Thermocouple This thermocouple is most commonly used for industrial purposes in the temperature range from 0 to 760 °C in both oxidizing and reducing atmospheres. The accuracy attainable is fair to poor (0.1 to 0.5 K below 300 °C, 1 to 3 K above 300 °C); the stability is fair below 500 °C but poor above 500 °C because of the higher oxidation rate of the iron thermoelement. 18.2.3 Type K Thermocouple This thermocouple has a wide temperature range ( -200 to 1260 °C) ; in the range : above 0 °C it is the most widely used thermocouple. It is recommended for continuous use in oxidizing or inert atmospheres. At high temperatures it and Type N are the most oxidationresistant base-metal thermocouples. In reducing, sulfurous atmospheres or in a vacuum it is necessary to use suitable protection tubes. Atmospheres with reduced oxygen content promote the so-called "green-rot" corrosion of the positive thermoelement because of the preferential oxidation of chromium; this causes large negative errors in calibration. This is most serious in the temperature range from 800 to 1050 °C, for example when the thermocouple is exposed to carbon dioxide in this temperature range. Green-rot corrosion frequently occurs when thermocouples are used in long unventilated protecting tubes of small diameter. It can be minimized by increasing the oxygen supply with large-diameter protecting tubes or ventilated tubes. Both elements of the Type K thermocouple are subject to oxidation in air above about 850 °C, but this normally leads to only a very slow increase with time of the thermocouple emf for a given temperature. Type K can be used above 1200 °C for very short periods in air without serious decalibration. Changes of up to 10 K can occur in the positive thermoelement due to short-range ordering in the
149 nickel-chromium solid solutions, especially if the thermoelements are annealed or used near 400 °C [Fenton (1972)]. The accuracy attainable is 0.1 K below 300 °C and 1 K up to about 1000 °C. Stability is fair. The accuracy and stability of this thermocouple are limited by its susceptibility to inhomogeneities and mechanical strain (cold work) and to previous heat treatment. 18.2.4 Type E Thermocouple This thermocouple is recommended for use up to 870 °C in oxidizing or inert atmospheres. In reducing atmospheres, marginally oxidizing atmospheres, and in vacuum it is subject to the same limitations as Type K thermocouples. Type E thermocouples develop the highest emf per kelvin (80 µV/K at 700 °C) of all the commonly-used types and are often used primarily because of this feature. Their accuracy is better than 1 K below 300 °C and 1 to 3 K from 300 to 1000 °C. 18.2.5 Type N Thermocouple The Type N thermocouple shows enhanced thermoelectric stability, particularly at high temperatures, compared to the other standard base-metal thermocouples (except Type T), obtained as a result of careful choice of composition of the two elements. By suitable adjustment of composition [Burley (1972), Burley et al. (1978), Burley et al. (1982)], it has been possible to achieve higher oxidation resistance at temperatures above 1000 °C; reduce short-term cyclic variations in thermal emf due to structural phenomena (temperature range from 300 °C to 500 °C); and reduce time-independent perturbation in thermal emf due to magnetic transformations (temperature range from 50 °C to 230 °C). Furthermore, the Type N thermocouple can now be obtained in a mineralinsulated metal-sheathed construction (Section 18.3.4) with a Nicrosil sheath [Burley (1987)]. The result is an overall better long-term stability, higher reproducibility, and higher accuracy than for all other base-metal thermocouples (except Type T). Below about 500 °C, however, these advantages are not dramatic; for example, from 0 °C to 500 °C they may be more reproducible than Type K by a factor of about 2. 18.2.6 Tungsten-Rhenium Thermocouple There are several tungsten-rhenium alloy thermocouples in use, but the only one standardized is tungsten 5% rhenium/tungsten 20% rhenium [CMEA (1978), GOST (1977)]. All have been used at 2500 °C to 2750 °C but general use is below about 2200 °C. These thermoelements are supplied as matched pairs guaranteed to meet an emf output of producer-developed tables within ± 1 percent or of standard reference tables
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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*
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62 transitions, but it could be app
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n ∑ i= 2 64 L x [ Π − k ] P =
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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,
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 132 and 133: 112 Table 11.1: Typical Time Consta
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
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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 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
Page 182 and 183: 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
Page 196 and 197: 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
Page 214 and 215: 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
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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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