techniques for approximating the international temperature ... - BIPM

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150 within ± 0.5 to 0.7 percent. Tungsten as the positive leg can pose problems because heating it to or above its recrystallization temperature (about 1200 °C) causes embrittlement, resulting in a loss of room-temperature ductility, an effect that does not occur with legs containing rhenium (3 5 percent). Use is only possible in a vacuum or in high-purity reducing (hydrogen) or inert atmospheres. 18.3 Construction A complete thermocouple temperature-sensing assembly consists of: a sensing element assembly including, in its most basic form, two dissimilar wires supported by an electrical insulator and joined at one end to form a measuring junction; a protection tube (ceramic or metal), sometimes referred to as a thermowell, which protects the sensing element assembly from the deleterious effects of corrosive, oxidizing, or reducing atmospheres; and a connector for the wire terminations of the sensing element assembly. It is, of course, necessary to ensure that the sheath and protection tube do not themselves contaminate the thermocouple. The reader should consult ASTM (1981) for details. 18.3.1 Thermoelements The positive and negative thermoelements for a given type of thermocouple, as supplied by anyone manufacturer, will conform to the calibration curve for that thermocouple within specified limits of error. However, because materials used for a given thermoelement by various manufacturers may differ slightly in thermal emf, larger errors may occur if positive and negative thermoelements from different sources are combined. Recommended maximum-temperature limits according to the wire diameter of the thermoelements are given in Table 18.2. These limits apply to protected thermocouples; that is, to thermocouples in conventional closed-end protecting tubes. In actual operation, there may be instances where the temperature limits recommended can be exceeded. Likewise, there may be applications where satisfactory life will not be obtained at the recommended temperature limit. 18.3.2 Sheaths A wide variety of materials are available for thermocouple sheaths. Up to about 100 °C the commonest include enamels and varnishes, cloth, plastics, and rubber. Most of these organic materials begin to decompose, or at least to conduct, at higher temperatures. Some silicone varnishes may be satisfactory up to about 300 °C, fibreglass up to 400 °C and with silicone as the binder up to 500 °C. The sheath has to ensure a sufficient electrical
151 Table 18.2: Recommended Upper Temperature Limits for Protected Thermocouples [ASTM (1987b), ASTM (1981)]. Thermocouple Diameter 3.2 mm 1.6 mm 0.8 mm 0.51 mm 0.33 mm _________________________________________________________________________ J (JP) 760 °C 590 °C 480 °C 370 °C 370 °C E (IN, TN, EN) 870 °C 650 °C 540 °C 430 °C 430 °C T (TP) -- 370 °C 260 °C 200 °C 200 °C K (KP, EP, KN), N 1260 °C 1090 °C 980 °C 870 °C 870 °C _________________________________________________________________________ Table 18.3: Properties of Refractory Oxides [ASTM (1981)]. Material Composition Maximum use Thermal stress Thermocouple Temperature Resistance Commonly used with _________________________________________________________________________ Sapphire crystal 99.9 AI2O3 1950 °C very good any Sintered alumina 99.8 AI2O3 1900 °C good any Sintered beryllia 99.8 BeO 1900 °C excellent high temperature Sintered magnesia 99.8 MgO 1900 °C fair-poor any Sintered mullite 72 Al2O3, 1750 °C good base-metals 28 SiO2 Sintered stabilized 92 ZrO2, 2200 °C fair-good high zirconia 4 HfO2, 4 CaO temperature Silica Glass 99.8 SiO2 1100 °C excellent base-metal Mullite porcelain 70 Al2O3, 1400 °C good base-metal 27 SiO2 High alumina 90-95 Al2O3, 1500 °C very good base-metal, noble porcelain 4-7 SiO2 metal below 1100 °C for a variety of constructions _________________________________________________________________________
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BUREAU INTERNATIONAL DES POIDS ET M
Page 3 and 4:
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,
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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
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 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
Page 218 and 219: 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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