techniques for approximating the international temperature ... - BIPM

More documents

Recommendations

Info

156 In the case of the Types K and E thermocouples and the negative legs of Types T and J thermocouples (all containing nickel or nickel-chromium), the thermodynamic equilibrium is limited by diffusion of constituents (temperature range from 300 °C to 500 °C) and magnetic-transformation processes (temperature range from 50 °C to 200 °C) so that a high accuracy (0.1 K) can be obtained only by special heat treatment, using specified calibration procedures, and in special application conditions [Burley et al. {1982)]. 18.5 Guidelines for Proper Handling and Use -Installation and Sources of Errors For accurate temperature measurements by means of base-metal thermocouples, in particular by metal-sheathed thermocouples, the following guidelines should be considered [ASTM (1981)]: - be sure that the sheath or protection tube material will survive the environment. - be sure that the thermocouple assembly is fully annealed for maximum life of sheath or protection tube and stability of the thermocouple calibration. - remember that the life of the thermocouple will decrease with higher application temperatures and smaller thermoelement and sheath diameters and is limited by grain growth (Tables 18.2 and 18.4). - in the case of metal-sheathed thermocouples, it must be possible to bend the sheath around a mandrel twice the sheath diameter without damage. - in metal-sheathed thermocouples, the appearance of moisture in the assembly is indicated by a diminution in insulation resistance; this can cause an error in temperature measurement by electrical shunting. Moisture can be removed by heating and sealing the exposed ends of the thermocouple. - metal-sheathed thermocouples should not be repeatedly bent at the same location as this work-hardens the sheath and may change the thermocouple calibration. To evaluate the performance of a thermocouple circuit, the numerous possible sources of error should be considered: thermal shunting; electrical shunting; calibration; decalibration (from instability or drift); extension wires; reference junction. A thermocouple, just as any other contacting temperature sensor, disturbs the temperature distribution of any object to which it is attached because it has a finite size and Iconducts heat away from (or to) the object. The thermocouple itself loses heat to (or gains heat from) its surroundings by conduction, convection, and radiation. This heat transfer can cause the thermocouple hot junction to be at a different temperature from that of the object. These effects cause a thermal-shunting error, the magnitude of which depends
157 largely on the method of thermocouple installation. This error is avoided when the portion of the thermocouple near the hot junction is isothermal and at the temperature of the object whose temperature is to be measured. This error can be especially large when the heat transfer from the measuring object to the thermocouple is poor and/or the heat transfer from the thermocouple to the surroundings is large (such as in surface temperature measurement). At higher temperatures, where the electrical resistivity of the insulator will be lower, an electrical shunting will occur and can cause temperature-measurement errors, especially at temperatures above 1500 °C. This effect is greater in metal-sheathed thermocouples than it is in the classic type of construction because of the larger area of electrical contact between the insulator and the thermoelements. Calibration errors depend upon the accuracy of the calibration standards and the calibration method applied (Sec. 18.6). Decalibration errors or drift, i.e. a change of the emf-temperature relationship with time, can occur even if the thermocouples are heat-treated, assembled, calibrated, and installed with utmost care. In the literature (e.g. [Kinzie (1973)]) many results are reported that show the same trends of decalibration but the magnitudes differ because of undetected differences in experimental variables that control the drift rates. Several factors cause a thermocouple to drift, such as: chemical reactions of the thermoelements with the gaseous environment, with the electrical insulator, or with the object whose temperature is to be measured (including impurities in the environment, insulator, or object); metallurgical transformations (such as order-disorder transformations or secondary recrystallization); loss of alloying elements by selective evaporation or oxidation at higher temperatures; transmutation by nuclear radiation. Drift rates generally increase rapidly with increasing temperature and are larger for smaller-diameter thermoelements. Extension-wire errors arise from the differences between the thermoelectric properties of the thermoelements and of the corresponding extension wires. In highprecision measurements the use of such wires should be avoided whenever possible. If extension wires must be used due to installation conditions, any error can be reduced by calibration of the complete thermocouple/extension-wire assembly and by ensuring a uniform temperature at the junctions. In thermocouple thermometry all emf measurements are referred to the temperature of the reference junction. Any error in this temperature, therefore, is directly involved in the error of the measured temperature. Reduction of this error is possible by ensuring that both thermoelement reference junctions have the same temperature and that this
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 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 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
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 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
Page 162 and 163: 142
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 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
Page 220 and 221: 200 Appendix F Interpolation Polyno
Page 222 and 223: - temperature range from 0 °C to 1
Page 224 and 225: 204 where: d0 = 0; d5 = -3.17578007
show all

techniques for approximating the international temperature ... - BIPM

Create successful ePaper yourself

Delete template?

Save as template?