techniques for approximating the international temperature ... - BIPM
techniques for approximating the international temperature ... - BIPM techniques for approximating the international temperature ... - BIPM
9.5 Approximations to the ITS-90 102 Platinum rhodium thermocouples can be calibrated by comparison with a laboratory standard thermocouple in an electric furnace by the techniques described in Chapter 2. Typical accuracies achievable in this way are listed in Table 18.7. A thermocouple can also be calibrated at fixed points according to the methods described in "Supplementary Information for the ITS-90" [CCT (1990)]. However, for most thermocouple measurements, the requirements described there are unnecessarily extreme. Because of the much lower accuracy expected in secondary measurements, the quality of the furnace can be lower, melts as well as freezes are adequate, the metal need not be as pure, the mass of metal may be smaller, etc. Especially at high temperatures, where it becomes difficult to provide proper furnaces and crucibles for freezing-point determinations, fixed-point calibrations can be made by the wire, or wire-bridge, method as described in Sec. 3.3.2. Similarly, the thermocouple can be calibrated using miniature fixed points as described in Sec. 3.3.3 [Tischler and Koremblit (1982)]. Whatever method is used, to complete the calibration the differences of the thermocouple emfs from the standard-reference-table emfs at the appropriate assigned temperatures on the ITS-90 for the particular fixed points are plotted and a smooth curve is drawn through all points including the origin. These differences are generally small and the curve is only slightly non-linear. It is recommended that a second- to fourth-degree polynomial be fitted to these differences [Jones and Hall (1978); Bedford and Ma (1980)]. This deviation curve in conjunction with the reference table [Powell et al. (1974); lEC (1977)] gives the emf-versus-temperature characteristics of the individual thermocouple. The accuracy of the calibration depends upon the methods used, the temperature range of the calibrations, and the thermocouple materials. The calibration accuracy, however, is seldom the limiting factor in thermocouple measurements, being outweighed by errors associated with the type of installation and with instability and drift of the thermocouple emf. As a rough guide, overall accuracies may be better than ± 0.5 K to the gold point (1064 °C), ± 1.0 K to the palladium point (1555 °C) and ± 2.0 K to the platinum point (1769 °C). The precision attainable with thermocouples under well-defined and constant conditions (immersion depth and temperature gradient) is somewhat better than these accuracies. If only the wire-bridge method of calibration is used, accuracies can be worse. For example, in a comparison of wire-bridge Au and Pd calibrations with optical pyrometer calibrations, agreement was within 0.7 K at Au and 1.5 K at Pd amongst three laboratories [Crovini et al. (1987)]. Comparison of quadratic interpolation of these differences of Au and Pd emfs from the standard table with the pyrometer calibrations showed disagreement of from ± 0.2 K to ± 0.9 K for different
103 thermocouples between the two fixed points. Use of this quadratic interpolation below 1064 °C or above 1554 °C was not recommended.
- 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 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
103<br />
<strong>the</strong>rmocouples between <strong>the</strong> two fixed points. Use of this quadratic interpolation below<br />
1064 °C or above 1554 °C was not recommended.
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