techniques for approximating the international temperature ... - BIPM
techniques for approximating the international temperature ... - BIPM techniques for approximating the international temperature ... - BIPM
128 rise decreases with time but increases with exposure of the thermometer to high temperatures. The maximum secular rise is unlikely to exceed 0.1 °C over several years in well-constructed thermometers provided the glass is not heated beyond its exposure limit. The reversible changes appear as a hysteresis on thermal cycling. The bulb expands on exposure to high temperatures and does not return to its original volume immediately on cooling, resulting in a depression of the ice-point reading (and all other readings). Recovery may not be complete for 24 to 72 hours or, if the thermometer is cooled very slowly, no depression may occur. It is to monitor and correct for these changes in bulb volume that ice points are taken before and after calibration, as described earlier. In a good thermometer the temporary depression is small. For a variety of reasons the mercury in the capillary tube may become separated, leading to spurious readings until the separation is removed. Methods for doing this are described by Ween (1968); among these are centrifuging, shaking, rejoining in the contraction chamber or in the bulb, and (a very slow process) distillation. Ice-point checks are an easy way of detecting abnormal thermometer behaviour: a substantial increase in icepoint reading frequently indicates a separated column; a substantial decrease may mean that mercury is trapped in the expansion chamber or that the thermometer has been overheated. A small amount of mercury trapped in the expansion chamber can be detected by careful visual observation. More detailed discussion of the calibration and use of liquid-in-glass thermometers is given by Wise (1976), ASTM (1985a), Ween (1968), Thompson (1968), and BS (1985).
129 16. Industrial Platinum Resistance Thermometers Industrial platinum resistance thermometers (IPRTs) are constructed from platinum of a lower quality than is used for SPRTs; typically, values of W(H2O b.p.) range from 1.385 to 1.3925 and values of residual resistance from 10 -3 to 2 x 10 -2 . Since IPRT design must provide sufficient reproducibility in the presence of shocks, vibration, high pressure and other hostile environments such as are found in industrial applications, it does not comply with some requirements of the ITS-90, such as strain-free mounting of the wire. Therefore IPRTs are less reproducible than SPRTs. Nevertheless, it is possible to select IPRTs that are only an order of magnitude less reproducible than SPRTs and in a restricted temperature range this difference might be even smaller. In particular environments an IPRT can be even more reproducible than an SPRT where, of course, an SPRT would not be considered for use anyway. Although this chapter delineates many of the drawbacks of IPRTs, we must emphasize that they are among the most-commonly-used thermometers. It is possible to purchase IPRT elements of a selected design or configuration at relatively low cost, to assemble them into sheaths with a minimum of laboratory practice, to anneal them at 450 °C, and finally to calibrate them according to one of the proposed schemes and so obtain an approximation to the ITS-90 to within about ± 50 mK between -180 to 0 °C, and within about ± 10 mK between 0 and 420 °C [Actis and Crovini (1982), Bass and Connolly (1980), and Connolly (1982)]. 16.1 Quality of Industrial Platinum Resistance Thermometers IPRTs are manufactured in a great variety of models and the actual fabrication has a definite influence on their metrological quality. The accuracies discussed in Section 16.2 for IPRTs can be achieved only with thermometers that are suitably stable. This variability led to the specifications codes discussed in Section 16.3 which allow a minimum level of uniformity in the thermometer characteristics. The tolerances allowed are wide (~ 0.2 °C to 2 °C), however, so they are satisfactory only for the more common industrial uses. A wide variety of techniques have been devised for winding the platinum wire (Fig. 16.1 a). Many thermometers are formed by winding a fine wire or coil on a glass support and imbedding the winding in glass. This can introduce strain into the wires and cause contamination of the wires at higher operating temperatures. The configuration so far introduced that offers the best stability [Actis and Crovini (1982)] and the lowest hysteresis [Curtis (1982)] is that in which a platinum coil is supported inside the
- 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.
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- 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
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- 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
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- 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 168 and 169: 18.2.1 Type T Thermocouple 148 With
- Page 170 and 171: 150 within ± 0.5 to 0.7 percent. T
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- Page 180 and 181: 160 19. Thermometry in Magnetic Fie
- Page 182 and 183: 162 Type of Sensor T(K) Magnetic Fl
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128<br />
rise decreases with time but increases with exposure of <strong>the</strong> <strong>the</strong>rmometer to high<br />
<strong>temperature</strong>s. The maximum secular rise is unlikely to exceed 0.1 °C over several years in<br />
well-constructed <strong>the</strong>rmometers provided <strong>the</strong> glass is not heated beyond its exposure limit.<br />
The reversible changes appear as a hysteresis on <strong>the</strong>rmal cycling. The bulb expands on<br />
exposure to high <strong>temperature</strong>s and does not return to its original volume immediately on<br />
cooling, resulting in a depression of <strong>the</strong> ice-point reading (and all o<strong>the</strong>r readings). Recovery<br />
may not be complete <strong>for</strong> 24 to 72 hours or, if <strong>the</strong> <strong>the</strong>rmometer is cooled very slowly, no<br />
depression may occur. It is to monitor and correct <strong>for</strong> <strong>the</strong>se changes in bulb volume that ice<br />
points are taken be<strong>for</strong>e and after calibration, as described earlier. In a good <strong>the</strong>rmometer <strong>the</strong><br />
temporary depression is small.<br />
For a variety of reasons <strong>the</strong> mercury in <strong>the</strong> capillary tube may become separated,<br />
leading to spurious readings until <strong>the</strong> separation is removed. Methods <strong>for</strong> doing this are<br />
described by Ween (1968); among <strong>the</strong>se are centrifuging, shaking, rejoining in <strong>the</strong><br />
contraction chamber or in <strong>the</strong> bulb, and (a very slow process) distillation. Ice-point checks<br />
are an easy way of detecting abnormal <strong>the</strong>rmometer behaviour: a substantial increase in icepoint<br />
reading frequently indicates a separated column; a substantial decrease may mean<br />
that mercury is trapped in <strong>the</strong> expansion chamber or that <strong>the</strong> <strong>the</strong>rmometer has been<br />
overheated. A small amount of mercury trapped in <strong>the</strong> expansion chamber can be detected<br />
by careful visual observation.<br />
More detailed discussion of <strong>the</strong> calibration and use of liquid-in-glass <strong>the</strong>rmometers is<br />
given by Wise (1976), ASTM (1985a), Ween (1968), Thompson (1968), and BS (1985).
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