Thermometry resistance

Temperature (R) Design of treatment/recovery Resistance thermometry... 

The ITS-90 scale is designed to give temperatures T90 that do not differ from the Kelvin Thermodynamic Scale by more than the uncertainties associated with the measurement of the fixed points on the date of adoption of ITS-90 (January 1, 1990), to extend the low-temperature range previously covered by EPT-76, and to replace the high-temperature thermocouple measurements of IPTS-68 with platinum resistance thermometry. The result is a scale that has better agreement with thermodynamic temperatures, and much better continuity, reproducibility, and accuracy than all previous international scales. 

The resistance thermometry is based on the temperature dependence of the electric resistance of metals, semiconductors and other resistive materials. This is the most diffused type of low-temperature thermometry sensors are usually commercial low-cost components. At very low temperatures, however, several drawbacks take place such as the low thermal conductivity in the bulk of the resistance and at the contact surface, the heating due to RF pick up and overheating (see Section 9.6.3)... 

The frequency response of the detection system is of low-pass type for characteristog-raphers and band-pass for bridges (see Section 10.4). In both types of measurements the narrowing of the bandwidth corresponds to a longer time of measurement. Depending on the chosen detection system, several problems (true traps) may be encountered in resistance thermometry. 

As we have seen in Section 9.5.3, in the case of resistance thermometry, the signal produced by a low-temperature thermometer is very low (microvolt range). Low-pass filters are not sufficient to narrow the detection bandwidth in order to get a suitable signal to noise ratio (S/N). Bandpass filters are needed. The most commonly used method is the synchronous demodulation, usually simply called lock-in technique, as shown in the block diagram of Fig. 10.7. 

Quinn, T. J., 1983, Temperature, Academic, London. A comprehensive review of thermometry over the temperature range 0.5-3000 K. The book includes chapters on resistance thermometry and thermocouples, and is extensively referenced. 

J-L. Riddle, G.T. Furukawa, and H.H. Plumb, Platinum Resistance Thermometry,... 

Waidner and Burgess, Optical Pyrometry, Bureau of Standards, Sci. Paper 11, 1905. Waidner and Burgess, Platinum Resistance Thermometry, Bureau of Standards, Sci. Paper 124, 1909. 

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