Stability and Calibration of Miniature Platinum Resistance Thermometers

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Abstract
Miniature platinum resistance thermometers have been evaluated for use as substitutes for the capsule type resistance thermometer in which smaller size but comparable precision and accuracy are required. The thermometers tested weigh about 1 g. The thermometers were calibrated between 12 and 373 K at fixed points and by comparison with an N.B.S. calibrated capsule type platinum resistance thermometer. The values obtained for the fundamental constants (α,β,δ) of the Callendar‐Van Dusen equation meet the requirements of the International Practical Temperature Scale above 90 K. At temperatures below 90 K, the RT/R273.15K drops less rapidly than for the capsule type thermometers, indicating a possible geometrically caused strain in the platinum. Because of this difference, it was not possible to interpolate between measured points by deviation plots in the manner ordinarily used when calibrating thermometers below 90 K. It was found that by using the data points as a basis for Lagrange's interpolation polynomials, a table could be generated that fit a smooth curve through the data to better than 0.002 K. Over a 12 month period the R0 of the miniature thermometers have been reproducible to 10 ppm.
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