Achieving 0.25 mK Uncertainty with an Integrated-Circuit Resistance Thermometer Readout

Document(s): 

Achieving 0.25 mK Uncertainty with an Integrated-Circuit Resistance Thermometer Readout (78.91 KB)

Resistance bridges, when used for temperature measurements with SPRTs, are able to achieve uncertainties better than 1 ppm. However, they have several shortcomings that prohibit their use in many applications. Among these are cost, size, slow speed, and limited range. An endeavor was made by the author to design a readout for resistance thermometers that achieves less than 1 ppm uncertainty in resistance ratio while overcoming some of the problems of resistance bridges. A new approach was taken with a design that uses the latest integrated-circuit analog-to-digital converters. This allows the instrument to have lower cost, smaller size, the capability of increased speed, and additional features. Special effort was made to reduce errors caused by component drift, thermoelectric EMF, component offset, electrical noise, and nonlinearity. The new resistance thermometer readout was tested to identify and evaluate sources of measurement uncertainty. The combined uncertainty was calculated for resistance ratio and W(T90) measurements of an SPRT with self-heating corrections. Measurements made with the resistance thermometer readout were compared with measurements made with a resistance bridge. The results show that the standard uncertainty of the new resistance thermometer readout is about 0.086 · 10-6 in measuring resistance ratio at 25Ω/100Ω and about 0.73 · 10-6 in measuring W(T90) at the melting point of gallium.