Precision Thermometer System for the Liquid Helium Region

A high-precision thermometer system has been developed using a carbon resistor (Allen-Bradley type) as the sensing element. The resistor is in the frequency determining circuit of a stabilized Wien bridge oscillator. Thus temperature changes are displayed as frequency shifts. The frequency is read directly from a Hewlett-Packard type 523B counter. When operated in the neighborhood of 100 kc at 4.2°K the sensitivity is approximately 50 kc/deg. This corresponds to a temperature resolution of 20 μdeg, when used with the above counter. Resolution can be improved by observing higher harmonics. The power input to the resistor is of the order of 10−2 μw. The time for a temperature measurement is 1 sec, and response times to changes are limited only by equilibrium in the thermometer. Absolute accuracy is essentially limited only by the determination of the helium vapor pressure during calibration.