A Thermometer for Fast Response in Cryogenic Flow

The measurement of transient temperatures in cryogenic fluid flow requires a highly sensitive, intrinsically fast sensor that is in good thermal contact with the fluid but in poor thermal contact with the solid walls confining the fluid. A resistance thermometer made from a 1 µm thick silicon layer on a 125 µm thick sapphire substrate has a calculated intrinsic response time of about 10 ns at 4 K, and its sensitivity is comparable to germanium or carbon thermometers in the range of 1 – 80 K. This paper describes a novel construction method to mount the small silicon-on-sapphire thermometer in an oscillating fluid flow. The large surface area of the thermometer provides good thermal contact with the fluid, while the suspension ensures poor thermal contact with the holder, maintains its fast response time, and withstands high velocities and frequencies of fluid oscillation. A self-heating response time of 300 ns was measured at 4 K in liquid and gaseous helium. Repeatability of the thermometer is ± 10 mK at 4 K. Examples of the performance of this thermometer for helium gas oscillations in the frequency range of 1 Hz to 12 kHz are given.