Frequency Response of a Thermistor Temperature Probe in Air

Abstract

An analytical study was conducted of the thermal frequency response of an atmospheric temperature probe consisting of a thermistor bead with two lead wires soldered to thin support posts. Such probes are used in aircraft temperature sensors and for surface-layer turbulence studies. The results show the effects of the lead wires on the frequency response (amplitude and phase) of the probe for two end conditions of the lead wires: 1) fixed temperature at the mean free-stream value, and 2) adiabatic. For the smallest commercially available thermistor bead of approximately 200-µm diameter and for 20-µm-diameter platinum lead-wire lengths of about 0.8 mm, the conduction to the supports was found to be minimal for both end conditions. It was determined, however, that the lead wires themselves act as heat transfer fins and actually improve the frequency response over that of an ideal isolated bead. Model calculations show that the inclusion of multiple lead wires (four and six) connected mechanically, but not electrically or thermally, to supports would further improve the response. The thermal analysis is also applied to small type-E thermocouple junctions made of 12.5-, 25-, and 50-µm- diameter wires, and the results show that the lead wires also improve the frequency response.