Correction for Self-Heating When Using Thermometers as Heaters in Precision Control Applications
- Tuesday, 01 March 2011
In precision control applications, thermometers have temperature-dependent electrical resistance with germanium or other semiconductor material thermistors, diodes, metal film and wire, or carbon film resistors. Because resistance readout requires excitation current flowing through the sensor, there is always ohmic heating that leads to a temperature difference between the sensing element and the monitored object.
In this work, a thermistor can be operated as a thermometer and a heater, simultaneously, by continuously measuring the excitation current and the corresponding voltage. This work involves a method of temperature readout where the temperature offset due to self-heating is subtracted exactly.
The true temperature of an object is Tobject = Tsensor – I × V × K, where I × V (measured current times the measured voltage) is the power dissipated in the sensor, and K is the thermal resistance. Because the relation between the sensor electrical resistance and its temperature is typically not approximated well by a single simple function over a wide temperature range, and because the thermal impedance is often temperature dependent, this solution is only easily implemented in hardware for thermistors mounted with small thermal resistance, and operating in a narrow range of set points. A software implementation is possible for a wider range of conditions, but a prior mapping of thermal resistance vs. temperature is needed.
This work was done by Konstantin Penanen, Michael E. Ressler, Hyung J. Cho, and Kalyani G. Sukhatme of Caltech for NASA’s Jet Propulsion Laboratory. NPO-46894