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
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Correction for Self-Heating When Using Thermometers as Heaters in Precision Control Applications
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Overview
The document titled "Correction for Self-Heating When Using Thermometers as Heaters in Precision Control Applications" (NPO-46894) from NASA's Jet Propulsion Laboratory addresses the challenges associated with using thermometers, particularly those with temperature-dependent electrical resistance, in precision control applications. These thermometers, which include thermistors, diodes, and various types of resistors, generate self-heating due to the excitation current required for resistance readout. This self-heating can lead to significant temperature measurement errors, especially when the thermometer is simultaneously used as a heater.
To mitigate these errors, the document proposes a method for accurately measuring temperature by accounting for the self-heating effect. The true temperature of the monitored object is calculated using the formula: Tobject = Tsensor – IVK, where I is the measured current, V is the measured voltage, and K represents the thermal resistance. This approach allows for the correction of temperature readings by subtracting the offset caused by self-heating.
The document emphasizes that while this correction method can be implemented in hardware for thermistors with small thermal resistance operating within a narrow temperature range, a software implementation is also feasible. However, the latter requires prior mapping of thermal resistance against temperature to ensure accuracy across a wider range of conditions.
The novelty of this method lies in its ability to facilitate high-accuracy temperature measurements while allowing the thermistor to function as a heater simultaneously. This dual functionality is particularly beneficial in precision control applications where maintaining accurate temperature readings is critical.
The document serves as a technical support package under NASA's Commercial Technology Program, aiming to disseminate aerospace-related developments with broader technological, scientific, or commercial applications. For further inquiries or assistance, the document provides contact information for the Innovative Technology Assets Management at JPL.
In summary, this technical brief outlines a solution to the self-heating problem in thermometers used as heaters, presenting a method for accurate temperature measurement that is essential for precision control applications in various fields.
