Self-Heating: Bondable Resistance Temperature Sensors

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Self-Heating
In order to obtain a useful output from passive transducers such as TG temperature sensors , it is necessary to apply an excitation voltage, which results in self-heating of the sensors. This will cause a certain temperature rise in the surface to which the sensor is bonded, thus creating an error signal. Since TG sensors have a high temperature coefficient of resistance, it is not necessary to utilize high excitation levels to develop large outputs, and self-heating errors can easily be kept to insignificant values. When it is necessary to use high excitation levels to obtain maximum output signals, it should be noted that the largest practical sensor grid size should be chosen. The thermal conductivity and thermal capacity of the specimen will then determine the highest excitation level that can be used for a given self-heating error.

It is usually very simple to measure self-heating errors directly with TG sensors because the excitation can be varied under constant ambient temperature conditions while observing the change in output indication in degrees. A bridge excitation of 0.25V or less will usually produce self-heating errors of a fraction of one degree for standard sensors mounted on metallic specimens. Special attention should be given to self-heating errors when accurate measurements must be made on low thermal conductivity materials such as plastic or glass.

The attenuation factor incorporated into LST networks greatly reduces the excitation voltage from strain gage instrumentation, and self-heating errors are seldom encountered when this readout method is used with TG sensors.

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