Thermometrics
U.S. Sensor
YSI
Considerations in the use and testing of thermistors
courtesy of Fenwal Electronics
1. Sensitivity to Temperature: Thermistors undergo a very large change in resistance for a small change in temperature, i.e., in the order of 4% to 5% per degree Centigrade at room temperature. This is one of its most valuable properties from a use standpoint and conversely is a severe difficulty when precise measurement of thermistor resistance is required. Be aware of this factor in any testing that you do -- even a 0.1 degree C change in temperature will change the resistance 0.5%.
2. Self Heating: Because a thermistor has electrical resistance, passing a a current through it will generate heat internally in accordance with Ohm's law. This property makes thermistors very useful in such applications as liquid level controls, fluid flow measurements, and gas analysis. This effect also influences test procedures. A typical glass coated thermistor will change its internal temperature 1 degree C for wach milliwatt of power dissipated within it. This in turn will change its resistance 4% to 5% as previously mentioned.
3. Measeurement of Resisitance: Precise measurement of thermistor resistance is usually done throught the use of an accurate Wheatstone bridge and associated equipment including a very closely controlled constant temperature bath. Less exact testing may be accomplished through the use of a VOM meter or an electronic VOM meter. The electronic meter is the preferred type of the two because it is more sensitive and consequently introduces much lower self-heating errors.
4. Summary:
A. In all use and testing of thermistors, give due consideration to their great sensitivity to temperature changes and to the ease with which they self-heat. In testing, accuracy of the test equipment itself provides a third source of error.
B. Thermistors are inherently very stable devices. They can be expected to change less than 0.5% per year under the most severe recommended service. Accordingly, if you find changes it is almost always associated with the three errors mentioned in "A" above.
This page originally created on July 14, 1998 by John Bennardo and Mike Victor