Part I of this article, in the May 1997 issue of Sensors, addressed NTC thermistor characteristics, materials, and configurations. Part II, in June, took up methods of determining the level of uncertainty. The photo and figure numbers in Part III are continuous from Parts I and II.

Photo 3. Hart Scientific "heat port" type baths are designed for precision thermistor calibrations because of their high stability and uniformity, typically less than ±0.005ºC. Some Hart baths feature stability specs less than ±0.0008ºC. (Photo courtesy of Hart Scientific.) The guidelines presented here on equipment and methods typically used for thermistor testing will help users accurately identify their application requirements and evaluate a supplier's capabilities. These recommendations are neither exhaustive nor meant to imply that every thermistor user should set up a complete temperature calibration laboratory. In many cases, such an exercise would be impractical and unnecessarily costly, and the services of an outside lab might be a better choice. Even when taking this route, though, the user can profit from knowing how to evaluate the capabilities of candidate labs.