As the current increases, the self-heating causes the resistance to decrease even more. This is the part of the slope where the graph has a negative slope, and is the negative- resistance region of the thermistor. There are two dominant factors that determine the resistance tolerance. The first is the manufacturing tolerance in the NTC's nominal resistance. The second factor is the tolerance in the material constant b , which determines the variation of resistance with temperature.
Manufacturers specify tolerances on both at the rated temperature.
When the tolerances for a thermistor are specified in this manner, the use of the equation is implied. This is another measure of tolerance and is expressed as a temperature tolerance over a temperature range. Interchangeability gauges how close the resistance-temperature curves of two thermistors match. High interchangeability helps keep costs down since equipment does not need to be calibrated or adjusted for individual thermistors. Interchangeability is also a major advantage where NTC thermistors are used as cheap, disposable temperature probes, for example, in medial applications.