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Terminology

Alpha (α) or Zero-Power Temperature Coefficient of Resistance - The zero-power temperature coefficient of resistance is the ratio at a specified temperature (T), of the rate of change of zero-power resistance with temperature to the zero-power resistance of the thermistor. Beta Value (β) - An indicator of the slope of the curve of a NTC thermistor's resistance-to-temperature characteristic. Since the Beta value is not a true material constant, it is adequate for use in the Beta equation only for narrow temperature ranges. The Beta equation has been replaced by the Steinhart-Hart equation for applications requiring more accurate resistance vs. temperature interpolation over wider temperature ranges. Dissipation Constant (δ) - The dissipation constant is the ratio, (in milliwatts per degree C) at a specified ambient temperature, of a change in power dissipation in a thermistor to the resultant body temperature change. Because the dissipation constant of an NTC thermistor is not a true constant, some manufacturers refer to this as the dissipation factor. Interchangeable - A term used to describe a family of tight-tolerance NTC thermistors that meet a manufacturer's published resistance-temperature curve characteristics within a specified temperature tolerance over a specified temperature range. “Interchangeable” implies that an NTC thermistor can be factory-installed or field-replaced without additional calibration. NTC - The NTC of a thermistor is the negative temperature coefficient expressed as the negative percent change in zero-power resistance per one degree Celsius increase in temperature. Resistance Ratio Characteristic - The resistance ratio characteristic identifies the ratio of the zero-power resistance of a thermistor measured at 25 °C to the resistance measured at another temperature. Typical industry standard resistance ratios are R0/R50, R0/R70, and R25/R125, where Rt is the resistance in ohms at temperature “t” in °C. R/T Curve Characteristic - The resistance vs. temperature relationship of an NTC thermistor over a temperature span is nonlinear; therefore, an NTC thermistor R/T characteristic is typically designated by its "curve". (i.e. Curve D characteristic) Resistance/Temperature Characteristic - The resistance/temperature characteristic is the relationship between the zero-power resistance of a thermistor and its body temperature. Self-heating Error - This is a possible error that can occur when the power applied to an NTC thermistor exceeds its ability to dissipate that power and thereby causes the thermistor to self-heat. When the intent is to measure temperature, NTC thermistors should be used in a zero power condition. Standard Reference Temperature - The nominal zero-power resistance of a NTC thermistor is specified at 25 °C. Steinhart-Hart Equation - An empirical expression that has been accepted as the best mathematical equation for calculating resistance as a function of temperature for NTC thermistors for 50 °C temperature spans within the range of temperatures from 0 °C to 260 °C. Thermal Conductivity - The measurable property of a material that quantifies the amount of thermal energy that can be conducted through the material under a given set of conditions. Thermal Time Constant (τ) - The thermal time constant is the time required for a thermistor to change 63.2 percent of the total difference between its initial and final body temperature when subjected to a step function change in temperature under zero-power conditions. Thermocouple - A device that converts thermal energy into an electrical voltage when a temperature gradient exists between the two end junctions of a pair of dissimilar metal wires. Stability - Stability of a thermistor is the ability of a thermistor to retain specified characteristics after being subjected to designated environmental or electrical test conditions. Zero-power resistance - The zero-power resistance is the dissipation constant resistance value of a thermistor measured at a specified temperature with a power dissipation by the thermistor low enough that any further decrease in power will result in not more than 0.1 percent (or 1/10 of the specified measurement tolerance, whichever is smaller) change in resistance.

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