# Glossary

## 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 The Beta equation has been replaced by the Steinhart-Hart equation for applications requiring more accurate resistance vs. temperature interpolation over wider temperature ranges.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.

## RoHS Compliance

RoHS is an abbreviation for “Restriction of the use of certain Hazardous Substances” in electrical and electronic equipment, as defined by “Directive 2002/95/EC of the European Parliament and of the Council of 27 January 2003.” For a complete description of QTI’s RoHS product capabilities, you may view or download QTI’s Statement Regarding RoHS Compliance.

## 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 Z 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 DC 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.