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Dissipation Constant (δ) or Dissipation Factor
The dissipation constant (δ) of a thermistor, expressed in units of milliwatts per degree Celsius (mW/°C), is defined as the ratio, at a specified ambient temperature, of a change in power dissipation to the resultant change in temperature of the thermistor. Typically, published d values are based on a thermistor at ambient immersed in a temperature-controlled oil bath or suspended by its leads in still air under equilibrium conditions. These values are for reference only since the δ of a thermistor is not a true constant, but will vary according to the type of encapsulation or housing, the temperature, and the type of medium being measured. Because the dissipation constant is not a true constant, it is sometimes referred to as a dissipation factor. In order to avoid introducing self-heat effects in thermistor temperature readings, it is a good practice to keep the self heat error to less than 1/4 to 1/10 of the desired measurement accuracy.

The following expression is useful for determining the maximum applied power.

δ (mW/°C) × Temp. Tol. (°C) × 0.1 = Maximum applied power


For example, a thermistor with a temperature tolerance of ± 0.2 °C and a dissipation constant of 1 mW/°C,

1 mW/°C × ± 0.2 °C × 0.1 = 0.02 mW maximum applied power


Generally speaking, in order to minimize or eliminate the effects of self-heating in thermistor sensors and to perform essentially zero power measurements, it is recommended that the excitation current be kept in the range of 10 to 50 microamps. Several factors affect a thermistor’s ability to dissipate power: the mass of the thermistor, the lead size and lead material, the thermistor coating material, how the thermistor is mounted, and the medium or environment in which the thermistor will operate. Therefore, careful consideration of all of these parameters is necessary to eliminate or at least minimize the effects of self-heat errors for temperature measurement applications. The following table is offered as a guideline for three different styles of Cornerstone Sensors thermistors tested in a still air chamber.


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