Other NTC Thermistor Specifications

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 δ 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.

 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 these parameters is necessary to eliminate or at least minimize the effects of self-heat errors for temperature measurement applications.

For thermistors with or connected to a relatively large thermal mass (such as a housing) it is possible to pulse the excitation current in the 250 microamp range for a few milliseconds during a measurement period and then allow the heat to dissipate for a few hundred milliseconds before the next measurement pulse. However, this does not work for micro-thermistors with relatively low thermal mass where a few milliseconds is enough to discernibly self-heat the thermistor during the measurement pulse. In such cases, it is better to keep the measurement excitation current in the 10 microamp range.

Time Constant (τ)

The time constant (τ) is defined as the time required for a thermistor to register a temperature change of 63.2% of the total difference between its initial and final body temperature when subjected to a step function change in temperature under zero power conditions.  Published time constant values are typically given for a thermistor immersed in a well-stirred, temperature-controlled oil bath or suspended by its leads in still air under equilibrium conditions.  These values are given for reference only since the time constant of a thermistor or thermistor probe is not a true constant, but is dependent upon the thermal properties of the materials, the mediums being tested and the temperatures being measured.  If the response time of a device is a concern, the time constant of the thermistor or thermistor probe should be measured in a test environment similar to field conditions. 

How to determine the time constant of a NTC thermistor 

To determine the time constant of a thermistor, its resistance values at three different temperatures must be known; a low temperature point, a high temperature point, and a mid-temperature point that is 63.2% of the difference between the high temperature and the low temperature.  A precision bridge is set for the middle temperature resistance with the bridge voltage supply set to provide zero power measurement.   An auxiliary bridge is set for the resistance at the higher tempera­ture. The thermistor is placed in a test medium (typically, a temperature-controlled oil bath or still air chamber) and is connected to the auxiliary bridge. The supply voltage is then adjusted to self-heat the thermistor until the auxiliary bridge balances to the higher temperature. The thermistor is immediately switched to the precision bridge. The time required for the precision bridge to balance is the time constant of the thermistor.