A constant expressed as the time for the temperature at the element of a thermistor, with no load applied, to change to 63.2% of the difference between their initial and final temperatures, during a sudden change in the surrounding temperature.
When the surrounding temperature of the thermistor changes from T₁ to T₂, the relation between the elapsed time t and the thermistors temperature T can then be expressed by the following equation. by ambient temperature change.

\[ \begin{eqnarray} T&=& \begin{pmatrix} T_1 - T_2 \end{pmatrix} exp \begin{pmatrix} -t / \tau \end{pmatrix} +T_2 \qquad \qquad・・・(3.1) \\[8px] &=& \begin{pmatrix} T_2 - T_1 \end{pmatrix} \begin{Bmatrix} 1-exp \begin{pmatrix} -t / \tau \end{pmatrix} \end{Bmatrix} +T_1 \qquad ・・・(3.2) \end{eqnarray} \]

The constant τ is called the heat dissipation constant.
If \( t = \tau \), the equation becomes: \( \begin{pmatrix} T - T_1 \end{pmatrix} / \begin{pmatrix} T_2 - T_1 \end{pmatrix} \fallingdotseq 0.632 \)

In other words, the above definition states that the thermal time constant is the time it takes for the temperature of the thermistor to change by 63.2% of its initial temperatrue difference.
The rate of change of the thermistor temperature versus time is shown in table 1.

Measuring conditions for parts in this catalog are as follows:

1. Part is moved from a 50 ̊C envirconment to a still air 25 ̊C environment until the temperature of the thermistor reaches 34.2 ̊C.
2. Axial and radial leaded parts are measured in their shipping form.

Please note, the thermal dissipation constant and thermal time constant will vary according to environment and mounting condition