Question

The thermal time constant is the time
1. To reach the final steady temperature if the initial rate of increase of temperature were maintained constant
2. To reach 63% of final steady temperature
3. To reach 66.66% of final steady temperature
4. To reach half of the final steady temperature

Answer 1

Correct Answer - Option 2 : To reach 63% of final steady temperature

Thermal time constant:

• Thermal time constant is one of the types of Times Constants which is used for any thermal system.
• The thermal time constant is the time measured in the response of a thermistor to change its ambient temperature.
• Under Zero power conditions, the time required for a thermistor to reach 63.2 % of its final state or final steady-state temperature when subjected to the step function change in the temperature.

When the ambient temperature changes from T₁ to T₂ then the relationship for the time taken t sec. to change the temperature and the thermistor temperature T can be expressed as-

\(T = \left( {{T_2} - {T_1}} \right)\left( {1 - {e^{ - \frac{t}{\tau }}}} \right) + {T_1}\)

Where, τ = thermal time constant