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.
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