Response Speed
Time required for a sensor to reach 63.2 percent of the final difference after a step change in temperature. Measuring the thermal time constant tau provides an indication of how quickly a thermometer or thermostat can track environmental shifts. The numeric value depends on the mass of the sensor and the heat transfer properties of the surrounding medium.
Dynamic Error
Sensors with a large mass respond slowly to rapid fluctuations. When the thermal time constant tau is too high, the recorded data will lag behind the actual temperature.
Measurement Condition
Values are typically specified in moving air or stirred water because the medium affects the rate of energy transfer. A thermal time constant tau measured in still air will be much larger than one measured in a liquid bath. Reference conditions must be clearly stated on the calibration certificate.
System Integration
Designers use this parameter to determine the sampling rate and the filtering requirements for a temperature control loop. If the thermal time constant tau is known, the true temperature can sometimes be estimated even before the sensor has reached equilibrium. A substantial error in this calculation leads to unstable behavior in the thermal regulation circuit.
