Thermocouple

A time constant has been defined as the time required by a temperature sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to approach 100% of the step change value. An exposed junction thermocouple offers the fastest response. Also, the smaller the probe sheath diameter, the faster the thermocouple response time, but the maximum temperature may be lower. Be aware, however, that sometimes the probe sheath cannot withstand the full temperature range of the thermocouple type.