Understanding Thermocouple Time Constants & Response Times

We are often asked about the responsiveness of sheathed mineral insulated Thermocouples.

Here are some important terms and factors to know...

Time Constant

When researching the responsiveness of a thermocouple you will most often see it expressed as a Time Constant. The Time Constant of a thermocouple is defined as the time required for the sensor to respond to 63.2% of its total output signal when subjected to a step change in temperature.

Response Time

The Time Constant of a thermocouple is not the same as the Response Time. When someone wants to know the Response Time of a thermocouple, they really want to know how long it takes the thermocouple to reach its full output. Five Time Constants are required for the thermocouple to respond to 99.3% of the total step change temperature.

Factors that Most Influence the Time Constant of Sheathed Mineral Insulated Thermocouples

• Type of Junction. Responsiveness depends on the ablility to conduct heat from its outer sheath to the element wires. Listed in order from quickest to slowest Time Constant: Exposed, Enclosed Grounded, Enclosed Ungrounded
• Outer Diameter of Sheath. A smaller sheath diameter results in a quicker Time Constant, but is not as durable as a larger sheath diameter.
• Type of Media. Liquid, Gas, or Air. In Liquid, due to its greater heat capacity, the Time Constant is quick. In Gas or Air, the Time Constant is dependent upon the mass flow rate. To determine the Time Constant of the thermocouple in Air or Gas the actual mass flow rate must be known.

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