How to check a thermocouple

Checking of thermocouple

We can check the thermocouple using hot water by dipping and connecting with a temperature indicator. Mostly we would be able to check the continuity and response of the thermocouple.

TC output signal is mV. So use a multi-meter and put Switch in mV then measure it. And see the TC reference Table to see the Temperature conversion. For type E is very special after you got the mV signal value then you should add it with cold junction Temperature. For measurements use dry block Calibrator to make sure it still normal or not.

The mV measurement of the output of a thermocouple without measuring the cold junction temperature and compensating for the temperature of the cold junction is NOT a valid check of a thermocouple.

All published reference tables ASSUME that the mV reading is the SUM of the hot-to-cold-junction mV measurement AND the mV equivalent for the same type thermocouple of the measured cold junction temperature.

Attempting to convert mV to temperature without cold junction will result in an erroneous low temperature value, low by the cold junction temperature.

In fact, a thermocouple left on a desk for a period ot allow it to equilibrate at a room temperature of say, 25 Deg C, will produce a mV reading of 0.0mV, because there is not thermal gradient between the hot end and the low end. Look up 0.0mV on the reference table and you’ll find equivalent temperature is 0.0°C. But you know the room temperature is 25°C. The cold junction temperature is 25°C and that is added to the hot-to-cold-junction measurement.

Frankly, even a mV test is really only good for continuity check because unless you have a trustworthy, known accurate measurement of the hot end against which the mV measurement can be compared, then a simple mV measurement will not detect drift, which is a serious problem with thermocouples.

Do not be confused by the ‘check your thermocouple’ tutorials on the web that are related to the thermocouples used for flame detection (absence or presence) in the gas shutdown safety circuit on natural gas-fired appliances, like water heaters or kitchen stoves. That’s not the same as a thermocouple making a temperature measurement in a process. That appliance test procedure has no relationship or validity for temperature measurement thermocouples.