How to Check a Thermocouple (Part 1)

Part 1 of a 2 part series with Wayne Cooke Sr.

A. Check for continuity by using a multimeter set up to measure ohms. Connect the multimeter test leads to the 2 thermocouple leads (polarity doesn’t matter for this test) and if the meter reads 0 or open circuit, then at least one of the thermocouple leads are broken. Could be at the tip and not visible to you or could be somewhere along the lead wire portion of the assembly.

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If you do get a reading it should be several ohms (for a short assembly) to 20, 30, 50 ohms or more depending on the lead length. Ex: a 4 ft. length of 20 gauge type J (iron-constantan) lead wire = 1.396 ohms (.349 / ohm per combined ft. x 4)

B. To determine if a thermocouple is grounded or ungrounded, perform this check. Using the multimeter in the resistance measuring mode, connect one lead of the meter to one lead of the thermocouple and touch the other meter lead to the sheath of the thermocouple. The meter should read 0 or open if that lead (or side) of the thermocouple is ungrounded. Repeat test on other lead of thermocouple and observe reading. If open that lead is also isolated from the sheath and the sensor is ungrounded. Grounded thermocouples are more responsive to a change in temperature while ungrounded thermocouples are useful when there is electrical noise in the area of the thermocouple and that noise could be induced into the sensor.

Note: When checking an “ungrounded” thermocouple, it is possible to have some leakage to ground in a mineral insulated thermocouple. It is difficult to keep the magnesium oxide insulation in a mineral insulated thermocouples (MgOs) completely dry. Most of the time this is not a problem when it’s connected to an instrument or PLC input module, even though you might see some resistance to ground in the megaohm or even kilohm range. When the thermocouple is heated in the process, it tends to drive the moisture out of the tube and create better isolation.

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