Tech Tips

How to Check a Thermocouple (Part 2)

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

C. Check for proper polarity. Many thermocouples are terminated with connectors or a transition where it is possible to have a reverse connection. That is, the negative wire is connected to the positive pin of the connector and positive wire is connected to the negative pin. When heat is applied to the thermocouple with the sensor plugged into a temperature tester, the reading will go down as the temperature goes up.

blogNote: It is not unusual to also see what is known as a “double reverse” connection. Here is an example. Customer has a type K thermocouple terminated with a screw cover head on top of a hot furnace. The electrician connects the wire in the head in reverse. Negative to positive and vice versa. He walks back to the controller 50 ft away and observes that the temperature is reading in the negative direction. Obviously he has a reverse connection. Instead of getting the ladder back out and climbing up on top of the hot furnace, he reverses the connection at the terminals of the controller and sure enough the controller reads in the positive direction. Problem solved? No. He must go back and correct the wiring because he still has the chromel wire connected to the alumel and vice versa in the run of wire from on top of the furnace (where the ambient temp in the screw cover head might be 150 F) and the terminals on back of the controller (where the ambient temp might be 70 F). The result is the same as it would be if he used uncompensated copper wire between the 2 connections and the reading will be off by 80 F (150-70 = 80). Remember red is always negative when working with a thermocouple and usually positive in a standard electrical connection so it is easy to get that mixed up.

D. If you want to determine if your thermocouple or the instrument it is connected to is bad, try the following. Disconnect the thermocouple from the temperature controller or PLC input module and short across the 2 input terminals at the controller. If the controller reads ambient at the terminals, the thermocouple is most likely the problem. This is not always true for all controllers as I’ve seen a few where you must connect a thermocouple to the instrument terminals so it is best to take a short piece of thermocouple wire, strip both ends and twist securely at one end to form a junction and connect the other end to the input terminals and see if you read ambient temperature.

Tech Tips

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.

blogIf 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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