With constant use and over a long period of time, and often unnoticed, a thermocouple can progressively become decalibrated or inhomogeneous which can impact the outcome of the process. The main reason for the decalibration is that the wires can become chemically attacked, which effects the wire composition, mechanical properties of the material and ultimately the temperature readings. If you suspect that you have a defective thermocouple, the best way to evaluate a used thermocouple is to place a new, or known good thermocouple adjacent to the suspect one in order to compare and document readings between both. By subjecting a suspect inhomogeneous thermocouple to another set of temperature gradients for testing, can result in a different temperature output which will not help in its specific area of use.
Always double check to make certain that the thermocouple that you have in question is of the same match, style and calibration type for the application / equipment that you are using. For example; a J type calibration thermocouple cannot be used on a specific application calling for a K type thermocouple. If your equipment calls for a J type, you will need to replace with a J type. This goes for all calibrations. (J, K, E, T, R, S, B, N).
If the Leads on your thermocouple are reversed, the temperature measured will show to be varying in the opposite direction relative to ambient temperature. Use the chart below to make sure that the positive and negative connections are in the proper order. The Red wire is generally the negative (-) wire in thermocouples, harnesses, and cables. This is often not realized and overlooked during troubleshooting.
Carefully complete a thorough inspection of the thermocouple probe looking for any pin holes, cracks, and areas with discoloration indicating internal contamination issues. Light green color, on a type K thermocouple indicates the occurrence of aging and a change in the chemical composition known as Green rot. An orange color from Iron Oxide on J and K types indicates internal moisture leakage. The wiring should also be inspected, as contamination on the surface of the wire can greatly reduce the accuracy of the temperature.
Often the thermocouple connection points are overlooked, but are critical for proper readings. Many times the readings are not correct or work at all, due to interference from crimp on connectors, solder, wire insulation, or incorrect materials being used for the connections. Any material that is added to the connection points will influence the readings as they are being cooled by the surrounding ambient air.
Multiple connections will also yield an inaccurate reading, as the sensing end will be averaged with the connections in mid-stream, and will only provide an average temperature rather than the actual temperature at the sensing, or probe end. Multiple connections should be avoided in order to improve accuracy.
You can check a thermocouple with a standard volt meter set to the Ohm's / Continuity function by checking across the positive and negative leads, in order to determine if the probe circuit is open or not. If finding that the thermocouple does not read, the problem is most likely a bad connection, broken wiring, or open internally within the thermocouple probe circuit.
If the thermocouple wiring is bare, missing insulation and is touch together anywhere between the sensing - probe end, and the connection points, the thermocouple will read at the area where the bare wiring is touching together. Make sure that the wiring is protected with insulation and is not damaged or broken anywhere in the circuit.
Learn about the Maintenance and Handling of Thermocouples