What is a thermocouple (TC) and how does it work?

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Cautions

What is a thermocouple (TC) and how does it work?

A thermocouple (TC) is a temperature sensor. It reads the temperature in the kiln and gives the information to the DynaTrol or pyrometer. A TC is made from 2 different metals. Type K TCs are alumel (-) and chromel (+). Alumel is magnetic, chromel is not. Also, when examined closely, alumel is a bit more copper colored and chromel is a bit more silvery. Type S TCs are Platinum and Platinum with 10% Rodium.

Once the two metals are welded together, a small millivoltage is actually generated from the welded joint. This millivoltage changes degree by degree as the kiln gets hotter/ colder. The millivoltage can be sensed by the DynaTrol or by a pyrometer, and each different amount of millivoltage is assigned to a different temperature. The DynaTrol can translate the millivoltage signal into a recognizable temperature.

Because the signal is in millivolts any point of resistance in the circuit will change the millivolt value and the corresponding displayed temp reading. Old TC wires, a poor connection, or overheated/ pinched TC wires are the most common ways to have an inaccurate TC reading. Because there is more than one TC in the kiln it becomes difficult for the controller to even out the different temps when one of them is not reading accurately. E- 1 and the elements seem fine is the result when this happens.

Additional Actions to Take

  1. Unplug kiln.
  2. Remove or hinge open the control box.
  3. Remove the Thermocouple Lead Wire from the Thermocouple.
  4. Unscrew the Thermocouple from the kiln.
  5. Remove Thermocouple.
  6. Install a new Thermocouple and screw in place.
  7. Replace Thermocouple Lead Wire and tighten. Be sure to get red matched to the MINUS (Negative) sign and the Yellow matched to the PLUS (Positive) sign.

See this video:

  1. Carefully examine thermocouple tip. This is the exposed welded joint at the end of the thermocouple that is not covered up by the ceramic tube.
  2. To do this you will have to remove the thermocouple from its protection tube (if it is a kiln that has one of our protection tubes). You can do this with the kiln disconnected from power.
  3. Look for corrosion - especially if it severe. These thermocouple tips will oxidize and otherwise corrode over time. That is normal. There is some point, however, at which the corrosion affects the ability of the tip to work (thermocouples work by generating a small voltage at the tip caused by two different metals reacting to each other).
  4. Corroded thermocouple tip

  5. Make sure the two wires are securely joined. One of the things that can cause an intermittent problem is a bad weld. If the two wires touch each other (even if they are not welded) they may work temporarily. However, if the weld is not secure then the wires could separate when the kiln heats up and cause an intermittent failure.
  6. If the thermocouple tip looks healthy then test the control board.