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Is my K-type thermocouple defective, or am I using it wrong?

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I have two K-type thermocouples that look exactly like the one below. One was sold as good to 800C, and the other was quoted as having a temperature range from 0-400C. I have been swapping them between a multimeter and PID controller while trying to get control of an electric lead furnace, which consists of a steel pot, resistance heating coil wrapped around it a few times, and an outer shroud.

I have tapped the shroud in a few places to thread the head of the controller's thermocouple tight against the pot. And I am getting very bad results, using infrared thermometers to check the actual temperature at the contact points.

  1. The thermocouple pressed tightly against the outside of the pot has very high latency, and it never catches up. E.g., the pot will heat to 400C in two minutes. The thermocouple will increase its reading more slowly, peak a minute or two after the pot reaches steady temperature, and never indicate much beyond 250C.
  2. If I drop the other thermocouple loosely inside the pot, touching the side, it will react more quickly and get further, but it will still stabilize about 100C lower than the infrared thermometer indicates for its contact point.

Since this occurs with two separate thermocouples (that, for all I can tell, are of identical manufacture) I'm wondering if I am doing something wrong? For example, is there supposed to be slack between the nut and the head of the thermocouple? Or are these just poor pieces of equipment and I need to buy something better?

Is my K-type thermocouple defective, or am I using it wrong?

Answers

The most likely cause of a relatively large error in that direction (if the junction is actually at the correct temperature) is a double-reversal of extension wire. Since color codes vary by region, the most reliable way to identify the polarity (assuming you are sure the extension is type K) is to match magnetic to magnetic (positive) and non-magnetic to non-magnetic negative).

Proper polarity extension or thermocouple wire must be run all the back to the measuring instrument.

It is extremely unlikely that anything is wrong with a thermocouple- they are far more reliable and stable than IR sensors. If thermal contact is poor the junction may well be reading correctly, but be considerably cooler than what you are trying to measure- the leads draw heat out.

Ideally, immersion length should be 10-15x the diameter of the probe.

Edit: If you cannot achieve that try shielding the probe from air (make an insulated cup against the vessel) and running the leads against the vessel so they don't leach as much heat from the probe tip. You would probably do better with a bead thermocouple- at least you can visualize it more easily.

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