Desperate for Help with Thermocouple

I'm using the Rocket Scream Reflow Oven Shield connected to a toaster oven with SSR and a Type K thermocouple. It's a screw-in type that I got off Ebay. I screwed it into the side of the oven wall and cut off the blades on the wire leads. Initially, it worked fine.

My problem: it works for a few seconds, or minutes, and then it starts reading 0 for the temperature. It's not an LCD display issue; the serial monitor shows this value too, and the PID controller reacts accordingly. (On the attached pic, you can see the 0-120 representing the actual versus desired temperature).

I'm attaching pictures of my setup. By the way, I'm not using it for reflow, but to hold a steady temperature. I have it connected to another Arduino with a data logger. All that stuff works fine.

Any ideas? Could the attachment of the TC to the oven wall be causing this? It has started happening at widely different temperatures--never the same value.

I saw one comment that a small capacitor across the TC helps, but the shield schematic already shows one.

Could I have damaged the board? (Well obviously I could have). But wouldn't that prevent it from working all together?

Thanks. I've always had great luck with this forum. Unfortunately, I only have a couple of days to get this fixed, and I don't have time to reorder the shield and additional thermocouples.

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I should also like to mention that I don't get any errors that the code is designed to trap, such as open or shorts.
Thanks.

Many multimeters have a thermocouple input capability. I'd suggest testing your thermocouple to make sure it's working properly.

You do know that on thermocouples the red wire is negative?

If you have a sensitive volt meter (millivolts) you can use a TC table to get an idea of whether or not the TC is OK, for example if the temp in the oven is 50C (122F) the mV will be 2.03 but you have to add the cold junction mV which (in a 25C (77F) ) room temp will be 1.000 mV, so you would read 3.03 mV.
Here's a link to a type K TC table.

thermocoupleinfo.com/pdf/type-k-thermocouple-reference-table.pdf

NOTE: You must disconnect the TC from the controller.

Thank you. I stuck the bare wires from the TC into my multimeter, and it appeared to operate properly. My suspicion is there's a connectivity issue at the screw terminal on the controller. Last night, I reconnected it with the wires less firmly pressed into the terminals and with the screws only lightly tightened. That's the opposite of my previous approach where I forced them in deep and cranked down on the screws as hard as I could.

Well, it worked perfectly for the two hours that I had it turned on. I don't think I could have been leaving an open connection, or grounding it, but maybe it was acting as another junction or thermal stresses were doing something...regardless, it's working fine at the moment.

Thanks. Oh, and thanks for the mV data. That will be helpful in the future.

outsider:
...
If you have a sensitive volt meter (millivolts) you can use a TC table to get an idea of whether or not the TC is OK, for example if the temp in the oven is 50C (122F) the mV will be 2.03 but you have to add the cold junction mV which (in a 25C (77F) ) room temp will be 1.000 mV, so you would read 3.03 mV.
Here's a link to a type K TC table.

thermocoupleinfo.com/pdf/type-k-thermocouple-reference-table.pdf

What are the various columns in this table?

To quote from Wikipedia: "The most common error in thermocouple construction is a circuit extension with dissimilar metals such as copper wires. As explained in the Principles... section above, it is crucial to know the temperature at this point of transition, as this point is the very definition of the reference junction. ".

I see you have crimp terminals on the ends of the thermocouple wires. Apparently they go to a screw terminal block.

You should maintain the thermocouple wire type to the point of measuring the voltage, or measure the voltage created by the dissimilar wire types. You may have to add or subtract this voltage from the thermocouple voltage.

Paul

Thermocouples only give the temperature between their ends. You have a hot junction and a cold junction. So you need to measure the temperature of the cold junction via a thermistor. Then add that temperature to the measured temperature to get the real temperature.

Thermocouples according to British standards are +/- 3 degrees C

You then have to add the accuracy of the measuring circuit. Thermocouples are not linear in their mV to temp output. See attached graph. Then add the accuracy of the method of measuring the cold junction's accuracy.

If you want a really accurate temperature measurement use a PT100 platinum resistance temperature sensor.

Attached is the graph of mV vcs Temperature for a type K thermocouple