Weirdness with the thermocouple in the big kiln

@Steve1 – I want to run this by you. The temp on the big glass kiln is “stuck”. We went back to run it, and it was sitting at 82 degrees. A bit warm for the warehouse. As we loaded it, it never changed. Then, when I fired it (because the oddity hadn’t struck me yet), it pretty quickly went to a “failure to heat” error. I fired it up one more time, and it went to an error again.

In your opinion, is it likely that this would be fixed by replacing the thermocouple? Or might it be an issue with the controller? The kiln is actually heating. I just went to move the glass to a different kiln, and it was probably over 100 degrees still. But the box still says 82.

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Either one is possible, however when thermocouple typically fail it goes to open. Still, it might just be a loose connection or it shorted in such a way that it always thinks it’s 82. Thermocouple is the cheap solution for sure, do you have a spare? If not, we could probably “borrow” one from one of the other kilns to find out if that’s the issue.

I’m not a kiln expert, but I do understand thermocouples and industrial temperature controls. I can take look if you’d like.

Oz (in DFW)

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Usually thermocouples have ‘cold junction compensation’ which involves a second sensor on the board they’re connected to. If the primary thermocouple is open and the controller doesn’t have a way to detect that, the math works out so it thinks everything is at the cold junction temperature.

I wouldn’t be surprised if it’s an open thermocouple.

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We’ll have to check with @PearceDunlap to see if we’ve got a spare. I suspect that we don’t have any spares, though.

@ozindfw – I’ll be around tonight. We’ll be back there doing a slumping kiln at 7:00. Maybe around 7:30? I’ll be there for the rest of the evening.

Can’t tonight.

Will be there tomorrow. I have some spare K type thermocouples I’ll try and dig out before I come. I don’t know what your kiln uses. I also have meter we should be able to use to test it. If it’s reading temperature, it’s almost certainly not the thermocouple. .If it’s heating at all it’s unlikely to be the elements or control switching.

Oz (in DFW)


I’m not sure that “reading” is what’s driving the temp that’s registering. As noted, it didn’t vary At All. For instance, when I opened the smaller kiln to load the glass, it read 60-something. Much more reasonable for the temp in the warehouse than 82. And the first time I opened the kiln to move the glass, it was toastier than I cared to pick up. So heating was occurring, but the thermocouple wasn’t reading any temp changes, so the program shut it down.

I was in this afternoon and took the temperature controller model number down to see if I find a manual or service info.

The kilm was padlocked. I wanted to measure the internal temperature with my thermocouple but couldn’t. I really didn’t want to mess with anything in the absence of adult supervision. I didn’t want to dig too far before I did that.

I’ll be in Saturday afternoon and hopefully will have looked at documentation by then.

Oz (in DFW)

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I’ll be in Saturday ~ 3 PM and take another look at this.

My suspicion is that the switch mechanism that controls the heating elements has failed and is partially on. If this is correct, I suggest the Kiln be unplugged until we get to it.

Oz (in DFW)

Just to be clear; I suspect the controller and thermocouple are fine. I’m guessing the solid state relay has partially failed. There is a failure mode where they are essentially a rectifier (think fixed light dimmer at ~30%) that might explain all of this.

I’ll need to open the kiln and measure it’s temp independently. If what I see matches what I expect, I’ll pull the controller cover and make some more measurements to confirm my hypothesis.

Oz (in DFW)

Well, the smaller kiln sitting next to it was registering … 68 when we opened it last night. The kiln itself doesn’t feel warm at the moment, although it got warm enough in the 2 tries where it quickly shut itself off because the thermocouple wasn’t changing temp. When I first went to move the glass, I shut the lid again and let it cool some because it felt uncomfortably warm to pick up and move the glass items. It was warm enough that the thin-fire paper had just barely started to turn brown, and the glue had turned black. Probably over 100, just at a guess.

Ok. I misunderstood then. I thought it was reading 80 degrees and seemed to be tracking. I’ll bring what spare thermocouples I have in case that’s the issue and I have something that will work.

We’ve had Art order a new one for us. I appreciate your effort! Jayson and Vincent are going to install it. Thermocouples are pretty easy to replace in kilns. At least, the old Paragon that Ceramics owned was pretty easy to plug a new one into.

Mind if I still take a look Saturday if it’s not already fixed? I should be able to confirm it’s the thermocouple.


The thermocouple was replaced when we changed the coils to 208. Then a few months later it acted up, We received a new thermocouple free as the vendor had received a bad batch. Also, during the install there was an issue with one of the wires not making a good connection. I would check the connection first.
This is odd, that kiln really has not had a lot of run hours.

Thermocouples are usually very reliable. It’s usually their connections or measurement circuitry that are the problem. I suppose it is possible to mess up the weld bead, but that’s not something I’d expect to fail over time. It kinda works or it doesn’t.

This is why I was so reluctant to suspect it when I understood (incorrectly) it seemed to be reading.

What time Tomorrow?

After lunch maybe one or two

Ok I’ll try and meet you there