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PID/thermocouple question
I have a P1 with an Auber PID that I've been using for a couple of years now. I used the K thermocouple that came with the PID. It has a woven sheath over the wires and the sheath wore through where it passes through the outer shell and was giving me intermittent readings. I replaced it with anoter K thermocouple from Ebay with a SS sheath and that's when the fun started. Instead of intermittent readings, I got temps that were very erratic. They seemed to rotate around a mid-point as the temp increased like this, 250, 241, 247, 252, 245, 250, etc. :( I've since replaced that thermocouple and order was restored. The replacement was similar to the original except the wire is a bit shorter. I've also wrapped some electrical tape around the wire to protect it where it passes through the shell.

Has this happened to anyone else? Or can anyone explain it?

There three possibilities, the thermocouple isn't a K, the thermocouple is deffective, or my PID didn't like the SS sheath.
I can't think of any reason why you would be getting erratic readings except an intermitten TC conntection at your PID, or a TC which was not welded correctly (perhaps just twisted?).

A different TC, like a T or J, will just give you false readings, not erratic ones. The type of sheath will make no difference.
Thanks Dan.

Here are my two cents worth-- Probably just plain stupid ideas, but hopefully something here will be helpful to you.

I wonder if you have the metal sheath grounded at any particular point in the system? I don't know if it should be grounded or not.

Another wild idea-- did it come with a standard plug, or did you have to connect the bare wires yourself? As you may know, any extra connections will cause additional 'thermocouple junctions'.

If you did connect to the bare wires, perhaps the polarity is reversed?

You could try testing the unit in some boiling water, away from motors and other sources of noise.

Also, with a sensitive volt-meter, you should be able to monitor the boiling water experiment in a stand-alone manner, and compare to NIST charts.

In my case, when I have a failure, I generally have an "open" or a "short", rather than intermittent readings.

I did dissect a J-type TC once, that had a metal sheath. It was filled with a white powder, and man that sheath was very tough to cut with the dremel tool. If it had been a K-type, I would be using it now :-)

Edited by bvwelch on 08/18/2008 10:16 PM
Island Addict
I'm not an expert, but I think erratic readings may be the result of a ground loop or electrical noise. Using an ungrounded TC may help. Omega gives some troubleshooting information here:


Island Addict wrote:
I'm not an expert, but I think erratic readings may be the result of a ground loop or electrical noise. Using an ungrounded TC may help. Omega gives some troubleshooting information here:

Thanks. I think that's the problem.
I think that is probably correct but these problems can also be due to a defective thermocouple. I used a thermocouple outside of its rated temp range and thereafter it produced erratic readings and had to be replaced.


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