Thermocouple measurements go crazy when current is applied.
Thermocouple measurements go crazy when current is applied.
(OP)
I have a problem with our thermocouples jumping the temperature readings either extremely high or low as soon as we apply a current to the test loop. Once the current is shut off the TC's go back to normal measurements. I was at a lab and they had grounded a part of their transformer to avoid this problem. Does anyone have any idea what to ground or how to fix this problem?
Thank you,
Thank you,





RE: Thermocouple measurements go crazy when current is applied.
TTFN (ta ta for now)
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RE: Thermocouple measurements go crazy when current is applied.
Bill
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"Why not the best?"
Jimmy Carter
RE: Thermocouple measurements go crazy when current is applied.
RE: Thermocouple measurements go crazy when current is applied.
RE: Thermocouple measurements go crazy when current is applied.
So, what did you expect? The few millivolts produced by the TCs will drown in the potential differences caused by your rather high test current.
DO NOT BOND TCs TO ANYTHING THAT CARRIES CURRENT
Insulate them or use NTCs or something like that. This is really very elementary. You should not ask questions like this.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Thermocouple measurements go crazy when current is applied.
RE: Thermocouple measurements go crazy when current is applied.
Insulated T/Cs
Keith Cress
kcress - http://www.flaminsystems.com
RE: Thermocouple measurements go crazy when current is applied.
RE: Thermocouple measurements go crazy when current is applied.
I get upset when someone who says that he is an engineering professional is as ignorant as you seem to be. And then I say so. Like it or not.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Thermocouple measurements go crazy when current is applied.
A classic way this was done was with flying capacitor front-ends. The logging/display instrument consists of a single front end but all the numerous inputs are hooked up to that front end individually, one at a time, and sequentially. That prevents any two from ever being connected simultaneously.
You can test this by hooking one of your TCs to a DMM with TC capability and seeing if it 'goes crazy' while having no current path thru the hand-held battery powered DMM.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Thermocouple measurements go crazy when current is applied.
RE: Thermocouple measurements go crazy when current is applied.
IR drops between the monitoring points in the test circuit may cause voltage differentials between the T/C's. This can cause inaccurate readings if the probes all go to a common front end.
-AK2DM
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"It's the questions that drive us"
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RE: Thermocouple measurements go crazy when current is applied.
It can be done, yes. But it requires quite a bit of experience. Much better to insulate the TCs. The good thermal coupling can be had by wrapping the stem of the TC around the metal piece.
Still somewhat surprised to see your question. And your reaction.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Thermocouple measurements go crazy when current is applied.
"No were did I claim to be an engineering professional"
There are forum rules and certain assumptions. One of the basic ones can be read at the top left corner (scroll to top).
I really do not understand why you call me your friend? I have not got that impression before. What made you change your mind?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Thermocouple measurements go crazy when current is applied.
RE: Thermocouple measurements go crazy when current is applied.
1. Keep everything as is.
2. Run current until the set-up is "hot enough"
3. Switch current off and determine the thermal time constant.
4. Continue running test and switch off periodically to read temperature. Use the measured time constant to correct readings, if necessary. I think not.
5. You may compensate for the reduced duty cycle by increasing current correspondingly. It is RMS that counts.
I can very well understand if you want to call me your friend now.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Thermocouple measurements go crazy when current is applied.
That said, there is still a possibility that some test current may flow across the thermocouple and distort your readings, depending on the mounting and contact method. (As mentioned by Gunnar.)
The danger here is that the readings may be off but not off enough to be noticeable.
That is rather than obvious errors, the readings may seem good but still be in error.
The insulated probes linked by Keith may be the best way to go rather than isolation.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Thermocouple measurements go crazy when current is applied.
RE: Thermocouple measurements go crazy when current is applied.
Still, I would agree that having the actual TC element in the current current path is rather silly. I wouldn't be surprised if that was a misinterpretation of what the UL engineer said.
RE: Thermocouple measurements go crazy when current is applied.
RE: Thermocouple measurements go crazy when current is applied.
The common construction of ungrounded thermocouples (like Moltenmetal suggests) is called "mineral insulated (MI) or MgO (magnesium oxide, an insulating powder), which insulates the thermocouple junction from the stainless sheath. Google will find dozens of vendors. The end result is universally call "ungrounded". Apparently, the lab has confirmed that they, too, use ungrounded thermocouples, for very good reasons.
The stainless sheath can be as small as 1/16" (0.063") diameter. The smaller the diameter, the faster the response.
2) Grounding
If your analog input is not extremely well isolated, you're very likely to have an error signal.
If you use a battery powered, handheld meter with a thermocouple input, your problem is likely to disappear for that particular measurement point.
I have used an adapter to 'float' an AC powered instrument that the thermocouple feeds when grounding creates an issue on several occasions.
RE: Thermocouple measurements go crazy when current is applied.
RE: Thermocouple measurements go crazy when current is applied.
The test setup UL mentioned had a transformer powering the circuit. The transformer lead where the TC was connected was grounded, which put the TC element close to ground potential. The TC electronics was likely a differential measuring setup but only for a small offset voltage from ground.
With isolated inputs you could bond the TC onto a live connector or wire and it could work. But, you still can't trust the reading if you're actually flowing current through the sensing element since that is still doing a dumb thing with a TC.