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k-type thermocouples bugging out at high temperature

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Ground loops, CMRR, EMI/RF interference, insulation breakdown...

I agree with all those as potential candidates and have experienced all those in my T/C experiences.

 

The most impressive effect I have observed was I had an exotic probe in a 2250C furnace with a purge gas.

Retracted the probe too fast into ambient air and the probe lit up like a sparkler! Never did work quite right again above 700C.

 

What is the max temp your T/C's are rated to and are they in ambient or a purged atmosphere?

 

-AK2DM

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"It’s the questions that drive us.”
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Message 11 of 19
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@WNM wrote:

Some materials that are electrical insulators (like ceramics) at room temperature can become more electrically conductive as the temperature goes way up. Combine that with high-voltage electrical heating elements and you may start seeing stray power-line leakage around your furnace. 


I had this very problem almost ten years ago, where the alumina ceramic that our furnaces were made of became conductive as they approached 1000 degC, leading to the thermocouples connecting to the furnace power.  We solved the problem by switching to units with high common-mode rejection (just as WNM suggested).

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Message 12 of 19
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Thanks to everyone for all the great ideas. This actually turned into a pretty interesting thread and I learned a lot from the various responses. In particular, I was interested to learn that ceramics can begin conducting electricity at high temperatures. It makes sense, when I think about it, since ceramics have metals in them. Our furnace is heavily insulated by ceramics, so it could definitely be an issue in the future, and I'm glad to know it.

 

HOWEVA, in this here case, that wasn't the issue. The wonky Tc was working fine when it was directly connected to a voltmeter.  So it turns out the quoted response here was the winner.  The PID controller was interfering with the Tc reading through the ground. Lynn was correct that the problem became worse as you approached the PID setpoint (although, interestingly, only if it approached the set point AND was at high temperature. For example, if the PID was set to 500K, there were no issues at all).

 

Thanks for the recommendation to check the AC and DC voltages between the Tc and ground. The DC voltages between the two thermocouple leads were always steady and as they should be, but at high temperatures, when the PID neared the set point, there was a clear AC voltage which oscillated between 0.5 and 1 V depending on whether the PID was sending current to the heating element.

 

Removing the ground cable from the Tc DAQ so that all the Tc's are on there own isolated loop solved the problem.

 

I feel kind of stupid, because it seems like electricity is kind of a mystery to me, but an old labmate of mine used to blame every 'unexplainable/weird' electrical event as 'some sort of grounding issue,' and usually implied that more things needed to be grounded, not less. Apparently that was not the correct lesson to take away : ) . For the record, because some people asked, most of the Tc's we are using are actually just k-type wires spot welded together in-house, in woven fiber insulation.

Message 13 of 19
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@gpsimms wrote:

Thanks to everyone for all the great ideas...

 

 

 

I feel kind of stupid, because it seems like electricity is kind of a mystery to me, but an old labmate of mine used to blame every 'unexplainable/weird' electrical event as 'some sort of grounding issue,' and usually implied that more things needed to be grounded, not less. Apparently that was not the correct lesson to take away : ) . ...

 


Thank you for the update!

 

It may help us someday.

 

Re: Grounding...

 

"Noise reduction Techniques in Electronic Systems" by Ott is a great book that addresses noise in measurements. He starts with each situation, makes some estimates to establish boundry conditions and then uses Maxwells equations to analyze each challenge and reduces each challenge to a rule of thumb.

 

A single ground with no parallel ground paths is what we want.

 

Ben

Retired Senior Automation Systems Architect with Data Science Automation LabVIEW Champion Knight of NI and Prepper LinkedIn Profile YouTube Channel
Message 14 of 19
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Quoting Henrik's tagline:

 

'“ground” is a convenient fantasy.'

 

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~
"It’s the questions that drive us.”
~~~~~~~~~~~~~~~~~~~~~~~~~~
Message 15 of 19
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@gpsimms wrote:

 

Our furnace is heavily insulated by ceramics, so it could definitely be an issue in the future, and I'm glad to know it.

 

HOWEVA, in this here case, that wasn't the issue.

Uh, no, it sounds like that was exactly your issue.  Your ceramics became conducting at high temperature, causing an AC power signal on both wires of your thermocouple.  Your TC DAQ is capable of doing a differential measurement so as to reject this "Common Mode" (as is your Votmeter, which is also differential), but you had set the DAQ up to do a single-ended measurement by grounding it.

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Message 16 of 19
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If you really do not understand electricity well, it might be a good idea for you to have someone who does review your entire setup. While you may have  it working for now, you could have other issues which will only become apparent when something changes slightly or something begins to fail due to age or long-term exposure to elevated temperatures. You do not want that issue to show up as a major catastrophe where someone gets hurt or a lot of expensive equipment goes up in smoke.

 

Lynn

Message 17 of 19
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Thanks for the advice, Lynn. While my previous reply may have indicated something troubling about my level of competence (thanks for clearing up my misunderstanding, drdjpowell!), I assure you this system is pretty robust and has been working for us for about 5 years. This thermocouple issue only presented itself because we had never before added one independent of the PID control to the interior of the furnace. Other than that, everything continues to run as expected. Once again thanks to the community for its gentle guidance, and I am glad the issue is resolved.


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Message 18 of 19
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Hi,

I'm facing a similar problem now, and I can relate to every trial you have done. Can you describe a little more in detail the solution that worked for you, please? It would be of great help. 

 

Thank you very much,

Prithvi

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