Thermocouple measurements go crazy when current is applied. 6

[camerontwilliams]

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,

 

[camerontwilliams]

LionelHultz, we have been using the raw element as you said, just the bare welded bead as well. It was not a misinterpretation, I flew to the UL Research Triangle Park Facility in Raleigh North Carolina and worked with both the PDE and our Project Engineer. They did not understand why we were having this problem here at our facility. Later I spoke with the Lab Manager and she has said that when building the lab in 2012 they were running into this problem when taking temperature measurements. An engineer was able to solve the problem by grounding out one of the two transformer buses. I have no idea how this was done or what was done there. The TC's used at UL are ungrounded Type J non insulated. Do you have any thoughts on what may have been done to the transformer?

 

[danw2]

1) ungrounded T/C

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.

 

[camerontwilliams]

This is great info, thank you danw2!

 

[LionelHutz]

Isolated inputs would let it work with power applied to the TC, assuming the TC doesn't have any current flowing through the element.

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.