1
\$\begingroup\$

My present project is a ~6kW switching converter in a sealed stainless steel box. The box is grounded well. When I put a thermocouple on the surface of the box, the reading is noisy. Even if I put some tape between the box and the thermocouple, I get the same result. All thermocouples in the box give reasonable results, but the one on the actual box surface gives me so much noise as to be useless. My PC temp logger box rejects all the results as outliers and never updates. A calibrated multimeter gives me readings 30C higher than three IR guns (pointed at black tape on the box) and a PN junction temp meter give me. This problem only seems to occur when the unit is running. Grounding or not doesn't seem to matter. Two brands of thermocouples give the same results.

Why would I be getting such bad thermocouple readings in that particular circumstance? Are thermocouples just that untrustworthy and I'm getting unlucky? Or am I missing something?

\$\endgroup\$
  • \$\begingroup\$ Maybe worth adding to the info in the question: some grades of stainless are ferromagnetic; some not. A fridge magnet will fall off the latter and it may not provide magnetic shielding. Which is yours? \$\endgroup\$ – Brian Drummond Jan 24 '14 at 15:01
  • \$\begingroup\$ Magnets don't stick to this box. \$\endgroup\$ – Stephen Collings Jan 24 '14 at 16:13
  • \$\begingroup\$ Are the thermocouples inside the box in the center/middle of the box? Or are they closed to the (inner) surface? \$\endgroup\$ – RawBean Jan 27 '14 at 15:03
  • \$\begingroup\$ @RawBean Some of each. \$\endgroup\$ – Stephen Collings Jan 27 '14 at 15:11
  • \$\begingroup\$ I was thinking that high frequency noise (not filtered in grounding the box) might impact the Seebeck effect of the thermocouple. The electrical field inside the box could vary on the location, that's why I asked. Could you please confirm if "This problem only seems to occur when the unit is running."? \$\endgroup\$ – RawBean Jan 27 '14 at 15:20
2
\$\begingroup\$

I think you need to use an instrumentation amplifier like the AD8221. It does sound like common mode noise. My company fit plenty of thermocouples on aircraft engines directly to blades and other hot and large pieces of metal and we still get noise but it's livable with when using an IA.

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.