Test and Measurement Forum

We are having trouble getting stable high temperature readings on a test furnace ( electrical heating elements with pulsed ON/OFF control using sold state relays ).

The furnace is controlled using Eurotherm ( control & overtemp ) instruments which work fine. In addition we have two thermocouples connected to a serial TR-08 which we are using to log the furnace chamber plus a 'workpiece' temperature. All thermocouples are Type R.

At temperatures below about 1000 °C, everything is fine, the logged temperatures follow the controller inticated temperatures quite well. Above about 1000 °C it all starts to go off the rails. The logged temperatures start to develop some kind of instability though the controller indicated temps remain absolutely solid.

Yesterday with a furnace temperature controlling steady at 1580 °C ± 1 °C on the Eurotherm we had a logged furnace temperature varying between 1520 and 1580 °C, and a workpiece temperature wandering between 1380 and 1490 °C. Turning the electric heating elements OFF immediately resulted in the logged furnace temperature showing steady within 1 °C of the Eurotherm indicated temperature ( both slowly cooling down together ), and the workpiece temperature showing steady at 1490 °C then cooling gradually from there.

Evidently, the heater circuit is affecting the logged temperature readings in a profound way, producing an error of up to 60 °C on one thermocouple and over 100 °C on the other, however it is not at all obvious to me (A) why this only happens at high furnace temperature above 1000 °C, and (B) why the Pico logger is affected in this way and the Eurotherm controllers are not affected at all.

I have earthed the negative terminals of the thermocouples at the TC-08.

I can confirm that the furnace & controllers plus the PC and logger are all supplied by the identical phase from the electricity supply and are all connected to the same earthing point.

What to do next ? Any suggestions gratefully received.

Thanks

Colin

Hello,

Try offering an earthing point where the thermocouples make contact with the workpiece. There may be a potential difference in earthing from the workpiece and the furnace.

The other possibility is EMI from the furnace elements that activate at higher temperatures.

It is difficult to determine the exact problem. But thermocouples traditionaly are affected either by earthing problems or EMI.

Please don't hesitate to contact me if you have any further questions.

Best regards,
Michael