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 Â°C, everything is fine, the logged temperatures follow the controller inticated temperatures quite well. Above about 1000 Â°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 Â°C Â± 1 Â°C on the Eurotherm we had a logged furnace temperature varying between 1520 and 1580 Â°C, and a workpiece temperature wandering between 1380 and 1490 Â°C. Turning the electric heating elements OFF immediately resulted in the logged furnace temperature showing steady within 1 Â°C of the Eurotherm indicated temperature ( both slowly cooling down together ), and the workpiece temperature showing steady at 1490 Â°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 Â°C on one thermocouple and over 100 Â°C on the other, however it is not at all obvious to me (A) why this only happens at high furnace temperature above 1000 Â°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.