Problem with thermocouple after change from in-air probe to thermowell

[Mtn_Brewer]

This is the problem/email I sent to Auber Instruments for advice:

"I'm having some trouble with my equipment I've gotten from you guys. I have 2 1/4 DIN PID's that I had hooked up to a couple of your thermocouples meant for kilns that were in the air in a fermentation chamber and worked great for over a year. I upgraded my system and needed to change to thermowell compatible thermocouples. I bought your k type thermocouples meant for thermowells, male and female adapters and your thermocouple extension wire. The temps were way off when I turned them on and they would slowly drift up by ~ 1 degree every 10 minutes for about 2 hours. At that point, I adjusted the temps with the correction setting and moving the probes from hot to cold glasses of water gave reliable and accurate readings for about an hour. I left the probes in a glass of water and let if sit overnight and the temp of both probes increased by exactly 9 degrees over the actual temp of the water. I recalibrated the temps again and left them for another night. The next day both probs underestimated the temp of the water by 9-10 degrees.

I know the previous set up with thermocouples connected directly to the PID worked great without drift, so the problem is either the extension wire, the connectors or the thermocouples themselves. Any advice or fixes you can think of?

I'm considering a change to RTD's. Can you get an RTD for a 1/4 inch thermowell with an 8 foot cable?"

Both thermocouples were type K. The connectors and extension wire were also type K. The gauge of the extension wire is smaller than the thermocouple.

Any have this problem or any other advice? Thanks.

 

[muhteeus]

Did you adjust the PID variable to reflect the new K-Type Thermocouple?

Also, you must add offset for every cold junction with thermocouples. Every terminal block and connector adds a bit of a temperature offset.