Thermocouple question please.

Thread Starter

boydage

Joined Oct 7, 2016
84
Repairing a fleet of warmer(s) that currently have an old and obsolete temp controller installed inside. There are lots and lots of spare controllers available on the market I have bought a couple to test. The exercise I am trying to repair quite an expensive unit in the best practical way.

So the replacement has arrived with a thermocouple that measures about 12k ohms where the installed thermocouple in our warmer measures about 4 ohms at the same temp.

My question is whether there is a solution to use this much lower resistance thermocouple as its way way better for us in the long run. Is there a linear resistance converter available to do something like this in an easy way? I would say roughly that for each 2k ohms the new thermocouple drops, the installed one will drop say approx 1 ohm.

The controller does come with a calibration solution but only +/- 10deg. Thoughts anyone?
 

Delta Prime

Joined Nov 15, 2019
1,187
Hello there. :) If you are using a thermal couple the controllers can only respond to the temperature changes it sees through feedback from the sensor. Changing the thermal resistance can introduce a lag or delay causing a wider swing between Maximum overshoot and minimum undershoot temperatures the controller should include the PID feature the anticipation and offset ability to compensate for these conditions. Changing the thermal resistance will influence the ability of the controller to regulate the temperature around a desired set point and may cause long delays in thermal transfer. Temperature varies from point to point what type of thermocouple are you using J,K,S?
 

Reloadron

Joined Jan 15, 2015
6,204
Sounds like we have a Thermocouple and Thermistor in the same mix. A very low resistance would be typical of a thermocouple, a few ohms while for example a 10K thermistor would read 10K Ohm at about 77 F and 12 K Ohm around 69 -70 F. I am thinking thermistor. A good picture would help.

Ron
 

Thread Starter

boydage

Joined Oct 7, 2016
84
Hi hey thanks for the replies I have gone with another controller and ordered it to suit the thermocouple we have which is a J type. interesting actually i learned a bit today. Thought the thermocouples were pretty much the same all except for the temp ranges. Also didnt think they would use a thermistor for temp control only using them for protection.
 

Reloadron

Joined Jan 15, 2015
6,204
Hi hey thanks for the replies I have gone with another controller and ordered it to suit the thermocouple we have which is a J type. interesting actually i learned a bit today. Thought the thermocouples were pretty much the same all except for the temp ranges. Also didnt think they would use a thermistor for temp control only using them for protection.
More to them than range as you see. Some types have a nicer linearity curve. The list goes on and on. Anyway, glad you got a handle on things. :)

Ron
 
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