PID Controller

[rsott]

I'm new to this forum and not exactly sure where to post this - I built the PID controller featured on the susanminor.org site. It has worked great for about a year - until yesterday. I noticed it seemed to never get up to temperature on the Auber PID - but a quick check of an external thermometer in the lid of the smoker showed the internal temperature to be way above the set point. When I unplugged the smoker from the PID controller box - the PID then read the correct temperature. When I plugger the smoker back in - the readout on the PID dropped by 100 degrees F. I noticed that "Rob" posted the how to on building the controller box on the susanminor site and mentioned that questions should be posted here. Hoping someone can help diagnose what may be going on...
Thank you,

Bob

[pensrock]

Not real sure what your actual problem is but I work with temperature controllers daily so here are a couple things to check.
I do not understand why unplugging the tower from the controller would change the temperature reading unless there is a grounding problem of some kind.
1) Make sure the tip of your thermocouple is not touching anything. For example not touching the wall of the tower, racks or any product you put inside. This will give a false reading to the controller.
2) Along the same path... make sure the tip is not touching any metal. Depending on the actual thermocouple it may cause noise on the line and cause a misreading.
3) A shorted thermocouple or the wire to the thermocouple being shorted will cause the controller to read low.
4) If possible try replacing the thermocouple, assuming you have not found the problem from my suggestions.

[Mr Walleye]

Hi rsott

I agree with Pens. It sounds like an issue with the thermocouple. That's certainly the first thing I would try. You could check the back of the controller and make sure the connections are snug and nothing on the thermocouple wires are shorting. It definitely could also be a ground issue. I had a ground loop that drove me nuts for sometime on one of my big smokers.

Mike

[rsott]

Thank you. It does appear to be a grounding issue. The tip doesn't appear to be touching anything but I have continuity between the tip and the metal of the smoker so it must be shorting somewhere. Thank you for the help.

[pensrock]

Some T/C's have a grounded tip so it would be best to find out what you actually have. But you do want to make sure the tip is not touching anything inside the smoker, like the wall, rack or meat. This will give a false reading to the controller. You can try using a couple wooden spring type clothespins to hold the T/C just below a rack and see if the problem goes away. That way you know the T/C is not touching any metal. Also look where the T/C wire goes through the smoker, you do not want any worn spots where the T/C wires inside the insulation may touch metal there as well.