Originally Posted by DougK
Depending on the unit, it will either have an internal temperature probe that they use to correct for the cold junction, or it will apply a constant correction and assume the controller will always be the same temperature. They may also ignore this transition, and assume the voltage is correct.
The cold junction is where you normally transition from thermocouple to copper. Normally 0 degrees c is used for this reference. If the cold junction occurs at 70 degrees c and is not corrected, the reading is going to be of by about one degree. For some applications, this amount of inaccuracy may be acceptable.
I'm an engineer in the aero industry, and our industry accuracy requirements are just a little more strict. I'm used to dealing with systems that have a 0.25 degrees of accuracy or less. I doubt you really need greater accuracy for brewing, however my experience in brewing is still pretty low.
Might just be a case of analysis paralysis. I'm thinking about it too hard and approaching it from the thought that any inaccuracy is unacceptable.
The problem is bigger than you might think. The Auber PIDs have a reference diode to attempt to correct for the temp of the cold junction. The cold junction is assumed to be at the back of the PID, so the reference is placed near the vent holes (you can actually see a leaded diode there). This works fine unless the temp at the reference diode is much different than the temp at the cold junction.
You can alleviate the problem by using thermocouple wire from the correct thermocoulple socket back to the PID. In my case, I also have a switch to select which TC my PID is looking at, so the switch became the cold junction even though I had the correct wire.
So, if you move the cold junction to, say, the outside of the control panel, then you might have a problem when the inside of the control panel starts to heat up.
So, the moral of the story is, if you can keep the back of the PID about the same temperature as your thermocouple plug, wherever that is, you'll be OK. If not, i.e., if you the back of your PID is near a hot SSR, you'll see several degrees of inaccuracy as the box heats up. Ask me how I know this.
BTW, I got a dual yellow TC plug on mcmaster carr. Here http://www.mcmaster.com/#thermocouple-connectors/=ld62bi