Important Info! Thermocouple wiring code

[Stacy E. Apelt - Bladesmith]

I had a question about a TC wires and though I would make a separate post on the subject.

In type K TC wires the cover is yellow. Inside are two leads - Red is negative and yellow is positive

Thermocouples are polarized. One lead is made from alumel. It is magnetic and is the negative lead ... which connects to the red wire in type K TCs . The chromel lead is not magnetic and is positive .. thus connecting to the yellow lead.

All other plugs, sockets, and TC blocks are polarized, too. Look for the + and - marks.

This is the color code chart for TC wires:
https://www.thermocoupleinfo.com/thermocouple-color-codes.htm

If in a past post or email I inadvertently typed the TC leads backward, I sincerely apologize. I don't think I did, but if you are using old threads or an email from me for information and having issues, check your polarity.

 

[Ken H>]

Inside are two leads - Red is negative and yellow is positive

Well, it depends. Looking at posted chart it seems RED is positive in more places than it's negative. The Chinese TC I've tend to follow the Japanese code where red is positive. The best method I've found to determine positive from negative is:

One lead is made from alumel. It is magnetic and is the negative lead ... which connects to the red wire in type K TCs . The chromel lead is not magnetic and is positive

That works everything without worrying about color code.

 

[Natlek]

 

[Drew Riley]

I recently posted a video in another thread about a $40 PID/TC setup I use to monitor my forge. The funny thing is, I accidentally put the red on the negative terminal and white on the positive. I fired everything up and temperature seemed to be reading just fine. After noticing my "mistake", I reversed the wires just to see what would happen, and everything appeared to work exactly as it had before.

I've been aware that TCs are normally polarity sensitive, as I used to change them out on heat seal machines somewhat regularly, but apparently it's not always the case?

 

[Ken H>]

Drew, that would be amazing - TC wiring due to the nature of the wire MUST be polarity sensitive.... well, it "should" be anyway {g}. Looks like you've proved that fabled exception to the rule that always comes up. That is interesting. Not doubting your word, just interesting. Could it have been an RTD?

 

[Drew Riley]

Drew, that would be amazing - TC wiring due to the nature of the wire MUST be polarity sensitive.... well, it "should" be anyway {g}. Looks like you've proved that fabled exception to the rule that always comes up. That is interesting. Not doubting your word, just interesting. Could it have been an RTD?

It read room temp both ways and seemed to react the same when in the forge. I know that the line connections for 110VAC weren't polarity specific, though the schematic for the TC connection states otherwise. I don't think it's an RTD, though that could make sense. It only has two wires, and I set it up as a K type TC in the parameters, though I'm not sure if they're meant to be any different for an RTD.

I'm almost doubting my own story now... hahah.

 

[Stacy E. Apelt - Bladesmith]

Just a possibility of what was happening - As long as the +/- wires of the TC go to the +/- terminals at the PID , wiring the TC wire backwards (switching red and yellow on both ends) would not greatly affect the reading if the TC wire was short.

Reversing them from +/- at the TC to -/+ at the PID will make it read wrong.

What happens when you make a connection between an alumel wire and a chromel wire is you get a thermally sensitive junction ... effectively a small TC. It creates a tiny current, either positive or negative, that will affect the current from the TC. This can either raise or lower the reading. Using any other wire than the same material as the TC (alumel/chromel), such as regular copper electrical wire, will also do the same thing. Because you don't know the amount of error, this affects the accuracy of the PID reading.