Thermocouple not reading accurately.

cpc123

I recently installed a type j thermocouple and watlow controller and am having issues with the accuracy of my temperature readings. I am using an analog guage as a reference to adjust my controller offset. The problem is that the temperature reading on the controller is not rising at nearly the same rate as the analog guage which is causing a significant difference in the read and actual temperatures. Could this be a grounding issue? I apologize as i am fairly inexperienced in this area.

Jamie Hall

Could be -- not so much a grounding issue as a wiring issue with the thermocouple and associated cold junction. They are absurdly sensitive to being wired properly!

Could also be that the two are actually measuring different temperatures, since thermocouples (when wired properly) measure the temperature exactly at the junction and nowhere else.

cpc123

The thermocouple is threaded into a water heater tank and so is the analog guage. They should be receiving the same temperature as they are only inches apart. Would it be problematic that i used wire nuts at the thermocouple connection instead of the terminals provided in the thermocouple housing? Also we ran a larger guage wire to the tc, I'm not sure if this could come into play. Our supplies were quite limited during this install so wiring had to be ran along with some 120v control wiring in the same conduit, maybe induction is a contributing factor in our issue?

Jamie Hall

The wire nuts aren't helping any. In fact, if you omitted the terminals provided, that's probably a big part of your problem. Thermocouples have to be wired exactly as they are intended to be. No short cuts. No substitutions.

cpc123

Makes sense, I'll land the wires properly and see if that helps. Thank you for your help.

BobC

I used to use thermocouples a lot (type j) and if we needed a long one I would splice a length of wire to the tc wire to make up the difference. Make sure to use the same type of wire for each side

This will introduce an error (a few degrees at best) but it's small AS LONG AS both sides of the splice are at the same temperature

Bob

ratio

Every junction of dissimilar wire is, in effect, a thermocouple junction. There's a lot of information out there, interesting reading.

I use K type wires to instrument new install equipment all the time. Sourced a few rolls from Omega, IIRC. I just twist the ends for the hot junction (longevity isn't an issue for me). It's cheaper to leave them in than to take 'em back out. Lately, I've stuck a few on refrigerant pipes, just jammed the junction into a dab of 15%, to see how long they'll last. My digital manifold uses K-type clamps for temp input, so I'll be able to validate them quickly.

cpc123

Thank you, one final question for you guys. I have another thermocouple available that has three wires (2 red 1 white) what is the purpose of the second red wire? How should it be connected for a 4-20 ma two terminal controller input?

ratio

Is it a thermocouple, or an RTD?

cpc123

Our parts room had it labled as a thermocouple, other than that i didn't see any lables on it. Is there a way in which i could distinguish the difference?

cpc123

After looking this up just now I'm pretty sure it is probably a rtd, i now see how it would need wired.

Leonard

3-rd wire might be a shield ground to protect circuit from stray voltages, use ohm meter to see if it's common with metal case of TC probe.

With thermocouples (TC) have to use the same wire materials all the way back to the controler. They have junction blocks made of separate metals for splicing the 2 wires. If introduce a 3-rd metal then accuracy goes out the window. ( can't use a copper zip wire for extension)

One issue I ran into with a wire TC in portable use is the wires crossed and touched intermittently at the controller, so it was reading temp intermittently from the end of the TC wire and at the controller.

I didn't do it but on a TC at work I think they just soldered the 2 TC wires at the end with standard ~ 40/60 tin lead electronics solder.

Also there are at least 6 different commonly used pairs of TC metals. ( J, K ,T, E ........)
http://www.omega.com/techref/colorcodes.html

DanHolohan

There's a good article here by Tim McElwain.