PID thermocouples

[HATCH]

That makes sense.
I thought you might have to tell it what temp rating the actual TC is like 0 - 900 or 0 - 1200. Etc...

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This chart should give you a rough idea



This is the same chart for ALL K type thermocouples regardless of who makes them or the length of the probe.

Whats different between a 400C rated probe and a 600C rated probe could be the outer metal of the probe. The general construction of the TC.

Doing the ice water test to verify 32 degrees F is fine but the problem is that isn't near the standard operating range that you would be using it at.
So imagine tuning a car to run perfectly at idle compared to a 4K RPM.

If the thermocouple is designed exactly to the spec then you are perfectly fine calibrating your PID at 32. But I doubt that a $5 Thermocouple will be built exactly to the K spec.
it will be close though..

We are casting boolits. It doesn't matter if we are off a few degrees.

What you want with a PID is repeat-ability.
Basicaly knowing that at a certain temp (lets say 700 F) that every time the PID reads 700 its gonna be the exact same temp as it was before.
The reason I say that temp doesn't really matter because if you aren't casting good boolits at what it says it 700 then you turn it up or down.
It doesn't matter if the PID is off a couple degrees or 50

[lwknight]

The actual length of the probe is only to make it fit the application and means nothing except how deep it will go. The thermocouple is in the tip.

[bangerjim]

Better forget trying to fix it! And the "weld" used is not just solder. The two bi-metal wires are fused together by a special process that has NO OTHER metals between them. Remember, wherever there is a metal to dis-similar metal junction, that forms a t/c! Soldering the wires would form at least 3 junctions, each reading off temp from the others. And the long sheath is normally filled with a dielectric to keep the wires in place.

Just better to buy a new one. And all these cheap-o t/c I hear about guys using ARE made to go in protective thermowells and not contact the process directly. The thin sheath over the junction is just not made to withstand the exposure/oxidization on the bare end.

I use to be an engineer with a company that made industrial grade high quality t/c's (not these ChiCom things) and it is a very complex process...if done right!

[BattleRife]

In practice adding a third metal has no significant effect. At a former job we routinely installed thermocouples to heated vessels by spot welding the dissimilar wires individually to the steel vessel. The wires weren't touching each other at all, but were only a couple of millimeters apart on the steel. They calibrated and gave reliable results throughout the test, just fine.

[AzElkhunter]

Ok. I got to put new TC in and it appears that it is not the problem....
When I start it it begins normal and as it runs a bit the temp screen seems to stop reading, (stays at one number) then I have to turn it off and back on then temp jumps up to whatever it should be then stops again shortly.

Also power light sometimes works and sometimes does not. It is working because pot is still on and heating. Just won't control temperature right. Sometimes acts like it's at temperature and turning on and off.

Could the my pin controller be bad?
Checked for loose wires and found none. Don't know if all wiring is correct as I have no schematic.

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[bbogue1]

I have no idea what PID you have I think they are basically all the same, but, Mine is very basic and it seems all PID's wire up similarly. My schematic: I had one wired wrong and it didn't work. I hope this helps.

[JonB_in_Glencoe]

Ok. I got to put new TC in and it appears that it is not the problem....
When I start it it begins normal and as it runs a bit the temp screen seems to stop reading, (stays at one number) then I have to turn it off and back on then temp jumps up to whatever it should be then stops again shortly.

Also power light sometimes works and sometimes does not. It is working because pot is still on and heating. Just won't control temperature right. Sometimes acts like it's at temperature and turning on and off.

Could the my pin controller be bad?
Checked for loose wires and found none. Don't know if all wiring is correct as I have no schematic.

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That does sound like a defective PID controller.

[bangerjim]

In practice adding a third metal has no significant effect. At a former job we routinely installed thermocouples to heated vessels by spot welding the dissimilar wires individually to the steel vessel. The wires weren't touching each other at all, but were only a couple of millimeters apart on the steel. They calibrated and gave reliable results throughout the test, just fine.

You can make a "T/C" (rough term by definition) by putting a paperclip under the PID terminals! That makes actually 2 different junctions, one each where the plated steel meets the screw terminal. I have done that many times to rough test PID functions. Not accurate but gives a ballpark functional test. Expensive PID's have iso-thermic connectors on them that eliminate those intermediary junctions. What we do with simple little ChiCom PID's and couples running a little lead pot - inaccuracies are not very noticable or important.

But accurate T/C science and construction is an art, just not slapping two wires together.

[AzElkhunter]

I will check wiring against diagram and also try jumper wire to further test things.
I have a feeling it is my pid. This is the one I have.

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[Mike W1]

Disconnect the TC and short those 2 pins with a small piece of wire. Should then read ambient temperature. If it does it's likely OK but you'd also best check the parameter settings to be sure they're correct. It's easy enough to have changed one by accident. Could be the one that determines that it's for a K type. You don't have an RTD type per chance?

[AzElkhunter]

Just k type TC

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[AzElkhunter]

It shows ambient temp with TC connected. But see how main power light on bottom is off. After it sets for a minute the temp stops working until turned off and back on.

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[Mike W1]

Looked back thru these posts and it's probably the PID itself. One would assume it was wired right in the first place. On a Mypin there's some differences which pins are the control leads to the SSR and also which ones to hook the TC to but 1 & 2 seem to be the normal ones for the main power for the PID. I can't really tell on the photo on post #31 but that SSR wiring looks strange to me. Looks like a white and a black wire on the output and they both should be black as that's the side that is being switched on/off to the pot. Maybe whoever built it didn't have enough black wire, who knows. Which model Mypin is the PID? Guessing here but it sounds like maybe it's never worked properly since you bought it?

[Retumbo]

I thought I had issues too, found a loose wire in one of the thermocouple connectors

[AzElkhunter]

MYPIN model TA7-SRR
The unit was working when I got it. I used 2 or 3 times then quit. I went through wiring all connections are tight. Tried different TC's and still same thing. I did find power indicator light is out but should not be an issue as it only indicates power is on.
Whoever wired it just did not use correct wire color.
I guess it's time for new controller. Is there a better one to get?

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[T Herder]

OK, being a retired technical manager for an accredited calibration lab I need to add some things at this point. Looking at the pic in post #31 I see a LOT of wiring issues and discrepancies! White wires connected to black wires as an example. Is the white wire connected to the SSR going directly to the negative (-) control output terminal on the PID and the black wire going directly to the positive (+) control output terminal on the PID? The blue and white wires going from the T/C connector to the PID look to be regular wire, this is a MAJOR NO-NO. It introduces a secondary cold junction in the thermocouple sensor to the PID. T/C wiring to a controller must be the same all the way to the PID to maintain accuracy.
Here's my suggestion to diagnose some of the problems you're having: Disconnect all power to the unit. Unplug the T/C from the input to the unit. Put a jumper wire across the T/C connector shorting out the white and blue wires at the connector. Power the unit on, wait a couple of minutes and the display should give a reading reasonably close to ambient temp. The reading displayed with the T/C input shorted will be the PID's internal cold junction compensation temp. This is a fast way to check if your T/C has failed, doesn't matter the brand of PID. If your PID won't read your sensor, simply unplug it and short the input terminal with what we termed the almighty paper clip. If your PID now shows a reading close to ambient with the T/C sensor input shorted, the T/C you had plugged in has failed. If the PID does NOT display a temp, then the PID has failed. The cold junction junction temp displayed with the sensor input shorted should be reasonably close to ambient. If it reads some really high or low temp the calibration of the PID is invalid and you can forget about any reasonable control.
Next: With the input T/C input shorted as above, read the DC voltage at the SSR input terminals 3 & 4. It should read at least 5 VDC. Our experience with SSR's has proven that even tho the SSR is rated at 3-32 VDC input a lot of them would not trigger ON below 5 VDC. IF you do not show VDC at the input terminals on the SSR, grab your PID manual and CONFIRM the correct settings for your application. TOO many times service calls were spent configuring controls simply because a simple setting got accidentally changed.
If you cannot find a short length of type K thermocouple wire to run from the connector to the PID send me a PM and I'll mail you a length at no charge. Just happen to have a lot of it laying around. But please correct the sensor wiring.
Best regards,
Butch

[bangerjim]

Thank God for the all mighty paper cilip! And duct tape.

And you confirmed my comments about multiple junctions created by non-TC wire. Those throw unkown amounts of error into your reading. AND........they multiply with each other!!!!!!

When dealing with any type of TC (j,k,n,t,s,r, etc) always use the correct type of iso-thermic connectors AND the correct type of tc extension wire.....NOT ordinary wire.

I am an applications engineer in the process industry and have dealt extensively with tc systems since 1974. I even taught classes to customers during seminar sessions we sponsored.

Banger

[Mike W1]

First do what T herder said. Probably would work if wired correctly. If not that rig is $195 on ebay and a TA7-SNR or TA4-SNR for $30 or whatever will do what you want for a lead pot.

[AzElkhunter]

After new TC and some aggravation ordered new pid and some time figuring out settings I have it up and running. Even works now with old TC.

I guess it's like everything else... if you have problems with it you have to learn more about it to fix it...

Thanks for everyone's input to this. Especially Mike W1 for the help on wiring.


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