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Old 11-01-2011, 06:02 PM   #1
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Default Thermocouple Frustration

I'm hoping some of you folks more familiar with the electric side of brewing can help me out.

In January I built a new system that incorporated a RIMS tube controlled by a PID and a thermocouple installed in the MLT. The first few batches went very well. And then on the third batch, I started getting random temp readings. It would suddenly drop ten or twenty degrees (instantly), but if I moved or even just touched the thermocouple wire it would go back to the accurate reading.

I assumed I'd messed up the wire by twisting it too much when removing it for cleaning, so instead of tracking down the issue, I bought a whole new thermocouple. Same thing, worked the first time, then started getting all wonky. Since I still had to unscrew the thermocouple from the MLT to dump it out, I assumed I'd done the same thing.

So...I replaced the whole thing again, this time with a disconnect for the Thermocouple wire so i would never have to worry about twisting the wire again. The very first brew, and I had the same issues.

Am I looking at a bad PID? Or is there some other issue I'm just clueless about?

I'm very close to insulating the MLT (should do that anyway) and scrapping the RIMS set-up.

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Old 11-01-2011, 06:13 PM   #2
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How are you recirculating in your MLT? You could be experiencing channeling issues in your grain bed. Pics would be helpful. I use a HERMS method and found a way to put my RTD in a thermowell into the return line and find it very consistent.

pics always help

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Old 11-01-2011, 06:14 PM   #3
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Definitely a wiring issue. Check any connectors you have in there.

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Old 11-01-2011, 06:15 PM   #4
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thermocouples work by generating a small voltage at the sensor, which is proportional to temperature being measured.

if something interferes with this voltage it will throw off the reading. getting the wires wet could do it, as could a poor connection somewhere. did you solder and heatshrink all connections?

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Old 11-01-2011, 06:18 PM   #5
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Quote:
Originally Posted by passedpawn View Post
Definitely a wiring issue. Check any connectors you have in there.
I've checked all that several times. The only connection is screws on the back of the PID, and now the disconnect I added.
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Old 11-01-2011, 06:21 PM   #6
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Quote:
Originally Posted by flananuts View Post
How are you recirculating in your MLT? You could be experiencing channeling issues in your grain bed. Pics would be helpful. I use a HERMS method and found a way to put my RTD in a thermowell into the return line and find it very consistent.

pics always help
I don't think it's a channeling issue. It's literally instantaneous jump of up to 20 degrees. Then a quick move of the wire puts it back where it should be. It's pretty obviously an inaccurate reading.

Pics may happen tonight. I'm at work at the moment.
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Old 11-01-2011, 06:26 PM   #7
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i do agree its a wiring issue. its clearly not the actual temperature swinging 20+ degrees in under a second. an internal controller wiring fault is possible if the other connections are definately good, since it has happened on more than one thermocouple.

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Old 11-01-2011, 06:38 PM   #8
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Quote:
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i do agree its a wiring issue. its clearly not the actual temperature swinging 20+ degrees in under a second. an internal controller wiring fault is possible if the other connections are definately good, since it has happened on more than one thermocouple.
I've been wondering about that. I hate to spring for a new PID if it's not the issue. I've already blown enough money on thermos.

Is it possible there's something else inside my project box causing interference?
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Old 11-01-2011, 06:59 PM   #9
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TC wire is brittle as hell and doesn't tolerate a lot of repeated movement. Throw it out and get an RTD. You'll never look back, I promise.

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Old 11-01-2011, 07:09 PM   #10
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TC wire is brittle as hell and doesn't tolerate a lot of repeated movement. Throw it out and get an RTD. You'll never look back, I promise.
[OP] Check first to make sure your PID model supports a RTD.

I agree, RTD is superior. I have thermocouples. I've experienced exactly what the OP is describing. Had to take apart the 2-prong thermocouple connector and rewire it.
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