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Old 12-03-2012, 06:48 AM   #1
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Default Pid temp reading way out

Hi

Let me start by saying that I know nothing about PID's or electronics so I'm way out of my depth.

I have a GERFRAN 500 PID controlling a 4000 w heater in my HLT. The temp readout on the unit does not agree with the temps I measure with 2 normal thermometers. The PID reads about 8 deg C high.

Funny thing is that when I first tested it it was pretty accurate about 1 or 2 degrees out. Then I swopped the J probe for new one and the new probe started to to read about 8 deg C high.

AH ha, I thought, the new probe is faulty but when I put the old probe back, it too was reading about 8 deg C too high.

I've checked the wiring as best I can and cannot see anything amiss.

I can't seem to find anyway to calibrate the GEFRAN 500.

Could there be some voltage leakage or something like that ?

Any suggestions would be much appreciated.

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Old 12-03-2012, 12:00 PM   #2
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Thermocouples are extremely finicky and prone to bad readings. Electrical noise can affect it and they can also be easily damaged. If the single loop controller accepts an RTD, I would swap out the Thermocouple for it. If not, I'm guessing you need a new thermocouple.

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Old 12-03-2012, 12:18 PM   #3
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Thanks Wubear.

I have swopped the probe's around with new ones.

This morning I had the PID unit with the probes tested at the supplier who gave them the A OK. I sat with them while they tested and the my PID with the thermocouple tested perfectly.

It must be some kind of interference. Any idea of how I test for this ?

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Old 12-03-2012, 12:26 PM   #4
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Don't let the PID get hot. There is a reference internally (the "cold junction") that needs to stay at room temp.

Is the thermocouple directly connected to the PID, or do you have intermediate connections?

I'll bet the PID has an offset that you can set.

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Old 12-03-2012, 12:41 PM   #5
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Passedpawn, the thermocouples (there are 2 - one for the MT and one for the BK) is not connected directly to the PID but goes via a 3 way switch (Element 1 ON - OFF - Element 2 ON) which a friend of mind wired up for me. It's that silver faced switch in the middle of the pic.

As I said, I know next to nothing about PID's or electronics.

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Old 12-03-2012, 01:18 PM   #6
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Love the pic.

That switch might very well be the problem. I have a similar switch with my thermocouples and it introduced a significant error, similar to yours.

A thermocouple is made of 2 dissimilar metals that generate a small voltage that changes with temperature. If you introduce other metals (i.e., your switch), you get errors.

The best solution is to change to RTDs or thermistors. You need to make sure your PID supports that first. And, of course, it will get you and your friend busy swapping them out. Not a picnic. I didn't do that either

What I did was get my mash BK to about normal strike temp (165F), measured with a good thermometer, then I adjusted the offset in my PID. I think I was off by about 3F.

Now my PID reads correctly at 165F. When I'm boiling, it reads about 210, which is of by 2 but I really don't care as long as it's boiling.

Good luck.

BTW, I had a much bigger problem with heat in my box. I moved my heat sinks outside the box and that made a big difference as well. This is also a problem specific to the use of thermocouples.

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Old 12-03-2012, 01:24 PM   #7
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Thanks very much, I'm going to put my buddy to work.

But what are RTD or thermistors ? And if I use them will I be able to run them through my switch as it is wired now ?

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Old 12-03-2012, 01:34 PM   #8
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Quote:
Originally Posted by Wheelspin View Post
Thanks very much, I'm going to put my buddy to work.

But what are RTD or thermistors ? And if I use them will I be able to run them through my switch as it is wired now ?
Yes, I think you can run them through your switch. They are both resistors. Your switch there has negligible resistance, so it won't create an error.

RTD and thermistor resistance changes with temperature. RTDs have a very linear resistance vs. temperature curve; thermistors curve is not linear. None of that matters as long as your PID supports these devices. RTDs are considered more accurate. RTDs typically have a third wire to eliminate a small error due to wire resistance, but I think you can just tie this to one of the other wires and use the thing as a 2-wire part.

Good luck and let me know how it goes.
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Old 12-03-2012, 02:04 PM   #9
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Thanks Passedpawn

So what you're saying is that RTD's won't be affected by any "leakage" or false readings due to dissimilar metals in the wiring like thermocouples are affected ?

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Old 12-03-2012, 02:10 PM   #10
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Quote:
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Thanks Passedpawn

So what you're saying is that RTD's won't be affected by any "leakage" or false readings due to dissimilar metals in the wiring like thermocouples are affected ?
Exactly. As long as the path to the RTD is low resistance, you'll be OK. Your switch should not introduce any problem here.
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