Thermocouple temp reading wrong?

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Cuzco_Brew

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I just put the first batch down to ferment in the new fermentation cabinet and hooked up the Temp controller and Thermocouple. Just to keep and eye on it all I also put my eclectic thermometer in the cabinet. I am getting a difference of 4 to 5 degrees C between the two. I tend to believe the thermometer over the thermocouple. Is this normal? Also when I put my hand on the thermocouple it has a bit of a buzzing feeling to it, like a current is running through it. Is that normal? What would explain the difference in the readings? 5 degrees C seems like a lot to me.

Any help would be much appreciated.
Cheers
Zac
 

Catt22

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Yes, 5 degrees C is a lot! Way too much of a discrepancy. I wouldn't jump to the conclusion that the thermocouple is the problem, but it well could be.

Check them both in a bath of finely crushed ice and water. That solution will be very close to 0 degrees C. That should tell you which is properly calibrated. You can also do a similar check in boiling water. I have a glass lab thermometer that I know is accurate and I use it to check my other thermometers.

I'm not sure about the electric current you are feeling from the thermocouple. Thermocouples do use a very small current to operate, but I would not think it would be enough for you to actually feel.

Which controller do you have?
 

Catt22

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I'm not familiar with that controller, so can't help you on that question.
 

kladue

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Are there any splices in the thermocouple leads between probe and controller?, if so the difference in wire alloys will generate an offset voltage and reading like you are experiencing.
 

McKBrew

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I'd verify your reading with another thermometer to figure out which one is bad. If the temperature controller is the issue and you can't get it repaired or replaced, you could always just note the difference and use it that way.

Another thing that comes to my mind (I can't read Spanish) is that the temperature controller may have a built in differential. I.E. you have it set to 40 degrees with a 5 degree differential. It could warm up to 45 degrees before the controller kicks on and starts cooling again. Your problem might be a simple as that.
 
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Cuzco_Brew

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The differential on it is about 1 degree. The read out shows the current temp according to the thermocouple. Will be checking with a 3rd and 4th thermometer tonight to see what the issue is. The wires don't appear to be spliced.

I used the digital thermometer over the weekend. It seemed to be ok, reading at 88 and 89 degrees C during the wort boil. Before anyone tells me that's wrong, I live at 3360m above sea level, water boils at that temp.
 
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Cuzco_Brew

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Update. Checked the temp with another thermometer, and it reads the same as the first, not the thermocouple. So any idea what would cause that to read 5 degrees C over? Have I wired it wrong? I initially had the + and - on thermocouple wired wrong and it was reading a lot higher. So I don't believe that is the problem.
 

pompeiisneaks

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I have one question... you tried w/ another thermometer but mentioned that the one is digital, are they both? You should make sure you test w/ a liquid thermometer, because I've got one digital, one liquid and one coil type and the liquid ones a good 2-4 degrees F lower than the digital and the coil type. Digital ones are notorious for being inaccurate. If you compared to two digitals, I'd try a liquid type to be sure.
 

Brickhouse

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You should be able to adjust the zero on your thermocouple readout whatever that is. Sometimes you'll get two points to adjust. Read through the documentation and you should find something.

I agree you should use an icebath and boiling water test points with your themocouple. It's easy to do and will stop the nonsense about what thermometer to believe - simply test your thermocouple.

Check the manufacture data for your thermocouple. They should be able to tell you the range for the thermocouple and accuracy. Generally thermocouple accuracy is on the average +0.75% of the measurement range.

Your statement that you feel voltage from the Thermocouple tells me you may be confused. You should make sure you have a Thermocouple and not a Thermistor or RTD:

A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature

A thermistor (and also RTD's) is a type of resistor whose resistance varies with temperature.

Is it possible that you could have a controller set up wrong ie., for a thermistor when you have a thermocouple?
 

Poobah58

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First make sure your thermocouple and input are the same. You probably have a type J or K thermocouple. You can usually tell a controller what type you have.
Second, does your control have cold-junction temperature compensation? If not, that could be your problem. Any time you have 2 dissimilar metals, it generates a voltage. This includes wiring the T/C wire to a terminal. This is OK if each junction is at the same temperature. If not, it must be compensated for.
A simple test is to put a jumper in place of the thermocouple. You should read ambient temperature. If not, it's time to troubleshoot...
 

JVD_X

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I doubt it is the thermocouple as the thermocouple is based on pretty sound physics. However, if you have a thermocouple that can be accidentally reversed then that will change the output.
 

tipicreeper

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IMHO, it sounds like you may have a lousy ground point somewhere, just judging by the way you stated that you can feel stray voltage through your forearm. It is a very sensitive area, I wouldn't be surprised at a millivolt leak to ground.
I would start disconnecting power and isolating items til you found the culprit, then we'll all figure out how to fix it. Use your arm as a testing rod.
Cheers
-David
 

Poobah58

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Holy crap... It never even registered in my brain that you stated the T/C had a buzzing feeling to it. That is not right. Thermocouples only generate a microvolt signal. If you feel some "juice" you got something hooked up wrong!
 
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Cuzco_Brew

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I will dig up the liquid thermometer tonight and test with that one aswell.

I have the thing wired up with out a ground wire. And before you all jump on me and tell me that is crazy, there are no earths or grounds on any of the power points in my house. The are all two pin sockets. The joys of living in South America.
 

tipicreeper

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I will not address the lack of proper grounding. You know what is right. (although you may be better served by running your own ground to a water pipe or something)
All things being equal the issue may be the type of thermocouple. Watlow make grounded & un-grounded versions. I know I have had compatibility issues with electrically isolating a heated platen for a plastics welder and using a grounded thermocouple. Once changed to a non-grounded version all problems went away.
Not saying that this is the issue. I still think you should do some systematic disconnecting of wiring to see if you still feel that stray voltage.
Cheers
-David
 
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Cuzco_Brew

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If I were to add a ground wire to this setup. What do I hook that up to? Obviously I ground it to a water pipe or something, but the other end?
 

tipicreeper

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I'm sorry Cuzco. I rather forgot what your set up entails. Refresh my memory.
 

ClaudiusB

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If you are in a hurry set F2 (offset) to -5.

According to the manual the controller excepts only type J thermocouple.
Did the thermocouple come with the unit?
What is the color code?
The US. Color code for J= RED & White
Color codes for the rest of the world.
Thermocouple Color Codes and Thermocouple Reference

tipicreeper wrote:
Once changed to a non-grounded version all problems went away.
Not saying that this is the issue.
Manny multi channel analog module are not isolated, ground loops create a big problem.
I use only non grounded thermocouples.


Cheers,
ClaudiusB
 
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Cuzco_Brew

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The thermocouple didn't come with the unit, but I bought it at the same place. Originally the guy sold me a K type, and then I went and changed it for the J type. The wires are red and blue. Which according to that page makes it a NETHERLANDS/GERMAN thermocouple. Which also says there in the notes, not recommended for low temperatures. Could this be the problem? Would it be worth getting a different thermocouple?

The buzzing/tingling feeling has gone now. I think this may have been from the thermocouple wire laying over the other electrical wires, moved and no longer an issue.

Cheers
Zac
 
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Cuzco_Brew

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Finally got around to checking the temp with a liquid thermometer, it reads the same as the digital. So the controller is wrong. Thanks to all for the help.

Cheers
Zac
 
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