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Erratic Temperature measurements with FP-TC-120

I have two FP-TC-120 modules on a furnace system. When the temperature reaches a certain level (930°C) my readings become very erratic. I have shielded all of the tc's and everything else that is connected to the FP bank to eliminate EMI noise. The interesting thing is that when I hook another temperature reader in Parallel with the Fieldpoint module, my readings are normal on both the new reader and Fieldpoint. Without the parallel reader, the readings will jump all over and abort my crystal growth runs due to a false high temp alarm. The problem affects one or more tc's at a time, and is not specific to only one of them. One time Tc #2 may fail, then next time may be #4. There is no rhyme or reason. I have been chasing this problem for three weeks to no avail. Please advise what could be causing this. I have several FP-TC-120 modules on other furnaces that operate with no problems. This is the first time I have had trouble.
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Message 1 of 13
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Sounds like grounding, especially given that it works fine with the parallel instrument.
When you say parallel are you talking about physically parallel (two TC, two electronics)
or electrically parallel (one TC, two electronics)?

Are the TC shields grounded only at the Fieldpoint terminal block?  Is the Fieldpoint
well grounded?  Is the furnace well grounded?

Do the readings become erratic around the target temperature of the oven, i.e.
when a controller starts cycling the heating elements off and on?

Sorry this is mostly questions but maybe there is an AHA! lurking somewhere.

Matt
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Message 2 of 13
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I have thouroughly checked all of the grounding throughout the entire system. Yes, the TC shields are grounded only at the fieldpoint bank and I am referring to one thermocouple that is attached to two seperate readers (an omega reader and the fieldpoint). I agree that the problem appears to be noise, but I have taken extensive steps toward eliminating the noise. Also, it makes no sense to me that when two readers are used, the noise is no longer a problem.
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Message 3 of 13
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Hmmm.  Is the Omega unit line or battery powered?  If it is line powered
then I would really suspect the grounding of the Fieldpoint somewhere.

At the terminal block is there a connection between the C or COM terminal
and the power supply common or ground?  My understanding is that there
should not be such a connection; the C or COM terminals should only be
connected to the sensor shields.

Are the erratic readings constant once the oven reaches temperature or are
they intermittent?

Sounds like a very frustrating problem.

Matt
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Message 4 of 13
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I apologize for the delay. I haven't been working on this system today. I'm not sure if it is an Omega unit or some other unit, but it is powered by the computer's USB port. I also have the same effect with the old mux board that the fieldpoint is replacing. If the Mux board is attached in parallel with the Fieldpoint, the readings are fine. I should note that the mux board is not even hooked up to anything other than the TC's (meaning nothing at all, no power, no PC, just sitting there with only TC's on it). The C or COM terminals on the Fieldpoint are not hooked to the power supply common. The erratic readings are intermittent, and when the power to the heaters is off, the problem goes away. Meaning, if I heat the oven up manually to 1000°C, and turn off the heaters, the TC's will read fine. It is only when I am trying to control the heat that the problem arises. This definitley points to noise, but as I said, I taken great measures to eliminate the noise. My next step tomorrow is to ground every thing that can can be grounded and even replace all TC's with shielded versions (currently I have a home-made shield set-up with standard TC's). If these do not work, I may have to look into purchasing, or developing a low pass filter to eliminate the noise.

Thanks for your help. If you think of anything else, let me know. I will post the solution when I find it so you know and others may benefit.

-Mark
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Message 5 of 13
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I'm not sure if your setup would promote this, but I have also had similar issues in the past and it is usually related to the thermocouple connection (i.e. wire extension connection) or the actual thermocouple itself at the end of it's life.  Check the resistance of your thermocouple.  I have a thermocouple that uses an extension wire and if there is even the slightest conncetion problem, or not enough contact, the same thing will happen.  The temperatures will begin to get erratic at higher temps.
 
I know this seems like an easy fix, but the first time it happened to me I spent a week looking for the problem and it ended up being that the extension wire was stripped a little to close causing a poor reading.
 
Hope this helps.
 
Brandi
 
FP-TC-120, LabVIEW 8.0, Win XP
LV 6.1, 8.2, 8.5, 8.6 beta
Fieldpoint
DAQmx
IMAQ Vision
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Message 6 of 13
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We have exactly the same problem.  cFP-TC-120s that become erratic at about 950 degC.   Appears to be related to power being supplies to the furnaces we are heating, and only happens if thermocouples from multiple furnaces go to the same TC-120 module. 

We would be very interested in knowing if someone has found a solution to this problem, or better identified the cause.
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Message 7 of 13
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We ended up fixing this problem with a low pass RC filter. The only problem with it is the response of the TC's is extremely slow and a failure may take some time to catch. But at least the furnace runs. Hardly an ideal fix, rather one that just gets us by.

-Mark
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Message 8 of 13
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Thanks for responding so quickly.

Could I ask what R and C you used?  What time constant?  Did you try a lower time constant to get resonable response times while still eliminating the (presumably 60 Hz) interferance?  

What kind of relay do you use?  We use "phase-fired" SSRs that are known to produce more noise, and were considering trying simple on-off SSRs, but we wont bother if you had the problem with ordinary Solid State Relays.

I presume that shielding the thermocouples didn't help?

-- James

PS  I'd really like to know why the problem only occurs above 900degC. Can't for the life of me think why the temperature matters to EMI noise.


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Message 9 of 13
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Found the problem!  I knew the temperature had something to do with it.

We use alumina (aluminum oxide) for our furnace tubes, which the heater elements and thermocouples both touch, and it turns out that the electrical resistance of alumina drops rapidly at high temperatures (surprisingly, it becomes a semiconductor).  Our problem only occur above 900 because that is the point at which the resistance has dropped enough for our main heater power (25 volts AC) to be conducted to the TCs (I read 17 Volts out of 25 volts on a TC attached to a 1 Mohm oscilloscope).  The resistance only has to drop into the megaohm range for this to happen, since the TC-120 input resistance is in the megaohms.  This high "common mode" voltage  causes problems on adjacent TC-120 channels (see attached NI note: "Troubleshooting the [c]FP-TC-120").  At lower temperatures, as well as when the TCs are not touching the alumina, the common-mode voltage is less than 1 volt.

I'm betting your furnaces use alumina also, and that this is at least related to your problem.

-- James
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Message 10 of 13
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