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Strange thermocouple readings at high temps

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I am having trouble reading my thermocouples at high temps. I have a 9074 cRIO with a 9211 thermocouple module connected to 3 K-type thermocouples. The thermocouples monitor 3 heat zones of a giant furnace, in which the zones can be turned on and off using a solid-state relay. When turned on, the thermocouples read the temperature normally up to 800 degrees C. However, when it goes past 800 degrees, it may seem to be reading normally, but when you switch different heater relays on or off, the thermocouple reading changes significantly some times. This does not happen below 800C. See attached pictures, and assume the first picture is the correct base temperature:

 

Furnace1.png

Furnace2.png

Furnace3.png

 

When I attached the first zone to an external thermocouple reader with its own digital display, I do not see this drastic change when switching the relays on and off. It stayed relatively constant.

 

Why am I seeing such a big change with the 9211 module? I am not processing the signal in any way, just a direct read-through, and the module itself is set to read K-type thermocouples. The heater contactor and the thermocouples are relatively far away.

 

Any help is appreciated.

Linus

 

 

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After some more testing, I have more info regarding the thermocouples.

 

It seems that if only one thermocouple is physically connected to the NI module, the temperature readings work fine, with no drops. However, with 2 or more connected, the problem returns. If I have connect all three thermocouples and disable two of them via software, the problem still remains. The only time I was able to make it work correctly is when I had only one connection to a thermocouple.

 

For some reason, having more than one thermocouple causes problems at temperatures above 800C. Is this a problem related to the module itself?

 

Linus

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Hi Linus,

 

After reading through your problem statement, I am still a bit unsure about how you have your system set up.  I understand you have your cRIO and 9211 with three thermocouples connected to a tank, but where does the solid state relay come into play?  A very quick or brief description of the schematic layout will be very beneficial.

 

Also, which solid-state relay are you using?

 

Thanks,


Nick

Applications Engineer
National Instruments
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To be more specific, a 9403 module controls 3 TTL relays, which in turn controls 3 solid state relays, which switches the heater coils on/off. Here is a quick schematic:

Furnace schematic.PNG

The solid state relay is a Crydom CRD2445. Its control voltage is 3-32 VDC, operational voltage  24-280 VAC, and operational current 45 Amps.

 

Linus

 

Message 4 of 14
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That's odd with it working fine when one is wired in, but not when more than one wired in. I'd be interested to hear what you find it is.

 

You've probably tried these:

  • check wiring is as it should be - and no shorts between the thermocouples, or shorts to ground
  • check thermocouple types are as they should be - can you swap any of thermocouples in case there is some fault with one
  • swap 9211 module for another - just in case it is faulty
  • check for any settings related to 9211 in the LV project - right click on the module  | properties. Some c-series modules have settings under there

 

Consultant Control Engineer
www-isc-ltd.com
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Hi Linus,

 

Did you ever resolve this issue as I am having the same problem.

 

Thanks

Craig

LabVIEW 2012
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Solution
Accepted by topic author ilnus

Hi Craig,

 

Yes, I was able to resolve this issue. a LabVIEW field engineer suggested this solution and it worked. See Section 3 Measuring Floating (Nonreferenced) Sources at this link:

 

http://www.ni.com/white-paper/3344/en/#toc3


The source of the problem was instrumentation amplifier input bias currents, which was causing the voltage level of the floating source to be moved out of range. I needed to add bias resistors to provide a DC path from the instrumentation amplifier inputs to the instrumentation amplifier ground and anchor this voltage level to some reference. I followed FIgure 10 and added two 100 kOhm resistors for each of my thermocouples. Since then, I did not get anymore fluctuating readings.

 

Hope this helps,

Linus

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Hi Linus,

 

Many Thanks for the quick reply.  I will do some wiring and let you know how i get on.

 

Many Thanks

Craig

LabVIEW 2012
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Hi Linus,

 

This did indeedsolve our problem

 

Thanks

Craig

LabVIEW 2012
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Dear all,

 

Currently I have the same problem that you discribed. I have tried the solution propocedure explained here. I used as a ground the COM pin of the 9211 module.But unfortunatly the problem is still there.

It always happens above 700 degrees celcius. My set up is almost the same as the described by ilnus.

 

I also observed that the problem desapears:

1. when I turn off the ''main power'' for the heaters,

2. when I use one 9211 modul for each thermocouple. 

 

Can someone help me to solve this problem?

 

Kind Regards,

Francisco

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