Type J TC cable - Temperature Variation??

[controlnovice]

We just migrated to a new DCS system that has Isolated Thermocouple Cards with Cold Junction Compensation built in. We are using Type J thermocouples.

During a cold snap (in Chicago), after about 2 months running on the new system, our operators noticed the process temperature was much lower than it should be. The temperatures in the DCS were reading about 20C below the actual. This could have been happening all along, but it took the cold snap to be noticed.

We thought the DCS was too cold (no actual temperature taken), as the room in not insulated, and the heater was not working. So, we brought in some temporary heat. They left it overnight (process was shutdown), came in the next morning to find the DCS room at 40C! The process temperatures in the DCS were reading 30C higher than actual.

Between the TC and the DCS, there are three junction boxes. They took a reading at the first one, closest to the TC, and found the reading was the same as the local gauge. The second termination in the second junction box also read the same as the local gauge. They went to the third junction box, and the reading was 13C higher than the local gauge. Note that the room temperature was no longer at 40C either.

The cable between the 2nd and 3rd junction boxes is Type J cable. The length is about 80ft, with 65ft outdoors in conduit, and 15ft indoors.

We are told all cable sections are Type J cable (I'm not on-site to verify). If one of these other cable sections were different (not Type J), is it possible this is the error we see?

Why do we see the temperature difference/error? Do you need additional information to answer?

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This is normally the space where people post something insightful.

 

[danw2]

You are running the thermocouple signal directly to the input of the input of the DCS, and you are not using transmitters, correct?

There are multiple junction boxes between any given thermocouple and the T/C input on the DCS, correct?

Is the wire between the T/C and the first junction box, and the wire between the remaining junction boxes up to the DCS input card T/C extension wire? Or is one or any of the wire runs copper wire?

This sounds like false junction reading(s) caused by the use of copper wire.

In answer to your question if copper wire were used instead of T/C extension wire, it would very likely cause the situation you describe.

 

[itsmoked]

That would be my guess too. Wrong wire which creates a 'junction' in one of the boxes. You end up reading that junction added or subtracted from the desired junction.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[controlnovice]

Okay. More testing:

They lifted the section of wire off the terminals between the 2nd and 3rd boxes.

At the 2nd box, they wired a Type J TC and read the value on the cable in the 3rd Jbox.

The actual temperature at 2nd box was 22C. The reading at the 3rd box was 35C.

Cable between 2nd and 3rd box is Beldon 20gauge TC wire for Type J.

Any other guesses?

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This is normally the space where people post something insightful.

 

[itsmoked]

Your description is not clear. I cannot make out exactly what the test was. Reading this 6 times didn't help ether.

With T/Cs subtle mistakes cause big reading errors. You have to be rather meticulous about your troubleshooting.

You removed the cable but then hooked a T/C at the box? Not following this.

I'd get a T/C meter with a J pigtail.

I would measure the temp at the original T/C's location.
Disconnect at the computer/instrument the wire coming from the field.

Connect the correct leads of my meter pigtail to these disconnected leads.

Wrong value?

Move to the next closest box.

Remove the lead going to the computer/instrument and replace with the pigtail leads.

Still wrong?

Go to the other side of this same box and remove the leads that are coming from the T/Cs direction. Hook the pigtail leads onto the T/C leads directly.

Step and repeat. Until you get a correct reading.

Your problem;
Can be a broken barely touching wire.
It can be a non-isothermal set of connections in any box.
It can be a loose screw.
It can be a wrong wire type.
Induced voltage.
Wet connections.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[GTstartup]

I think you have a problem with the extension cable. It may not be J type like you think. Insulation color can be misleading.

Take a piece of the extension wire (18" will do} twist it at the end and put it in a hotbox with the type J T/C you mentioned in the last post. They should read the same. If they don't then you know what your problem is.

 

[controlnovice]

Thank you for your suggestions and answers. Sorry my description was not clear. I tend to ramble with no sleep.

Today, they told me they are done troubleshooting, and are going to install transmitters at the TC.

We may never know.

______________________________________________________________________________
This is normally the space where people post something insightful.

 

[itsmoked]

HAHAHA!! Oh well.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[JLSeagull]

The terminal material is another possible source of error. Also, this seems like too many terminations for thermocouple connections.

 

[itsmoked]

How can terminal material affect this? The Law of Intermediate Metals removes this constraint, does it not?

http://www.omega.com/temperature/Z/pdf/z021-032.pdf

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[JLSeagull]

Don't assume that the terminals are the same material such as copper, that both terminals are at the same temperature or that the terminals are made well - without oxydation, etc. We can usually eliminate a temperature difference between the junctions. Consider the affect if they terminals were actually selected for the thermocouple material - and that they are reversed.

Does anyone actually buy terminals made of iron and constantan? They are available and some people buy them.

http://www.connectwell.com/PDF/ctt2-5u.pdf

One conductor in a Type J thermocouple is iron. Iron rusts (oxydizes). A small amount of rust in the termination would affect the low millivolt signal.

 

[roydm]

Rust or corrosion might effect one or two but the original post indicated all were out. Perhaps the best way to approach this problem would have been using a simulator, starting at the DCS and working out to the distant J Box. Changing to transmitters seems a bit drastic, are you also changing all the cables etc?
I miss the old calibrators that had manual CJ compensation, not as convenient but they made you think and use your tables.

Roy