Thermocouple readout fluctuations

leitmotif

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Join Date
Nov 2004
Location
Seattle Wa. USA
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Have situation that has both me and the instructor at school stymied.

PROJECT OVERVIEW Simultating using water pumped thru pipe embedded in concrete to melt snow or ice on sidewalk.

EQUIPMENT Allen Bradley SLC (forgot to bring home model number) with NT 4 thermocouple mV module. Using only two of the 4 thermocouples. Thermocouples are type K.

Each thermocouples is immersed in its own bath (warm water and ice bath). Over the short time frame temp in each bath may vary by a degree or two. Thermocouples are not reversed (checked by heating with match - temp goes up).

The LES (less than) instruction is used to compare thermocouple readout to a desired temp - in case of ice bath 40 F and in case of warm water bath 100. One LES instruction for each case.
PROBLEM
1. The temp readout displayed by the LES instruction fluctuates wildly ie from -454 to + 100 over a matter of seconds.
2. The LEDs on the thermocouple go out occasionally indicating a fault.
3. When the LEDs on the thermocouple module go out the outputs also fail stopping the system.

Instructor thinks there is a mismatch between the PLC processing speed and the processing speed fo the thermocouple module ie the PLC is outrunning the thermocouple module.

SOLUTIONS attempted
1. Adjusted the filter rate - did not help.
2. Ran thermocouple leads (they are continuous no splices) as far away from any power conductors as possible

3. He added a timer so that each LES instruction is activated every five seconds.
4. This eliminated the fluctuations
5. BUT then caused the outputs to short cycle every five seconds.
6. The short cycling can be overcome using another timer configured to TOF
7. We then used the Move (MOV) to put a copy of the input into another file (N11:10 and 11) in hopes that this would yield a stable reading - no help

This methodology strikes me like putting bandaid on gangrene. I want to understand what is causing the fluctuations (gangrene) and eliminate the problem.

THINGS I NEED to do (after rereading the manual)
1. Make dang double sure the NT 4 is compatible with the PLC. There is a table in teh manual for this.
2. Trouble shooting section in manual says I can check bits in the inputs for trouble codes.
3. Heck if I know

QUESTION
1. What is causing the fluctuations?
2. How do I overcome the problem?
Thank you for the help

Dan Bentler
 
I have experienced this myself, and it was caused by the use of a mixture of grounded and ungrounded-junction thermocouples on the same NT4 module. The grounded-junction T/C's were introducing noise into the card. The manual stated that you could use one or the other, but not both. Tech-support, however told me to stay away from grounded-junction T/C's altogether. I understand that they now have an NT4I (I=isolated), that can handle both types at the same time.
 
I think that mgvol is on the right path ... I've seen the same thing happen ... for troubleshooting, try using only one thermocouple as a test ... and take it out of the water ... if the problems start to sort themselves out, then maybe ungrounded thermocouples are the answer ... I know that it fixed my similar problem a couple of years ago ...
 
Thanks for thoughts guys.
One of the underlying problems is that this is classroom equipment set up by previous students. I "cleaned the slate" and rewrote the program to make it simpler (they had been using PID or attempting to). I also wanted to make it just thermocouple sensing just to learn the initial steps of dealing with them in a PLC.

Mix n match of sensors seems to me to be a quick way to foul yourself up. I took out the thermocouples that were installed by person who set this thing up and installed my own. I have used these TCs on a Fluke data logger with no problems in the past. They are almost identical. I am going to go to my storage locker and pull out a couple that are IDENTICAL and have used in past with no trouble on the data logger. They are just two wires no ground conductor in em, single length (no splices) so should (in theory) be as trouble free as I can get em.

After I KNOW that the TCs are identical then I may have narrowed my list of problems to chase down.

The manual also mentioned that if the CJC (cold junction compensating thermistor assemblies)that are installed are loose etc etc they can contribute to problems so I need to check these.
Any thoughts on these??
 
I had this problem on an extruder with a slc5/04 and a 1794-NT8 module on a rio link

The thermocouples were J types and the extruder had a lot of noise on it, everything we tried didnt really cure the problem.

In the end we changed the standard J types to Insulated J type thermocouples this solved our problem 100%

In your case with water might be a different scenario though!

NB. By Standard thermocouples i mean the type thats connected (earthed) to the jacket it sits in, by Insulated i mean the welded thermocouple tip is totally Insulated from the outer probe case
 
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Is the T/C assembly in a stainless steel sheath, or are you working with thermocouple wire that is bare wire, twisted or welded at one end?

The T/C wire in a sheath can be either grounded to the sheath or ungrounded, insulated by mineral insulation (MgO) from the sheath.

If you've got sheathed assemblies, use an ohm meter to find out whether the sheath has continuity to the T/C legs. If it does, it's grounded and the grounding can contribute to this kind of problem, particularly when a 2nd or 3rd T/C element is connected.

Dan
 
Using K becaues that is what I have. I think instrument outfit I bought them from recommended them because they were the cheapest (been 10 years we woooont discuss memory). They worked real well on data loggers. They are just two conductor covered in PVC.

THE PROBLEM was electro magnetic interferance from the heaters (120 VAC 4 amp) in the warm water. Took several hours to chase down and finally identify the CULPRIT by process of elimination. I have read on this time and time again but it is the first time I have seen it.
The CAUSE will actually entail a rebuild of the system - but in a school lab - who wants to take the time ??

IT IS LATE and I am brain tired (dead?).

IF I were to rectify and drive the heaters on DC would that reduce the problem? They are turned on and off by solid state relays - which with DC control and DC load may not turn off - would probably substitute electromechanical relays.

Thank ;you for the help

Dan Bentler
 
Ken Moore

After we id'd the problem, I thought of using and average or running (rolling) average. BUT there is such a wide variation that I rejected the idea. ALSO it seemed to me the solution is to eliminate the problem and not try to "treat it"
We put this project on hold (I get an A on it). Later on I will (NOT BY PM) contact you with a proposal on how to do it and you can correct my thinking as needed.

Next step is to use PLC to control a variable freq drive and motor. This way I can laern more on PLC and VFD while I am at it. Give me some thoughts on doing more than making the motor go round or reversal.
Thinking current sensing (load sensing ??) OR load vs speed (something like a motor starting a car on a hill - need torque but not so much as to make wheels slip)

As soon as I get that dang Win Zip (is it ??) figgered out I will look at your washer program and send you mine.

Thanks for help

Dan Bentler
 
Hi Dan,
You can download Winzip HERE. It's an eval version, has nag messages, but fully functional.

The rolling average would not have helped with your TC problems, I ususally use it when I have a signal that bounces up and down 5-10%, may not be noise, but caused by the system design, splashing, foam, vibration etc... A lot depends on the application, averaging introduces system lag, and sometime that can't be tolerated.

Three real world applications for a VFD and PLC.
1. Flow loop, using a flow transmitter, PLC and VFD.
2. Level loop, using a level transmitter, PLC, VFD.
3. Cascading flow loop, using a flow transmitter, PLC and VFD, but setpoint is based upon a Reactors temperature.

There are a LOT of hard wired VFD's out there in the real world, but if possible I prefer using some type of communication network. RIO, devicenet, profibus etc....
A little more difficult to set up the first time, but after that they're eaiser, and uses a lot less wiring.

Just my $1.298

Ken
 
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Ken

Agree on rolling average when I get into 5 to 10 % fluctuation will ask. Did not find "average" command in Allen Bradley so looks like would have to do a sum and divide. That will be another learning evolution. Hmmm maybe I should tinker with it right now.
Nahh I think I will do the VFD thing first and wait until I need it and then learn how to do it under a time crunch - that is how I have learned almost every thing so far (slight humor there).

Thanks for ideas. What is / does RIO stand for??
 
What is / does RIO stand for??
RIO = Remote I/O

Did not find "average" command in Allen Bradley so looks like would have to do a sum and divide.

The Enhanced PLC-5's have an average instruction, but with lessor processors you have to roll your own. Don't know about the ControlLogix platform, I would assume it does.
 

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