# Problem in measuring of a panel room temperature by a thermocouple connected with a P



## splatz (May 23, 2015)

The thermocouple makes such a tiny voltage it's very possible there's something going on one one of the signal wires or grounding that's screwing up the signal on one but not the other. Maybe watch for whether the readings drift apart when something else switches on. 

You can put one thermocouple on two inputs at the same time. It's generally not recommended but I think it would be a good troubleshooting step.


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## glen1971 (Oct 10, 2012)

Is the card configured properly? Simulate the thermocouple to the PLC at the thermocouple and do a minimum of a 3 point check on it.. 
I ask, since we recently found that after several cards were replaced on a plant DCS, they were only single point checked and now just over a year has passed and they were good on the positive side, but once they were ran in the negatives their error got around 10 degrees.. This was part of a huge process issue, costing thousands..


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## varmit (Apr 19, 2009)

If the two instruments are connected in parallel on one thermocouple, there will usually be variations from actual temperature and between the two instruments. Replacing the thermocouple with a double ended thermocouple (2 thermocouples in one unit) would be best. The other option would be a signal splitter with 1 T/C in and 2 T/C out.

You will probably need to work on calibrating both inputs to where the will read the same temperature.


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## gpop (May 14, 2018)

You may need to trend it in the plc to see whats really going on. You need to know if its drifting or spiking out of calibration.


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## MDShunk (Jan 7, 2007)

I have a fair bit of disdain for thermocouples. An RTD is the way to fly. This is 2018.

I feel like I should ask, though... did you use actual thermocouple wire between the thermocouple and the PLC thermocouple input module, or did you use ordinary Belden-type cable? What sort of distance between the two is involved?


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## glen1971 (Oct 10, 2012)

MDShunk said:


> I have a fair bit of disdain for thermocouples. An RTD is the way to fly. This is 2018.
> 
> I feel like I should ask, though... did you use actual thermocouple wire between the thermocouple and the PLC thermocouple input module, or did you use ordinary Belden-type cable? What sort of distance between the two is involved?


To add.. Any terminals need to be Thermocouple terminal blocks..

Each has a place.. Furnaces, and engine temp are two places that come to mind for thermocouples... For ambient temperature measurement, I'd be voting on the RTD too..


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## Rora (Jan 31, 2017)

If I understand correctly, you're measuring with two entirely separate systems with components (including sensor leads) that might be located in different places. There are several things which could generate a different reading, although there is already a combined uncertainty of 4 C (assuming +/- 2 C each).

- Cold junctions in different locations and/or misconfigured cold junction compensation

- Different length and routing of the sensor leads, leading to different lead resistance and exposure to different noise profile

- Incorrect type of lead extensions

- Different structure of the probe, which may respond faster or slower to non-contact temperature changes

Thermocouples are popular because of their large measurement range and cheap cost... I'd have to strongly agree with the others here, an RTD makes much more sense. Especially considering the relatively mild range of temperature measurement and the need for more accuracy, plus the fact that you only have to put in one of them. Try to find one with a probe that is made for ambient air measurements/HVAC.


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## MDShunk (Jan 7, 2007)

Something just dawned on me. Has either the PLC input module or the meter you're using to double-check during commissioning had a proper calibration? Even just ice water and boiling water check? It's completely within the realm of possibility that one or both is out of calibration. 

Even still, I think a this is a dumb application for a thermocouple.


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## splatz (May 23, 2015)

I wasn't even thinking about RTDs, but maybe that would make sense, they don't cost that much and they handle the noise - if it is indeed noise that's the issue. 

This may just be my own bias but. I was thinking a thermocouple, a TC to 4-20ma signal converter near the thermocouple, and 4-20ma looped through both the meter and the PLC input. 

All said and done, you have to buy PLC module, sensor, maybe a signal converter, and panel meter. 

If the PLC already has 4-20ma inputs available, you save money because you don't have to buy a module. If the PLC doesn't have available 4-20ma ports, the 4-20ma module you add is more versatile, you'll be able to do more with the leftover inputs.


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## Signode (Oct 9, 2016)

Hi

Thanks for inputs. The PLC thermocouple module has possibility only to connect TC not RTD. Both TCs have been installed at same location. But Autonics meter is just 1.5m away while PLC is almost 7-8m away.
Moreover, in the specs of PLC module, this is written _*Built-in cold junction compensation*_

Its mean there is no any other option available other than K type thermocouple for present condition.


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## just the cowboy (Sep 4, 2013)

This is what I am using to sense air temperature in my plant. It is an RTD and transmiter all in one ss housing. https://www.automationdirect.com/ad...ers/Temperature_Transmitters_(Integral_Sensor)

For $121 you can't beat it, add a $179 click PLC with digital & analog IO with Enet, and you can do almost anything.


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## Signode (Oct 9, 2016)

just the cowboy said:


> This is what I am using to sense air temperature in my plant. It is an RTD and transmiter all in one ss housing. https://www.automationdirect.com/ad...ers/Temperature_Transmitters_(Integral_Sensor)


The page is not opening, there is some error with it.


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## just the cowboy (Sep 4, 2013)

*try this*



Signode said:


> The page is not opening, there is some error with it.


https://www.automationdirect.com/ad...smitters_(Integral_Sensor)/XTP25N-030-N40140F


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## Signode (Oct 9, 2016)

After having discussion, i have decided to move to RTD because TC is not good for low temperature measurements.
Yes, i will have to change the PLC module from TC to RTD and thermocouple also.


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## splatz (May 23, 2015)

That's what's nice about the RTD and transmitter combo that @Just the cowboy mentioned, the signal converter comes with the temperature probe so you can use a regular 4-20ma input rather than a special temperature module. 

That's nice in general but especially for your setup, since you can loop the 4-20ma output in series through both the PLC input and the panel meter.


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## Signode (Oct 9, 2016)

Dear Friends;


Thanks to spend your valuable time. I need a bit help to select the right instruments.
Fatek PLC and modules are not much expensive like AB/Siemens etc. So moving from TC to RTD is not a big issue. For selection of RTD, kindly guide;




As you people know well about my application (measuring of panel room temperature, range 20-70c) where PLC monitors the exhaust air temperature and take the programmed action, should i select PT100 or PT1000?
There are different kind of RTDs available, GENERAL PURPOSE RTDs is ok for me? (https://www.sterlingsensors.co.uk/rtds.html)
Can I use regular Beldon 3C X 0.5mmq shielded cable as an extension cable? (Instrument will be almost 7/8m far from PLC)
There are also Class A, B, 1/3 DIN, 1/5 DIN and 1/10 DIN RTDs available. Which class will be ok for my application? For me temperature variation within 0.5C+/- is ok.
Any other instruction?


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## just the cowboy (Sep 4, 2013)

*4-20ma*



Signode said:


> Dear Friends;
> 
> 
> Thanks to spend your valuable time. I need a bit help to select the right instruments.
> ...


If you use the one I selected for you you would just use a 4-20ma analog input card. It uses a m12 cordset no special wire or setup. Just change scaling in PLC to calibrate. Easy to use.


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## Signode (Oct 9, 2016)

I have arranged the PLC RTD Module and RTD PT100 temperature sensors. I have some questions;




What could be the maximum length of cable from RTD to PLC?
Can we use normal un-shielded 3core cable as extension?
What should be the size of cable from RTD to PLC (0.5, 0.75 or 1mmq)?


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