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Old August 19th, 2018, 04:29 AM   #1
AB2005
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Problem in measuring of a panel room temperature by a thermocouple connected with a P

Hi;

In a panel room of a flexo graphics printing machine, there are 11 panels having servo drives and VFDs. There are industrial air conditioners mounted at all panels. Then all the panels have been covered by an aluminum cabin (panel room). For cross ventilation in panel room, a 5000cfm blower has been installed with corresponding 03nos exhaust fans. A FATEK brand PLC FBs-20MA has been installed with a thermocouple module FBs-6TC. A regular Chinese made round tip rod “K” type thermocouple has also been installed and connected with thermocouple module to measure the EXHAUST AIR Temperature. PLC monitors the exhaust air temperature and regulates the speed of fresh air blower and switches the exhaust fans. As the system is in commissioning phase and almost completed, so I have also installed an Autonics (Korean) brand temperature meter with same “K” type thermocouple connected to verify the same temperature reading both at PLC and Autonics meter.

My problem is that some time the temperature display of both PLC and Autonics meter is equal and some time there is a difference up to +/-6°C. I am confused why this is happening. Anyone can help me out?
I also need info which kind of thermocouple is suitable at such position to measure correct temperature?
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Old August 19th, 2018, 05:02 AM   #2
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Type K is fine. Did you use Type K thermocouple extension wire? Type K connectors (yellow)?
Both meters should have Cold Junction compensation. Sounds like one of them drifts with local temp. You need to figure out which one is wrong.

If it's not that, then it might be noise between units. Try two thermocouples.

Last edited by keithkyll; August 19th, 2018 at 05:10 AM. Reason: More info
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Old August 19th, 2018, 02:39 PM   #3
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Are you reading the same TC with the PLC and the instrument?
Sometimes the way the device detects an "open Thermocouple" condition causes interference in the reading in the other device.
Also the the cold junction compensation on 1 device can interfere with the other reading.
If you are using both devices on 1 TC. Disconnect the TC and see how each device responds.

If you are jumping the Thermocouple cable from 1 device to the other. make sure the wires are together under the screws, and not on each side of screw. This can cause an additional cold junction error.
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Old August 20th, 2018, 04:21 AM   #4
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Both devices (Autonics meter and PLC) have different thermocouple. Thermocouple-1 (K type) for Autonics meter.
Thermocouple-2 (K type) for PLC.
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Old August 20th, 2018, 06:00 AM   #5
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Are both thermocouples next to each other measuring the same physical location ?
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Old August 23rd, 2018, 11:22 AM   #6
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Quote:
Originally Posted by danw View Post
Are both thermocouples next to each other measuring the same physical location ?
Yes, both have been installed at same location. But Autonics meter is just 1.5m away while PLC is almost 7-8m away.

Its mean there is no any special thermocouple for said application. Correct?
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Old August 25th, 2018, 12:14 AM   #7
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A standard ”Limit-of-error” Type K thermocouple has an accuracy of +/- 2.2 Deg C. ”Special Limit-of-error” reduces that to +/- 1.1 Deg C. Are your Chinese Type K’s Special Limit-of -Error?

An RTD, also called a Pt100, is a better choice for ambient temperature measurements because RTD’s are inherently more accurate than thermocouples, as sensor elements and due to the fact that RTD’s do not use cold junction compensation so there is no compound error component from the CJ measurement.

I suspect that temperatures changing due to air current changes (VFD and ’switches’) inside the panel create a changing CJ temp which probably lags the hot junction measurement on one of the units which results in a delta until the CJ catches up.
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Old August 25th, 2018, 11:31 AM   #8
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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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Old August 26th, 2018, 01:22 AM   #9
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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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Old August 26th, 2018, 06:40 AM   #10
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Quote:
Originally Posted by AB2005 View Post
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.
Good decision!
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Old August 27th, 2018, 12:07 PM   #11
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Quote:
Originally Posted by AB2005
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?

Whichever your PLC's RTD input card is comfortable with. HVAC applications tend to use 1000 ohm RTDs, probably because the IR voltage drop is 10x higher for any given source current, so a cheaper amp/A/D in the analog input can be used. The higher level signal also magnifies small temperature changes because the expected range for HVAC is not as wide as industrial temp ranges. Industrial apps tend to use 100 ohm RTD. If the plant already uses RTDs for other apps, I'd go with whatever else they use. I personally favor 100 ohm because it seems to me (in the industrial world) that 100 ohm are more readily/widely available. Either will work (assuming your card handles the range)

Quote:
Originally Posted by AB2005
There are different kind of RTDs available, GENERAL PURPOSE RTDs is ok for me? (https://www.sterlingsensors.co.uk/rtds.html)

Yes.


Quote:
Originally Posted by AB2005
Can I use regular Beldon 3C X 0.5mmq shielded cable as an extension cable? (Instrument will be almost 7/8m far from PLC)

yes

Quote:
Originally Posted by AB2005
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.

Class is an accuracy spec, meaning how the temperature measured by a field unit compares to the "true" temperature measured by the nations standards laboratory. An RTD will be very repeatable, so any error from national standards (if you could know what that was) will be consistent from day to day, week to week, month to month. I'm not a metrologist and I see the world through the lens of 'good enough', not the nit picking that metrologists love to argue about. For me, the "accuracy" of panel temperature measurement with a B class RTD is good enough. If you need to assure your customer that your measurement is "accurate" (by national standards) within ±0.5°C, then you'd have go to Class A (and your AI would have to be capable of that performance,too), but I can't believe it makes a fig of difference in the end.

Quote:
Originally Posted by AB2005
Any other instruction?

The most important - use only an AI card capable of handling a 3 wire RTD and a 3 wire RTD element. 2 wire has a constant positive offset and true 4 wire AI cards are for laboratory grade measurements, not needed in your case[/quote]
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