Thermocouple Cold Junction Compensation Query
Thermocouple Cold Junction Compensation Query
(OP)
Hello,
I am working with control instrumentation for the first time and I am looking into Type K Thermocouples.
My question is this, I understand the need for cold junction compensation - am I correct in assuming that I can use a RTD to measure to measure the temperature at the cold reference junction, and that it is ok for the temperature at the terminals of the cold reference junction to go up and down (together), and that this won't have a negative impact on the readings from the hot end of the thermocouples.
Thanks a bunch
Jericho.
I am working with control instrumentation for the first time and I am looking into Type K Thermocouples.
My question is this, I understand the need for cold junction compensation - am I correct in assuming that I can use a RTD to measure to measure the temperature at the cold reference junction, and that it is ok for the temperature at the terminals of the cold reference junction to go up and down (together), and that this won't have a negative impact on the readings from the hot end of the thermocouples.
Thanks a bunch
Jericho.





RE: Thermocouple Cold Junction Compensation Query
RE: Thermocouple Cold Junction Compensation Query
Just to confirm, the cold end of the thermocouple can be wired to the inside a box that fluctuates in temperature, as long as the fluctuation in temperature is measured by a RTD.
Thanks.
Jericho
RE: Thermocouple Cold Junction Compensation Query
What temperature measurement system are you using that doesn't have built-in cold junction compensation for a thermocouple?
This is the 2nd decade of the 21st century. Nowadays, rolling your own CJ is either a lab exercise or a 1952 instrumentation history sidelight in a trade journal.
Yes, you can make your own CJ measurement with an RTD or thermistor and do the math, but why would you?
RE: Thermocouple Cold Junction Compensation Query
Thank you for the response.
We are actually using a fairly expensive PLC + HMI system (Modicon M340 + Magelis XPT GTO4310) and to tell you the truth I was surprised to see there was not built in CJC for the analogue input cards the Thermocouples are connected to.
There is an additional card you can buy in conjunction to the analogue input cards that has internal CJC - however this costs quite a lot in comparison to buying a RTD and having the PLC do the maths.
For anyone that may have used the same PLC system - the thermoelectric voltage is being sent to a BMXART0414 analogue input card.
Thank you all for your time in responding.
Jericho.
RE: Thermocouple Cold Junction Compensation Query
http://www.alliedelec.com/m/d/2949e4bd41f6095fa93c...
-AK2DM
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"It's the questions that drive us"
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RE: Thermocouple Cold Junction Compensation Query
That's what marketing calls 'unbundling'.
RE: Thermocouple Cold Junction Compensation Query
AK2DM - I saw the support for CJC, sorry I guess I did not make that clear. So the idea is to connect a PT100 to it to provide the CJC.
Danw2 - do you think that any PT100 would do? On the bottom of page five it says:
"The CJC sensor detection input is connected to CJ+ if the sensor type is DS600. It is not connected (NC) if the sensor type is LM31"
I bought a 'DS600' sensor type without really understanding what it was, it turned out to be an extremely tiny pcb chip, which in my eyes is impossible to wire to the module. I cannot find any information on the LM31 type - in reality, i feel a PT100 would suffice?
Thanks
Jericho.
RE: Thermocouple Cold Junction Compensation Query
The DS600
https://datasheets.maximintegrated.com/en/ds/DS600...
has a voltage output.
A Pt100 is a passive resistance only, the AI card pumps a constant current through the RTD resistance to create an IR drop that the card can read.
Where is the current going to come from for an RTD input?
I don't know. Someone more astute than I needs to figure that one out.
RE: Thermocouple Cold Junction Compensation Query
RE: Thermocouple Cold Junction Compensation Query
Another, similar, description: http://www.mstarlabs.com/sensors/thermocouple-cold...
The PTC is how the instrument determines the amount of compensation needed to account for the cold junction voltage offset. done a transfer calibration
TTFN (ta ta for now)
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RE: Thermocouple Cold Junction Compensation Query
The DS600 is a tiny PCB chip, with roll-your-own mounting and wiring.
There is no reference, other than one mention of it in the Modicon spec sheet, as to what an LM31 is.
The Modicon spec infers that CJ can be accomplished with a Pt100: "Cold junction compensation External by Pt100 probe".
I did not find a hardware manual in a quick Google search. But I'm speculating that for thermocouple CJ, that maybe one has to use one of the available channels for an RTD for external cold junction reference temp, and then in the thermocouple channel setup software there's some sort of selection for CJ applied to the T/C channel, like a) DS600, b) LM31, c) Pt100 channel _n__
I'm amazed that there is not internal CJ as a standard.
Usually references to "external CJ" are for those relatively rare situations when, like Hacksaw says, an icebath CJ cal is needed. Or the terminal blocks are in a different panel and copper wire comes back to the thermocouple card and the CJ has to be the remote terminal block temperature, not the local terminal block temperature.
RE: Thermocouple Cold Junction Compensation Query
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Thermocouple Cold Junction Compensation Query
Thank you again for taking the time to leave responses.
I have managed to come across what i believe is the official manual for how to operate this analogue card it states quite clearly that a PT100 probe can be used in the first channel of the module to act as the cold junction compensation of the module.
IRStuff - I read the links, I have come across them before and they are useful so thank you for that. From them I take away that any type of 3 - wire RTD PT100 will suffice, so long as it is calibrated well.
Additionally, Danw2 - I found in the PLC software the smallest of tabs that allow you to set the CJC to either Internal (using the additional hardware i mentioned in previous post), or external by PT100. This must be wired to channel 0/4.
Thanks again for all the help,
If anyone has any more comments i'd be happy to discuss further
Jericho.
RE: Thermocouple Cold Junction Compensation Query
I assume this HAS to come from the analogue module?
Jericho.
RE: Thermocouple Cold Junction Compensation Query
>supply of power to the PT100. I assume this HAS to come from the analogue module?
Yes. Any of the inputs that claim they are useable with a Pt100 will have an internal constant current source, provided the point is wired correctly and configured for an RTD.
RE: Thermocouple Cold Junction Compensation Query
There's an easy test to see if the board has internal CJ or not.
Short the thermocouple input with a copper wire or a paper clip and the point should read close to ambient temperature (technically the terminal temperature but it depends on where the CJ is located at what temperature sees).
The short is 0.0mV, typical of an isothermal thermocouple at ambient temperature and the CJ is supplying the difference between the freezing point of water and whatever temperature it sees.
If the reading with a short is near the freezing point of water, then there is no internal CJ or it hasn't been turned on for that channel.
I strongly suspect that the board has internal CJ. The presence of internal CJ is 'inferred', the board being a thermocouple input board, and the other references to external CJ are for the 0.03% of people who need external CJ.
RE: Thermocouple Cold Junction Compensation Query
Based on a cursory survey of webpages, probably only about 1/4 of TC instruments use an actual TC for the CJC. In either case, an isothermal block is required, and a temperature measurement is required. For most instruments there is no advantage to having a 2nd thermocouple; only when calibrating would an ice bath be desirable.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Thermocouple Cold Junction Compensation Query
You could never achieve suitable ice reference functionality using the an externally applied reference to the module's terminal blocks. The module's terminal blocks will have a built-in isothermal structure to help with basic reference accuracy. The only rational use is as mentioned above where the TCs all terminate out somewhere else.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Thermocouple Cold Junction Compensation Query
Dan - I like the idea of the test and it is something I will be trying.
I do think that it will be necessary to provide external CJC as literally no where on this manual it suggests that internal CJC is already provided (without the additional accessory that is).
I have included an image of the manual that focuses on CJC.
Thanks
Jericho
RE: Thermocouple Cold Junction Compensation Query
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: Thermocouple Cold Junction Compensation Query
Seeing is believing. I did not think that there was a commercial thermocouple AI card that did not have internal CJ as a standard feature in this day and age. Yet, here it is.
What a pain to not only have to supply a CJ sensor for each 4 channel card but to have to position the sensor for a decent CJ reading.
I'd probably opt for small diameter 2 wire Pt100's. The connections are so close to where the measurement needs to be made that the lead wire error will be insignificant.
And it appears that one gets to use all four input channels for thermocouples, when using the 2 wire RTD option (across EXO+ and EXO- in the graphic below).