Thermocouple Cold Junction Compensation Query

[Jericho93]

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.

 

[Compositepro]

Yes, that is how it works.

 

[Jericho93]

Thank you for the prompt reply.

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

 

[danw2]

> 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.

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?

 

[Jericho93]

Hi Danw2,

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.

 

[analogkid2digitalman]

Looks like the module does support CJC on the CJ+ and CJ0 terminals:

http://www.alliedelec.com/m/d/2949e4bd41f6095fa93c798467bba08f.pdf

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
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[danw2]

That same document on page one says, "Cold junction compensation External by Pt100 probe".

That's what marketing calls 'unbundling'.

 

[Jericho93]

Thank you for the responses,

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.

 

[danw2]

You got me. It is not at all clear.

The DS600

https://datasheets.maximintegrated.com/en/ds/DS600.pdf

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.

 

[hacksaw]

Ultimately you will have to perform an "ice bath" calibration to confirm important measurements.

 

[IRstuff]

Maxim's app note explains what's going on:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/4026

Another, similar, description:

http://www.mstarlabs.com/sensors/thermocouple-cold-junctions.html

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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[danw2]

I think Jericho93's issue is how to implement CJ with this hardware.

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.

 

[IRstuff]

That's why I suggested the OP read the links. The standard approach nowadays is a single TC with PRT measurement of the junction block. No cold TC is required, because it contributed nothing in the original approach, since you needed an external measurement of the CJ TC temperature anyway. If the PRT is well calibrated, you don't need an ice bath calibration either.

TTFN (ta ta for now)
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[Jericho93]

Good afternoon folks

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.

 

[Jericho93]

Daw2 - I just remembered a previous comment of yours regarding the supply of power to the PT100,

I assume this HAS to come from the analogue module?

Jericho.

 

[danw2]

Thanks for the update on what's available in the software setup for CJ. That makes sense.

>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.

 

[danw2]

I've pondered this and can't believe that in a competitive marketplace that an analog input board from a major manufacturer which is represented as being a thermocouple input board does not have internal CJ. EVERY other T/C input in the marketplace has internal CJ. I joked about marketing unbundling CJ to save $0.87 on a CJ chip, but it presses credulity to not have internal CJ on a thermocouple input board.

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.

 

[IRstuff]

A separate cold TC is redundant, since the resulting voltage output is essentially the same without it. That's because the M1/M2 couple in series with the M2/C couple is electrically and mathematically equivalent to a single M1/C couple.

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)
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[itsmoked]

I fully agree with Dan's assessment and suggest you try the test. Lack of a built-in ice point is for lack of a better word absurd. The "internal" comment in the manual alludes to an internal reference as I see it.

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

 

[Jericho93]

Good morning gentlemen,

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