Thermocouple Cold Junction Compensation?

Well, stop and think about what it is that cold junction compensation does.

The thermocouple's "cold end" connection point is not likely to remain at 0°C, is it?

Yet all the thermocouple tables and the polynomial equations derived from them assume the cold end is referenced to 0°C.

So the field cold junction measurement is the temperature of the terminal connections at the 'cold end' of the thermocouple, ie where the thermocouple connects to the analog input device.

What is the typical DCS analog input? 4-20mA

What do the process industries use to convert thermocouple readings to 4-20mA? temperature transmitters

So, where does a thermocouple used for temperature measurement occur in a DCS and where does the cold junction compensation take place ?

Hello,

First of all I'd like to thank you for your answer. I guess you didn't get what exactly I had asked about. I know that a Temperature Transmitter will do this by itself, by means of the electronics installed in it. But what about the Thermocouples that are directly connected to the DCS without any Temperature Transmitter in their way to Control Room. So, What I wanted to know is what card does this compensation, and obvioulsly it is electronically compensated by the card it is connected to.

Same deal.

Wherever the thermocouple connects to an analog input, the cold junction compensation needs to measure the temperature of the terminal connection point.

If the T/C terminates at the transmitter, CJC happens there.

If the T/C terminates at a thermocouple analog input, then CJC happens there.

They do it using electronics. There are various ICs that do precisely that. http://www.emesystems.com/OL2therm.htm#MAX667x_description These ICs can be put into temperature transmitters, in which they convert TC voltage into higher-level voltage, after which the latter gets converted to 4-20 mA, 0-10 V, or whatever (in the transmitter). Otherwise, TCs are extended all the way to the DCS by using TC wire. In that case, the voltage-conversion/junction-temperature-compensation ICs are input cards that are part of the DCS. Cheers! DZ

Many Thanks.

Scream,

Download this book

Lessons In Industrial Instrumentation. By Tony R. Kuphaldt

CHAPTER 20. CONTINUOUS TEMPERATURE MEASUREMENT

20.4 Thermocouples

that chapter have a nice explain about cold junction compensation

Regards

JP

Thank you so much.

hi,

This is very correct that Thermocouple Cold Junction Compensation takes place at either Transmitter end or at the card level (for T/C directly connected to DCS).

My doubt is what about the compensation for junction which occurs at the T/C head (what is material of Terminal block at T/C head) and at Junction boxes for using multi-pair cables.

Thanks in advance.

Shyam

There's no cold junction compensation at head blocks or junction blocks.

The assumption (when using head blocks or junction blocks) is that the block is isothermal, meaning that its temperature is the same across the block.

When the terminals or block remain isothermal, there is no gradient created between the terminals.

The law of intermediate metals says that a terminal block made of a material other than the two dissimilar thermocouple conductors does not introduce an error unless there is a temperature difference between the connection points.

Thank you....Iris

Hello Everyone,

I am looking at this post of mine after a long time and it's good to see that you have answered in different ways and everyone shared what they knew. So, I just wanted to tell you people that I have figured it out myself and it's the CTU (Conditioning and Termination Unit) in a DCS, where the Cold Junction Compensation takes place.

I would've added more but I can't seem to find my notebook right now.

Thanks everyone for participation.