Control.com - Nerds in Control

We have a controversy at our plant regarding how much error (if any) is introduced by using standard isothermal terminal blocks on thermocouple heads, rather than special terminals matched to the thermocouple type.

We always run thermocouple extension cable directly from the thermocouple head terminal block right up to the PLC thermocouple input module, where cold junction compensation takes place.

However, the terminal blocks on our thermocouples are general purpose isothermal blocks, not special type R polarized terminal blocks, and therefore the thermocouple element is not in direct contact with the extension cable and two additonal junctions are introduced. Thermocouple literature always insist on not adding intermediate metals to t/c lines because of this parasitic junctions, but I also understand that if the temperature on a terminal block is exactly the same on both ends, the net effect of the extra junctions in series cancel out and the net effect is zero error.

One of my collegues states that even if we have those additional junctions on the head terminal block (which is very hot by the way), no error is introduced because it is an isothermal block.

It makes sense to me, but I want to be sure, can anybody enlighten me on this?

I agree with your colleague, the mV generated by one joint is cancelled by the other. Isn't there something in the law of intermediate metals about that?

Roy

As long as the junctions are the same temperature, and the same material is used, then no error is introduced.

OR similar material is used for the compensating cable. We have a small number of platinum/rhodium TCs and there is no way we would run identical cables in. We just use standard compensating cable matched to the TC (can't recall exactly what it is off the top of my head). But needless to say it works really well.

If we had a really critical temperature that required accurate measurement we would use RTs as standard anyway.

It's called the law of intermediate metals. See http://www.wici.com/technical_info/articles/temp_prm/tmprmch1.htm for some discussion or http://www.ariindustries.com/thermocouples/how_thermocouples_work.php3

In this case you have two instances of intermediate metals in your circuit, one for each leg of the circuit. This will work to the extent that the isothermal block really is one temperature with no gradient. If you have a gradient of say 3 C, then you will have a reading error of roughly that size.

Eric
www.icpdas-usa.com

I am a supplier of terminal heads & terminal blocks. All terminals on the block are made of brass coated with nickel. And it works well in plants.

http://www.taisuo.com
taisuo@cnool.net