R
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?
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?