Compensating and Extension Cable for Thermocouple

Compensation cables for the thermocouples are of the same type as the thermocouple. They are a little bit smaller in size, depending of the distance to be covered and a X suffix. So, a T type thermocouple extension wire is TX cable.

A smart transmitter compensates for the thermocouple created by the junction of the extension wire to the connection screws by measuring the ambient temperature and adding or subtracting the corresponding millivolts in the smart transmitter software running in the microprocessor. Below is image for this cable

The smart transmitter cannot compensate for a copper extension wire, it's not a current practice.

If you have to install terminal blocks in a thermocouple junction box, you have to use special compression type terminals (Entrelec manufacture them) which compress the thermocouple wire with the extension wire, so no cold junction generating voltage is created.

If you have a lot of thermocouple to send to a remote site and in order to lower the extension wire costs you can use copper wires. But, here's the trick. If you look at thermocouple table, you can see that all thermocouple types generates zero millivolts at zero degrees centigrade. The first solution (which is not practical) is to freeze at zero the interior of your junction box. The result is zero millivolts generated at the cold junction. The other solution is to heat the interior of the junction box at a known temperature over the maximum reachable ambiant temperature and compensate for the known error millivolt generated. It's a solution I have once seen but we had to determine the error generated by copper to type K thermocouple. The junction box was heated at 150 degrees Fahrenheit.

Its suggested that we must used extention dan compesating cable with their resistifitay is less than 100 Ohms (i.e. total of both legs) .

One example is that of Type K extension cable which has a combined loop resistance of 4.5 Ohms with 7/0.2mm conductors; in this case, 20 to 25 (100/4.5) meters is the maximum permissible cable run. The use of larger diameter wires will permit greater lengths of course.