Thermocouple are simply dissimilar wires joined at hot end.Thermocouple wire is composed of wire from sensing tip to the cold junction compensation end.But for practical purpose grade of these wires are different.It means that there will difference between thermocouple wire and extension wire.Features which is given for the wire of which sensing part is made will be different from extension wire.For ex Maximum ambient temperature of extension wire will be far less then thermocouple probe wire.
Some basic procedures to install a thermocouple wires are.
1.)Wires are fabricated into an accurate and dependable thermocouple by joining the Thermo elements at the sensing ends.
2.)To extend thermocouples extension wire of the same type must be used to indicating or Control and instrumentation.
3.)The color code for negative is RED through out.
4.) Connectors are used to extend the wire if required, they are made of the
same alloys and have the same color codes as extension wire.
5.) Exception to these codes is Hi-Temp connectors which are colored red regardless of
For extension Wire to be used are of same metals as that of thermocouples.These wires are used to connect the thermocouple to the measuring instruments.It is taken into consideration to not introduce any additional joints as it would create unwanted voltages.Since the connecting wires are of same metals they do not generate voltages.It is not necessary to use same metal but the thermal coefficient of emf the metals connecting should be same for a narrow range of temperature.For example platinum thermocouples, extension used is copper alloy because using platinum extension would be quite expensive.
Total allowable length
Theory of circuit says the input impedance of the measuring instruments(amplifying instruments) should be high so that the it draws minimum current from thermocouple.This will prevent voltage drop in wires due to wires own resistance.But still while selecting the allowable length of thermocouple wire two factors have to considered.
Total loop resistance-Generally kept less then 100 ohms.Measured by multiplying resistance per unit length and wire length.
Electrical noise immunity-Wires should be kept away from electromagnetic field source.Since the voltage generated by thermocouple is few millivolts.
Also any change in system like termination type ,size will cause unwanted junction and will effect its accuracy.