why we need compensation cable if law of intermediate metal works? is it required when transmitter is near to sensor?

 

simply because the various junctions are at unknown temperatures, so it depends on your design and acurracy required. In extreme cases the measurement error could be 50-100 DegC.

 

> is it required when transmitter is near to sensor?

How near is "near"?

 

Thermocouple extension cable is required because it completes the measurement, which is the EMF generated across entire thermal gradient from the 'hot' end all the way to the 'cold' end. The 'cold' end temperature is measured as 'cold junction compensation' and its value is added to the temperature value from the EMF generated across the thermal gradient.

Copper wire has no thermocouple characteristics because it is not two dissimilar metals. Therefore copper wire cannot complete a thermocouple's thermal gradient, copper wire terminates the thermal gradient.

If copper wire is used (thermocouple extension wire/cable is not used), the actual 'cold junction' is where the thermocouple connects to the copper wire. That junction temperature is not measured and could be very different from the measurement made at the terminals of the analog input where cold junction is measured. Any claims of accuracy are out the window if extension wire is not used.