Very long thermocouple runs are possible as long as certain precautions are followed.
Most instrumentation today have a high input impedance and thermocouple wire is relatively low resistance.
For example: A 1000 ft run of type J 20AWG thermocouple wire has a resistance of 350 ohms. Typical instrumentation have an input resistance of 10M ohms.
In the above case the approximate error due to the thermocouple run would be 0.0035% (350 / 350 + 10,000,000)
Thermocouple resistances can be found in our Thermocouple Catalog. Input resistance of the monitoring device should be in the documentation for that device.
It is recommended that shielded wire be used on long thermocouple runs.
You must use thermocouple wire of the correct alloy for the entire run.