Answer: Thermocouples are low-level millivolt sensors that typically come in contact with the process. Very long thermocouple runs can be affected by what's know as "immersion error." Immersion error occurs when the thermocouple extension wire acts like a heat sink to hotspots near the wire run, i.e., hot water pipes. The wire will conduct the heat causing the dissimilar metal junctions to change in temperature, thus causing more errors. Therefore, to increase the integrity of a thermocouple measurement, an accurate and stable amplifier combined with isolation should be placed as close to the sensor as possible.
For systems with widely distributed measurements, single-loop, field-mount transmitters are typically used. Otherwise, for centrally wired panel applications, DIN rail-mount transmitters are ideal.