RTDs
Resistance temperature detectors use the predictable change in electrical resistance of a material, commonly platinum, with temperature. Lead resistance and wiring configuration affect measurement, which is why multi-wire arrangements are commonly used.
Thermocouples
A thermocouple produces a voltage related to the temperature difference between its measuring junction and reference junction. The receiving system therefore needs reference-junction or cold-junction compensation to infer the measurement-junction temperature.
Thermowells and protection
Protective wells isolate temperature sensors from process fluid and can make removal easier, but they add thermal mass and affect response time. Mechanical suitability for the actual process is an engineering matter, not merely a sensor choice.
Response time
A sensor can be accurate at steady state yet respond too slowly for the intended control or protection function. Sensor construction, insertion depth, flow, protective hardware and thermal contact all influence dynamic response.
Temperature transmitters
Transmitters can linearize the sensor, compensate wiring effects, convert to a standard current or digital signal and provide diagnostics. Sensor type and transmitter configuration must agree.