RTDs (resistance temperature detectors) measure temperature based on the resistance changes in a metal resistor. Pick Heaters typically uses a 100 ohm platinum RTD that has a resistance of 100 ohms at 0 degrees C.
Thermocouples use 2 different metal wires to produce a voltage relative to the temperature present in the junction between them.
Pick Heaters uses RTDs for most Constant Flow Heater applications and Type J Thermocouples for Variable Flow Heater applications.
RTDs are typically more accurate, stable and reliable than Thermocouples, but Thermocouples are much more responsive (nearly 3 times faster) to process temperature changes, hence they can keep pace with Variable Flow heater applications and maintain accurate temperature control when using electronic temperature controllers.
Thermocouples can also have a tendency to drift over time due to chemical changes (oxidation) in the sensor.
It is a good idea to check with the Pick Heater factory whenever you have questions about the type of sensor required.