A thermocouple is made of two dissimilar metals joined together. This joint creates a positive temperature coefficient with a millivolt output signal that is interpreted by a temperature control, or a high limit device. Bucan thermocouples follow the standard ISA calibration with types “J” and “K” calibrations being the most common ratings for Industrial Process Heating. For other calibrations, please check the factory. The two thermocouple constructions are mineral insulates (MI) and tube and wire, each having features suited to specific process applications.
The magnesium oxide filling gives this construction great strength, durability and a toughness needed in demanding process heat environments. MI thermocouples can be welded or brazed onto the sheath of the heating elements; they can be field-bent to suit specialized mountings; they can work well with compression fittings, and withstand the operating pressures when used in flanged or screw-plug immersion heaters.
An RTD has a resistor that changes resistance with temperature and offers better accuracy than thermocouples. RTD’s also have better stability over time. For Industrial Process Heating the RTD sensor is inserted into a tube and looks like a tube-and-wire thermocouple. The RTD usually has 3 lead wires; 2 red and 1 white. This gives it the ability to negate the lead wire resistance and the RTD and offer you extended lead lengths without loss of temperature accuracy.