Calibration types have been established by the American Society for Testing and Materials(ASTM)according to their temperature versus EMF characteristics in accordance with ITS-90,in standard or special tolerances.
A thermocouple circuit has at least two junctions: the measurement junction and a reference junction. Typically, the reference junction is created where the two wires connect. This second junction is really two junctions but considered as one (thermal) junction.
Calibration types are designed to deliver as close to a straight line voltage curve inside their temperature application range as possible. This makes it easier for an instrument or temperature controller to correctly correlate the received voltage to a particular temperature.
The smaller the diameter, the faster the thermocouple responds. Grounding the junction also improves response time by approximately 50 percent based on the sensor achieving 63.2 percent of the final reading or to the first time constant.