There are several graduation numbers of thermocouples: S, R, B, N, K, E, J, T and so on. Among them, S, R, and B belong to precious metal thermocouples, and N, K, E, J, and T belong to inexpensive metal thermocouples.

The characteristics of the S are strong oxidation resistance, and it is suitable for continuous use in an oxidizing and inert atmosphere. The long-term use temperature is 1400C and the short-term 1600C. Of all thermocouples, the S graduation number has the highest accuracy level, and is usually used as a standard thermocouple;

Compared with the S graduation number, the R graduation number is about 15% larger than the thermal electromotive force, and other performance is almost the same; The B graduation number is extremely small at room temperature, so it is generally not necessary to compensate the lead when measuring. Its long-term use temperature is 1600'C and short-term 1800C. Can be used in an oxidizing or neutral atmosphere, or short-term use under vacuum conditions.

The characteristics of the N index number are strong resistance to oxidation at 1300C, the long-term stability of the thermoelectromotive force and the reproducibility of the short-term thermal cycle, and the resistance to nuclear radiation and low temperature.

K index number is characterized by strong anti-oxidation performance, suitable for continuous use in oxidizing, inert atmosphere, long-term use temperature of 1000 "C, short-term 1200 ° C. Most widely used in all thermocouples;

The characteristic of the E graduation number is that among the commonly used thermocouples, the thermoelectric force is the largest, that is, the sensitivity is the highest. Should be used continuously in an oxidizing, inert atmosphere, using temperature 0-800C;

The characteristics of J index number are that it can be used in both oxidizing atmosphere (upper temperature limit 750 "C) and reducing atmosphere (upper temperature limit 950C), and is resistant to H2 and CO gas corrosion.