A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature.
Type R Thermocouples are high rhodium used for temperature range 0 to 1600°C & sometimes lower temperature applications for its high accuracy +/-0.25% and stability.
Type B Thermocouples are suited for extremely high temperature applications where temperature goes up to 1800°C with accuracy +/-0.50%.
Type S Thermocouples, similar to type R Thermocouple are used temperature up to 1600°C & sometimes lower temperature applications for its high accuracy +/-0.25%.