What are the differences and selections of thermocouples of different calibration numbers (S, R, B)?

Difference
Material composition
S scale: The positive electrode is a platinum-rhodium alloy, containing 10% rhodium and 90% platinum, and the negative electrode is pure platinum, commonly known as a single platinum-rhodium thermocouple.
R scale: The positive electrode is a platinum-rhodium alloy, containing 13% rhodium and 87% platinum, and the negative electrode is pure platinum.
B scale: The positive electrode contains 30% rhodium and 70% platinum, and the negative electrode contains 6% rhodium and 94% platinum, commonly known as a double platinum-rhodium thermocouple.
Temperature measurement range
S scale: The maximum long-term use temperature is 1300℃, and the maximum short-term use temperature is 1600℃.
R scale: The maximum long-term use temperature is 1300℃, and the maximum short-term use temperature is 1600℃. The temperature measurement range is similar to that of the S type, but the thermoelectric potential is slightly higher than that of the S type at the same temperature.
B scale: The maximum long-term use temperature is 1600℃, and the maximum short-term use temperature is 1800℃, which is the highest temperature measurement limit among the three types.
Accuracy and stability
S scale: It has the characteristics of high accuracy and good stability in the thermocouple series, and is often used for high-precision temperature measurement and as a standard thermocouple.
R scale: The accuracy and stability are equivalent to S type, and the reproducibility is slightly better than S type.
B scale: High accuracy, good stability, especially excellent stability at high temperature.
Thermoelectric potential characteristics
S scale: Thermoelectric potential and thermoelectric potential rate are small, and the sensitivity is relatively low.
R scale: Thermoelectric potential is about 15% larger than that of S type, and other properties are almost the same as S type.
B scale: Thermoelectric potential is extremely small at room temperature, and the thermoelectric potential is less than 3μV in the range of 0~50℃, and usually no compensation wire is used for compensation.
Environmental adaptability
S scale: Applicable to oxidizing and inert atmospheres, very sensitive to pollution.
R scale: Applicable to oxidizing and inert atmospheres, sensitive to pollution.
B scale: Applicable to oxidizing and inert atmospheres, and can also be used in vacuum for a short period of time, but not suitable for reducing atmospheres or atmospheres containing metal or non-metallic vapors.
Cost
S scale: Due to the use of precious metal materials, the one-time investment is large.
R scale: Like the S type, the cost is higher due to the use of precious metals.
B scale: The amount of precious metal materials used is large, the cost is high, and the price is relatively more expensive.
Selection
According to the temperature range
S scale: Applicable to medium and high temperature measurements, the temperature is in the range of 500℃-1300℃, such as kiln temperature measurement in the glass, ceramics and other industries.
R scale: Similar to the S type, it is often used in temperature measurement occasions of 500℃-1400℃, such as some heat treatment furnaces with high temperature accuracy requirements.
B graduation: Mainly used for ultra-high temperature measurement, in environments with temperatures above 1600℃, such as in some high-temperature processes such as high-temperature melting and glass fiber production.
According to measurement accuracy
S graduation: Commonly used in situations where high-precision measurement is required, such as scientific research laboratories, metrology and calibration institutions, etc., and its error can be controlled within ±0.25%.
R graduation: Suitable for scenarios with high accuracy requirements and slightly larger thermoelectric potential requirements, such as temperature monitoring of some precision instruments.
B graduation: It can still maintain high accuracy and stability at high temperatures, and is suitable for situations where ultra-high temperature measurement accuracy is extremely high.
According to the use environment
S graduation: It can work stably for a long time in oxidizing and inert atmosphere environments, such as heating furnaces in steel plants.
R graduation: Similar to the applicable environment of S type, it is more suitable in oxidizing and inert atmosphere environments such as high-temperature sintering furnaces for the production of some electronic components.
B graduation: In addition to oxidizing and inert atmospheres, it can also be used for short-term vacuum environments, such as high-temperature measurements in vacuum coating equipment.
According to the cost budget
S Graduation: If there are certain restrictions on cost, but higher accuracy and stability are required, and the temperature is within its applicable range, you can choose S type.
R Graduation: It can be selected when you are not very sensitive to cost and want a slightly larger thermoelectric potential.
B Graduation: When the upper limit of temperature measurement is extremely high and the budget is sufficient, you can choose B type.