Three Kinds of High Temperature Precious Metal Pt-Rh Thermocouples

Platinum rhodium thermocouple, also known as high temperature noble metal thermocouple, is divided into three types: S-type thermocouple (platinum rhodium 10-platinum thermocouple), R-type thermocouple (platinum rhodium 13-platinum thermocouple), B-type thermocouple (platinum rhodium 30-platinum rhodium 6 thermocouple).

It mainly divided into single platinum rhodium and double platinum rhodium, which are suitable for various high temperature situations and widely used in glass, ceramics and industrial salt bath furnaces. And so on in the production process temperature measurement. 

Among them, S-type thermocouple is the most commonly used one in domestic industry. Because the measuring temperature is high, the stainless steel protective pipe can only withstand high temperature up to 1000 degrees, and the stainless steel protective pipe withstanding 1000 degrees or more is expensive. Therefore, corundum ceramic protective pipe with high purity can withstand high temperature up to 1600 degrees, and its shortcoming is fragile.

S-type thermocouple:

Platinum rhodium 10-platinum thermocouple (S-type thermocouple) is a precious metal thermocouple. The diameter of the couple wire is 0.5mm and the allowable deviation is -0.015mm. The nominal chemical composition of the positive electrode (SP) is platinum-rhodium alloy, which contains 10% rhodium and 90% platinum. The negative electrode (SN) is pure platinum, so it is commonly called single platinum-rhodium thermocouple. The maximum long-term service temperature of the thermocouple is 1300 C, and the maximum short-term service temperature is 1600 C.

The S-type thermocouple has the advantages of the highest accuracy, the best stability, the wide temperature measuring range and the long service life in the thermocouple series. It has good physical and chemical properties, thermoelectric stability and oxidation resistance at high temperature. It is suitable for oxidation and inert atmosphere. Since S-type thermocouple has excellent comprehensive performance and meets the international temperature standard, it has long been used as an interpolation instrument of international temperature standard. Although ITS-90 stipulates that it will no longer be used as an internal check instrument of international temperature standard in the future, the International Temperature Advisory Committee (CCT) believes that S-type thermocouple can still be used to approximate International temperature. Mark.

The shortcomings of S-type thermocouple are thermoelectric potential, low thermoelectric potential ratio, low sensitivity reading, low mechanical strength at high temperature, very sensitive to pollution, expensive precious metal materials, and large one-time investment.

R-type thermocouple:

Platinum-rhodium 13-platinum thermocouple (R-type thermocouple) is a precious metal

thermocouple wire. The diameter of the couple wire is 0.5mm and the allowable deviation is -0.015mm. The nominal chemical composition of the positive electrode (RP) is platinum-rhodium alloy. The nominal chemical composition of the positive electrode (RP) is platinum-rhodium alloy. The rhodium content is 13%, platinum content is 87%, and the negative electrode (RN) is pure platinum. The maximum service temperature for a long time is 1300 C, and the maximum service temperature for a short time is 1600C.

R-type thermocouple has the advantages of the highest accuracy, the best stability, the wide temperature measuring range and the long service life in the thermocouple series. It has good physical and chemical properties, thermoelectric stability and oxidation resistance at high temperature. It is suitable for oxidation and inert atmosphere. From 1967 to 1971, three major research institutes, NPL, NBS and NRC, carried out a cooperative study. The results show that the stability and reproducibility of R-type thermocouple are better than S-type thermocouple.  

The shortcomings of R-type thermocouple are thermoelectric potential, low thermoelectric potential ratio, low sensitivity reading, low mechanical strength at high temperature, very sensitive to pollution, expensive precious metal materials, and large one-time investment.

Type B thermocouple:

Platinum Rhodium 30-Platinum Rhodium 6 thermocouple (B type thermocouple) is a precious metal thermocouple. The diameter of the couple wire is 0.5mm and the allowable deviation is -0.015mm. The nominal chemical composition of the positive electrode (BP) is platinum-rhodium alloy, which contains 30% rhodium, 70% platinum, and the negative electrode (BN) is platinum-rhodium alloy with 6% rhodium, so it is commonly called Double

platinum-rhodium thermocouple wire. The maximum long-term service temperature of the thermocouple is 1600 C, and the maximum short-term service temperature is 00 C.

Type B thermocouple has the advantages of the highest accuracy, the best stability, the long service life and the high upper limit of temperature measurement in the thermocouple series. It can be used in oxidizing and inert atmosphere or in vacuum for a short time, but not in reducing atmosphere or atmosphere containing metal or non-metal vapor. A distinct advantage of B-type thermocouple is that compensation wire is not needed, because the thermoelectric potential is less than 3_V in the range of 0-50 C.

The shortcomings of B-type thermocouple are thermoelectric potential, low thermoelectric potential ratio, low sensitivity reading, low mechanical strength at high temperature, very sensitive to pollution, expensive

precious metal materials, and large one-time investment.

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