Thermocouple

1. General Knowledge of GWL-B Double Platinum-Rhodium (Platinum-Rhodium 30 - Platinum-Rhodium 6) Thermocouple

• The thermoelectric potential of this thermocouple is extremely small at room temperature (-2mV at 25℃ and 3mV at 50℃). Therefore, compensation wires are generally not needed during measurement, and the impact of temperature changes at the reference end can be ignored. Its long-term service temperature is 1600℃, and short-term service temperature is 1800℃. The melting point of platinum-rhodium 6 alloy is 1820℃, which limits the upper limit of its service temperature. The electromotive force rate of the double platinum-rhodium thermocouple is small, so it needs to be equipped with a display instrument with high sensitivity.
• Type B thermocouples are suitable for use in oxidizing or neutral atmospheres, and can also be used in vacuum environments for a short time. Even when used in reducing atmospheres, their service life is 10-20 times that of R and S type thermocouples. Because R and S type thermocouples will have the phenomenon of platinum-rhodium positive electrode diffusing to the negative electrode at high temperatures, causing thermocouple degradation. To prevent this phenomenon, rhodium is added to platinum to make platinum-rhodium alloys, which not only improves heat resistance, but also increases the thermoelectric potential rate of the alloy to platinum. When the rhodium content is below 20%, the thermoelectric potential of the platinum-rhodium alloy to platinum increases sharply, but beyond this value, with the increase of rhodium content, the change is small, and the alloy becomes significantly hardened, making processing difficult. Therefore, the rhodium content in such alloys cannot exceed 40% (by weight).
• Platinum-rhodium alloys have a smaller tendency to grain growth than pure platinum, and this tendency decreases with the increase of rhodium content, which can make the thermoelectric performance more stable and the mechanical strength higher.
• Therefore, double platinum-rhodium thermocouples are widely used in high-temperature measurement at 1800℃. Thenumber of the double platinum-rhodium thermocouple was previously LL-2, and now it is B.

2. General Knowledge of GWL-S Single Platinum-Rhodium (Platinum-Rhodium 10 - Platinum) Thermocouple

• This type of thermocouple is characterized by stable thermoelectric performance and strong oxidation resistance, and is suitable for continuous use in oxidizing and inert atmospheres. The long-term service temperature is 1400℃. When exceeding this temperature, even in air, the pure platinum wire will have coarse grains due to recrystallization. Therefore, the long-term service temperature is limited to below 1400℃, and the short-term service temperature is 1600℃. Among all thermocouples, it has the highest accuracy grade and is usually used as a standard or as a thermocouple for measuring high temperatures. It has a wide temperature range, good homogeneity and interchangeability.

3. General Knowledge of GWL-K Nickel-Chromium - Nickel-Silicon (Nickel-Aluminum) Thermocouple

• The positive electrode of this thermocouple is a nickel-chromium alloy containing 10% chromium (KP), and the negative electrode is a nickel-silicon alloy containing 3% silicon (KN). Its negative electrode is magnetophilic, and based on this characteristic, a magnet can be used to easily identify the positive and negative electrodes of the thermocouple. It is characterized by a wide temperature range, stable performance at high temperatures, an approximately linear relationship between thermoelectric potential and temperature, and low price. Therefore, it is currently the most widely used thermocouple.
• Type K thermocouples are suitable for continuous use in oxidizing and inert atmospheres. The short-term service temperature is 1200℃, and the long-term service temperature is 1000℃.
• In China, nickel-chromium - nickel-silicon thermocouples have basically replaced nickel-chromium - nickel-aluminum thermocouples. Nickel-chromium - nickel-aluminum thermocouples are still used abroad. Although the chemical compositions of the two thermocouples are different, their thermoelectric characteristics are the same, and the same  table is used.
• Type K thermocouples are base metal thermocouples with strong oxidation resistance. They are not suitable for bare wire use in vacuum, carbon-containing, sulfur-containing atmospheres, or alternating oxidizing and reducing atmospheres. When the oxygen partial pressure is low, chromium in the nickel-chromium electrode will be preferentially oxidized (also called green corrosion), causing great changes in thermoelectric potential. However, metal gases have little impact on it. Therefore, metal thermocouple protection tubes are mostly used.

Type K thermocouples have the following disadvantages:

1. The high-temperature stability of thermoelectric potential is worse than that of precious metal thermocouples. At higher temperatures, they are often damaged due to oxidation. In an oxidizing atmosphere, a K-type thermocouple with a diameter of 3.2mm will exceed the allowable error of class 0.75 after about 650 hours at 1100℃ and 1200℃.
2. The short-term thermal cycle stability in the range of 250-550℃ is poor. Even at the same temperature point, the thermoelectric potential values during heating and cooling are different, and the difference can reach 2-5℃.
3. The negative electrode of the K-type thermocouple will undergo magnetic transformation in the range of 150-200℃, resulting in the 分度 value often deviating from the 分度 table in the range of room temperature to 230℃, especially when used in a magnetic field, there are often time-independent thermoelectric potential interferences.
4. When long-term in a high-flux neutron flux irradiation environment, due to the decay of elements such as Mn and Co in the negative electrode, its stability is poor, leading to large changes in thermoelectric potential.

Standard Thermocouple Temperature-EMF Reference Tables