Types and working principles of thermocouples

Types and working principles of thermocouples
Working principle of thermocouple:
When two different conductors and semiconductors A and B form a circuit with their two ends connected to each other, as long as the temperature at the two nodes is different, with one end having a temperature of T, called the working end or hot end, and the other end having a temperature of TO, called the free end (also known as the reference end) or cold end, a current is generated in the circuit, and the electromotive force present in the circuit is called the thermoelectric potential. The phenomenon of generating electromotive force due to temperature differences is called the Seebeck effect. There are two effects related to Seebeck: firstly, when a current flows through the connection between two different conductors, heat is absorbed or released (depending on the direction of the current), which is called the Peltier effect; Secondly, when a current flows through a conductor with a temperature gradient, the conductor absorbs or releases heat (depending on the direction of the current relative to the temperature gradient), known as the Thomson effect. The combination of two different conductors or semiconductors is called a thermocouple. The thermoelectric potential EAB (T, T0) of a thermocouple is composed of contact potential and thermoelectric potential. Contact potential refers to the electric potential generated at the contact point between two different conductors or semiconductors, which is related to the properties of the two conductors or semiconductors and the temperature at the contact point. Thermoelectric potential refers to the potential generated at two ends of the same conductor or semiconductor at different temperatures. This potential is only related to the properties of the conductor or semiconductor and the temperature at both ends, and is independent of the length, cross-sectional size, and temperature distribution along the length direction of the conductor. Both contact potential and thermoelectric potential are generated due to the different numbers of electrons concentrated at the endpoints of the contact, and the thermoelectric potential measured by a thermocouple is a combination of the two. When the circuit is disconnected, there is an electromotive force difference △ V between the disconnected points a and b, whose polarity and magnitude are consistent with the thermoelectric potential in the circuit. And it is stipulated that at the cold end, when the current flows from A to B, A is called the positive electrode and B is the negative electrode. Experiments have shown that when Δ V is small, Δ V is directly proportional to Δ T. The differential thermoelectric potential of Δ V versus Δ T is defined as thermoelectric potential rate, also known as Seebeck coefficient. The sign and magnitude of the Seebeck coefficient depend on the thermoelectric properties of the two conductors that make up the thermocouple and the temperature difference at the junction. Types of thermocouples
Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The so-called standard thermocouple refers to a thermocouple that has a national standard that specifies the relationship between its thermoelectric potential and temperature, allows for errors, and has a unified standard scale. It has a matching display instrument available for selection. Non standardized thermocouples are not as widely used or of the same order of magnitude as standardized thermocouples, and generally do not have a unified calibration table. They are mainly used for measurements in certain special occasions. Since January 1, 1988, all thermocouples and thermistors in China have been produced in accordance with IEC international standards, and seven standardized thermocouples, namely S, B, E, K, R, J, and T, have been designated as unified design thermocouples in China.
1. K-type thermocouple nickel chromium (nickel silicon (nickel aluminum) thermocouple)
K-type thermocouple is a low-cost metal thermocouple with strong oxidation resistance, which can measure the temperature of the medium from 0 to 1300 ℃. It is suitable for continuous use in oxidizing and inert gases, with a short-term use temperature of 1200 ℃ and a long-term use temperature of 1000 ℃. The relationship between its thermoelectric potential and temperature is approximately linear, making it currently the most widely used thermocouple. However, it is not suitable for bare wire use in vacuum, sulfur-containing, carbon containing atmospheres, and alternating oxidation-reduction atmospheres; When the oxygen partial pressure is low, chromium in the nickel chromium electrode will preferentially oxidize, causing a significant change in thermoelectric potential. However, the influence of metal gas on it is relatively small, so metal protective tubes are often used.
Disadvantages of K-type thermocouple:
(1) The high-temperature stability of thermoelectric potential is inferior to that of N-type thermocouples and precious metal thermocouples, and it is often damaged due to oxidation at higher temperatures (such as over 1000 ℃);
(2) The short-term thermal cycling stability is not good within the range of 250-500 ℃, that is, at the same temperature point, the thermoelectric potential readings are different during the heating and cooling process, and the difference can reach 2-3 ℃;
(3) The negative electrode undergoes magnetic transformation within the temperature range of 150-200 ℃, resulting in deviations from the calibration table in the temperature range of room temperature to 230 ℃. Especially when used in a magnetic field, time independent thermoelectric interference often occurs;
(4) Long term exposure to high-throughput system irradiation environment can cause significant changes in thermoelectric potential due to the degradation of elements such as manganese (Mn) and cobalt (Co) in the negative electrode, resulting in poor stability.
2. S-type thermocouple (platinum rhodium 10 platinum thermocouple)
The positive electrode of this thermocouple is composed of platinum rhodium alloy containing 10% rhodium, and the negative electrode is pure platinum.
Its characteristics are:
(1) Stable thermoelectric performance, strong oxidation resistance, suitable for continuous use in an oxidizing atmosphere, long-term use temperature can reach 1300 ℃. Even in air, pure platinum wire will recrystallize, causing coarse grains and fracture when exceeding 1400 ℃;
(2) High precision, with the highest accuracy level among all thermocouples, usually used as a standard or for measuring higher temperatures;
(3) Widely applicable, with good uniformity and interchangeability;
(4) The main disadvantages are: the differential thermoelectric potential is small, resulting in low sensitivity; Expensive price, low mechanical strength, not
Suitable for use under reducing atmosphere or conditions with metal vapor.
3. E-type thermocouple (nickel chromium copper nickel [constantan] thermocouple)
E-type thermocouple is a relatively new product, with nickel chromium alloy as the positive electrode and copper nickel alloy (constantan) as the negative electrode. Its biggest feature is that among commonly used thermocouples, it has the highest thermoelectric potential, that is, the highest sensitivity; Although its application range is not as wide as K-type thermocouple, it is often selected under conditions that require high sensitivity, low thermal conductivity, and tolerance for high resistance; The limiting conditions during use are the same as those of K-type, but it is not very sensitive to corrosion in environments with higher humidity.
4. N-type thermocouple (nickel chromium silicon nickel silicon thermocouple)
The main features of this thermocouple are: strong temperature and oxidation resistance below 1300 ℃, good long-term stability and short-term thermal cycle reproducibility, good resistance to nuclear radiation and low temperature performance. In addition, within the range of 400-1300 ℃, the linearity of the thermoelectric characteristics of N-type thermocouples is better than that of K-type thermocouples; However, the nonlinear error is relatively large in the low temperature range (-200~400 ℃), and the material is hard and difficult to process.
5. J-type thermocouple (iron constantan thermocouple)
J-type thermocouple: The positive electrode of this thermocouple is pure iron, and the negative electrode is constantan (copper nickel alloy). It is characterized by its low price and is suitable for vacuum oxidation reduction or inert atmosphere. The temperature range is from -200 to 800 ℃, but the commonly used temperature is only below 500 ℃, because beyond this temperature, the oxidation rate of the ferroelectric electrode is accelerated. If thick wire diameter is used, it can still be used at high temperatures and has a longer service life; This thermocouple is capable of withstanding corrosion from hydrogen (H2) and carbon monoxide (CO) gases, but cannot be used in high temperature (such as 500 ℃) sulfur-containing (S) atmospheres.
6. T-type thermocouple (copper copper nickel thermocouple)
T-type thermocouple: The positive electrode of this thermocouple is pure copper, and the negative electrode is copper nickel alloy (also known as constantan). Its main characteristics are:
Among low-cost metal thermocouples, it has the highest accuracy and good uniformity of the thermoelectric electrode; Its operating temperature is -200~350 ℃. Due to the easy oxidation and detachment of the oxide film of the copper hot electrode, it should generally not exceed 300 ℃ when used in an oxidizing atmosphere. Within the range of -200~300 ℃, they have high sensitivity. Another characteristic of copper constantan thermocouples is that they are inexpensive and are the cheapest among several commonly used standardized products.
7. R-type thermocouple (platinum rhodium 13 platinum thermocouple)
The positive electrode of this thermocouple is a 13% platinum rhodium alloy, and the negative electrode is pure platinum. Compared with the S-type, its potential rate is about 15% higher.
(1) Stable thermoelectric performance, strong oxidation resistance, suitable for continuous use in an oxidizing atmosphere, long-term use temperature can reach 1300 ℃. Even in air, pure platinum wire will recrystallize, causing coarse grains and fracture when exceeding 1400 ℃;
(2) High precision, with the highest accuracy level among all thermocouples, usually used as a standard or for measuring higher temperatures;
(3) Widely applicable, with good uniformity and interchangeability;
(4) The main disadvantages are: the differential thermoelectric potential is small, resulting in low sensitivity; Expensive price, low mechanical strength, not
Suitable for use under reducing atmosphere or conditions with metal vapor.