Principle and structure
The thermocouple temperature sensor is connected by two different conductors to form a closed loop. When the temperature of two contacts is different, a thermoelectric electromotive force will be generated in the loop. This phenomenon is called thermoelectric effect, and the electromotive force is the famous "Seebeck temperature difference electromotive force", or "thermoelectric electromotive force" for short. Thermocouples measure temperature by measuring thermoelectric EMF. When this measuring point is heated, it will generate a force to make the thermal current move. Its size is equal to the temperature difference between the measuring point with different temperatures and the other end (reference contact). The direct measuring end is called the working end (hot end) terminal end is called the cold end. When there is a temperature difference between the hot end and the cold end, the thermal current will be generated in the circuit. Connect the display instrument, and the instrument will indicate the corresponding temperature value of the generated thermoelectric EMF, The electromotive force increases with temperature. Thermocouple temperature sensor (thermoelectric thermometer) is an instrument that takes thermocouple as temperature measuring element and displays temperature by instrument. It is widely used to measure the temperature in the range of - 200~1300 ℃. Under special circumstances, it can measure the high temperature of 2800 ℃ or the low temperature of 4K. It is also the most widely used and has the largest market share.
The thermoelectric flow movement force of the thermocouple is independent of the wire diameter or length. It is related to the material of thermocouple and the temperature at both ends.
Thermocouple classification, temperature measurement range, accuracy, advantages and disadvantages
Graduation
Material Science
Measuring range
Level I tolerance
Level II tolerance
Advantages/Disadvantages
Type K
NiCr NiSi
-200~+1300℃
± 1.5 ℃ or ± 0.4% t
± 2.5 ℃ or ± 0.75% t
It is cheap and widely used, and is suitable for use in oxidizing and inert atmospheres/the bare wire is not suitable for use in vacuum, carbon, sulfur and alternate redox atmospheres; The stability of high temperature thermoelectric EMF is not as good as that of N type.
Type J
Fe Cu Ni
-200~+950℃
± 1.5 ℃ or ± 0.4% t
± 2.5 ℃ or ± 0.75% t
The price is cheap. The thermoelectric electromotive force rate is larger than that of Type K, which can be used in both oxidation and reduction atmospheres. It is resistant to H2 and CO corrosion/cannot be used in sulfur containing atmospheres. After the temperature exceeds 538 ℃, the iron electrode oxidizes quickly, and the temperature resistance is not high enough to be used in high temperature areas
Type E
Nickel chromium copper nickel
-200~+850℃
± 1.5 ℃ or ± 0.4% t
± 2.5 ℃ or ± 0.75% t
Cheap price, maximum thermoelectric EMF, high sensitivity/insufficient temperature resistance, no
Other characteristics of the method are similar to those of K type
Type N
Nickel Chromium Silicon Nickel Silicon Magnesium
-200~+1300℃
± 1.5 ℃ or ± 0.4% t
± 2.5 ℃ or ± 0.75% t
Cheap price, high temperature oxidation resistance, nuclear radiation resistance, ultra-low temperature resistance, good long-term stability of thermoelectric EMF/small thermoelectric EMF, relatively late launch time compared with other types, not widely used
T-type
Cu Cu Ni
-200~+350℃
± 0.5 ℃ or ± 0.4% t
± 1.0 ℃ or ± 0.75% t
Low price, high precision/poor oxidation resistance, and high temperature resistance
S-type
Platinum 10 Platinum
-200~+1600℃
± 1.0 ℃ or ± 0.4% t
± 1.5 ℃ or ± 0.4% t
Ultra high temperature resistance, suitable for use in oxidizing and inert atmospheres/expensive, low thermoelectric potential at room temperature, not suitable for low and medium temperature measurement
Type R
Platinum 13 Platinum
-200~+1600℃
±[1+(t-1100)x0.3%]℃
± 1.5 ℃ or ± 0.25% t
Ultra high temperature resistance, suitable for use in oxidizing and inert atmospheres/expensive, low thermoelectric potential at room temperature, not suitable for low and medium temperature measurement
Type B
Platinum 30 - Platinum 6
-200~+1800℃
---
±0.25%t（600-1700℃）
Ultra high temperature resistance, suitable for use in oxidizing or neutral atmosphere/expensive, low thermoelectric potential at room temperature, not suitable for low and medium temperature measurement