How the thermocouple works

 

       When there are two different conductors and semiconductors A and B to form a circuit, the two ends of the connection, as long as the two nodes at different temperatures, one end of the temperature T, known as the working or hot end, the other end of the temperature TO , Known as the free end (also known as the reference end) or cold side, there is a current in the circuit, as shown in Figure 2-1 (a), that is, the electromotive force exists in the circuit called thermoelectromotive force. This phenomenon due to the different temperature generated by the electromotive force called the Seebeck effect. There are two effects associated with Sebeck: First, when there is a current flowing through the junctions of two different conductors, the heat is absorbed or released (depending on the direction of the current), called the Peltier effect; Second, when there is a current flowing through the existence of temperature gradient of the conductor, the conductor to absorb or release heat (depending on the current relative to the temperature gradient direction), known as the Thomson effect. The combination of two different conductors or semiconductors is called a thermocouple. The thermoelectric potential of the thermocouple EAB (T, T0) is synthesized by the contact potential and the temperature difference potential. The contact potential is the potential generated by two different conductors or semiconductors at the contact with the nature of the two conductors or semiconductors and the temperature at the contact point. The thermoelectric potential is the potential of the same conductor or semiconductor at both ends of the temperature. This potential is only related to the nature of the conductor or the semiconductor and the temperature at both ends, regardless of the length of the conductor, the cross-sectional size, and the temperature distribution along its length The Regardless of whether the contact potential or the temperature difference potential is due to the potential generated by the number of electrons concentrated at the end of the contact, the thermocouple measured by the thermocouple is the synthesis of the two. When the circuit is disconnected, there is an electromotive force difference ΔV between the opening a and b, whose polarity and size are consistent with the thermoelectric potential in the circuit, as shown in Figure 2-1 (b). And the provisions of the cold end, when the current flow from A to B, said A is a positive, B is negative. Experiments show that △ V is very small, △ V and △ T is proportional to the relationship. Define △ V for △ T differential thermoelectric potential for the thermoelectric power rate, also known as the Seebeck coefficient. The sign and size of the Seebeck coefficient depends on the thermoelectric properties of the two conductors that make up the thermocouple and the temperature difference between the junctions.

 

       2. Types of thermocouples

 

       At present, the International Electrotechnical Commission (IEC) recommended eight types of thermocouples as a standardized thermocouple, namely T-type, E-type, J-type, K-type, N-type, B-type, R-type and S-type.

 

       Thermal resistance

 

      1. Characteristics of thermal resistance materials

 

    The resistance value of the conductor changes with the temperature change, by measuring its resistance to calculate the temperature of the measured object, the use of this principle is composed of the sensor resistance temperature sensor, this sensor is mainly used for -200-500 ℃ temperature range of temperature measuring.

 

    Pure metal is the main manufacturing material of thermal resistance, the thermal resistance of the material should have the following characteristics:

 

    ① resistance temperature coefficient should be large and stable, resistance and temperature should have a good linear relationship between.

    ② high resistivity, heat capacity is small, the reaction speed is fast.

    ③ material reproducibility and process is good, low prices.

    ④ in the temperature range of chemical and physical properties of stability.

 

    At present, the most widely used in the industry of platinum and copper, and has been made into standard thermal resistance.

 

    2. Platinum resistance

 

    The relationship between platinum resistance and temperature is close to linearity, and Rt = R0 (1 + At + Bt2) (2-1) is in the range of 0 ~ 630.74 ℃ in the range of -190 ~ 0 ℃ Rt = R0 (1 + At + Bt2 ten Ct3) (2-2) where RO, Rt is the resistance value of platinum resistance at temperature 0 ° and t °, t is the arbitrary temperature, A, B, C is the temperature coefficient, , A = 3.9684 x ​​10-3 / C, B = -5.847 x 10-7 / C 2, C = -4.22 x 10-l2 / From (2-1) and (2-2) see, when the R0 value is different, at the same temperature, the Rt value is also different.

 

    3. Copper resistance

 

    In the temperature measurement accuracy is not high, and the temperature range is relatively small, you can use copper resistance made of thermal resistance material instead of platinum resistance. In the temperature range of -50 to 150 ° C, the relationship between the resistance and temperature of the copper resistance is linear with the temperature. Rt = R0 (1 + At) (2-3) where A = 4.25 × 10- 3 ~ 4.28 × 10-3 ℃ for the copper resistance of the temperature coefficient