Detailed introduction of the hottest couple thermi

2022-08-09
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Thermocouple thermal resistance detailed introduction

thermocouple is a kind of temperature sensing element and a primary instrument. It directly measures the temperature, converts the temperature signal into thermoelectric EMF signal, and converts it into the temperature of the measured medium through electrical instruments (secondary instruments). The basic principle of thermocouple temperature measurement is that two different material conductors form a closed loop. When there is a temperature gradient at both ends, there will be current passing through the loop. At this time, there is electromotive force - thermoelectric electromotive force between the two ends, which is the so-called Seebeck effect. Homogeneous conductors with two different components are thermoelectric electrodes. The higher temperature end is the working end, and the lower temperature end is the free end. The free end is usually at a constant temperature. According to the functional relationship between thermoelectric EMF and temperature, a thermocouple graduation table is made; The graduation table is obtained when the free end temperature is 0 ℃, and different thermocouples have different graduation tables

when the third metal material is connected to the thermocouple circuit, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it will not be affected by the third metal connected to the circuit. Therefore, when the thermocouple measures temperature, the measuring instrument can be connected. After measuring the thermoelectric EMF, the temperature of the measured medium can be known

] working principle

two conductors with different components (called thermocouple wire or Thermoelectrode) are connected to form a circuit at both ends. When the temperature of the junction is different, electromotive force will be generated in the circuit. This phenomenon is called thermoelectric effect, and this electromotive force is called thermal electromotive force. Thermocouples use this principle to measure the temperature. The end directly used to measure the medium temperature is called the working end (also known as the measuring end), and the other end is called the cold end (also known as the compensation end); The cold end is connected with the display instrument or supporting instrument, and the display instrument will indicate the thermoelectric potential generated by the thermocouple

thermocouple is actually an energy converter, which converts heat energy into electrical energy, and measures temperature with the generated thermoelectric potential. For the thermoelectric potential of thermocouple, we should pay attention to the following problems:

1: the thermoelectric potential of thermocouple is the difference of temperature [2] function at both ends of thermocouple working end, rather than the function of temperature difference at both ends of thermocouple cold end and working end

2: when the thermocouple material is uniform, the thermoelectric potential generated by the thermocouple has nothing to do with the length and diameter of the thermocouple, but only with the composition of the thermocouple material and the temperature difference at both ends

3: when the material composition of the two thermocouple wires of the mini thermocouple is determined, the thermal potential of the thermocouple is only related to the temperature difference of the thermocouple; If the temperature of the cold end of the thermocouple remains constant, the thermoelectric potential of the thermocouple is only a single value function of the working end temperature. The conductors or semiconductors A and B of two different materials are welded together to form a closed loop, as shown in the figure. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, electromotive force will be generated between them, thus forming a large and small current in the circuit. This phenomenon is called thermoelectric effect. Thermocouples use this effect to work

features

◆ simple assembly, convenient replacement

◆ spring loaded temperature sensing element, good seismic performance

◆ large measurement range

◆ high mechanical strength, good pressure resistance

◆ high temperature resistance up to 2400 degrees

thermocouples - Types and structure formation

(1) types of thermocouples

common thermocouples can be divided into standard thermocouples and non-standard thermocouples. The standard thermocouple used refers to the thermocouple whose national standard stipulates the relationship between thermal potential and temperature, allowable error, and has a unified standard graduation table. It has its supporting display instrument for selection. Non standardized thermocouples are inferior to standardized thermocouples in the scope of use or order of magnitude. Generally, there is no unified graduation table, which is mainly used for measurement in some special occasions. Standardized thermocouples since January 1st, 1988, all thermocouples and thermal resistors have been produced according to IEC international standards, and seven standardized thermocouples, s, B, e, K, R, J and T, have been designated as thermocouples of unified design in China

(2) structural form of thermocouple in order to ensure the reliable and stable operation of the thermocouple, its structural requirements are as follows:

① the welding of the two thermoelectrodes constituting the thermocouple must be firm

② the two thermoelectric poles should be well insulated from each other to prevent short circuit

③ the connection between the compensation wire and the free end of the thermocouple should be convenient and reliable

④ the glass fiber composite dealers in Asia are also growing at an average rate of 18% per year. The tubes should be able to ensure that the Thermoelectrode is fully isolated from harmful media

common thermocouple materials

thermocouple graduation Thermoelectrode materials

positive and negative electrodes

s Platinum Rhodium 10 pure platinum

r platinum rhodium 13 pure platinum

b platinum rhodium 30 platinum rhodium 6

k nickel chromium nickel silicon

t pure copper nickel

j iron copper nickel

n nickel chromium silicon nickel

e nickel chromium copper nickel

types of thermocouples: assembly thermocouples, armored thermocouples, end face thermocouples, spring fixed thermocouples, high temperature thermocouples, Platinum rhodium thermocouples, anti-corrosion thermocouples, wear-resistant thermocouples, high-voltage thermocouples, special thermocouples, hand-held thermocouples, micro thermocouples, precious metal thermocouples, fast thermocouples, tungsten rhenium thermocouples, etc

its advantages are:

① high measurement accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium

② wide measurement range. Common thermocouples can be continuously measured from -50~+1600 ℃, and some special thermocouples can be measured as low as -269 ℃ (such as gold iron nickel chromium) and as high as +2800 ℃ (such as tungsten rhenium)

③ simple structure and convenient use. Thermocouples are usually composed of two different kinds of metal wires, which are not limited by the size in recent years and the beginning. There is a protective sleeve outside, which is very convenient to use

the basic law of thermocouples

1, the law of homogeneous conductors

a closed loop is formed by welding the two ends of the same homogeneous material (conductor or semiconductor). No matter how the cross-section of the conductor and how the temperature is distributed, there will be no contact potential, the thermoelectric potential will offset, and the total potential in the loop is zero

it can be seen that the thermocouple must be composed of two different homogeneous conductors or semiconductors. If the thermal electrode material is uneven, due to the existence of temperature ladder, there will be an attached heating potential

2, and these materials should have recyclable potential. Interconductor law

connect the intermediate conductor (the third conductor) in the thermocouple circuit. As long as the temperature at both ends of the intermediate conductor is the same, the introduction of the intermediate conductor has no effect on the total potential of the thermocouple circuit, which is called the intermediate conductor law

application: according to the law of intermediate conductor, in the actual temperature measurement application of thermocouple, the form of hot end welding and cold end open circuit is often used, and the cold end is connected with the display instrument through the connecting wire to form a temperature measurement system

some people worry that if the copper wire is used to connect the cold end of the thermocouple to the instrument to read the MV value, the contact potential generated at the connection between the wire and the thermocouple will cause additional errors in the measurement. According to this law, there is no such error

3, intermediate temperature law

the thermoelectric potential between the two contacts of the thermocouple circuit (temperature T, t0) is equal to the algebraic sum of the thermoelectric potential of the thermocouple at temperature T, TN and at temperature TN, t0. TN is the intermediate temperature

application: due to the nonlinear relationship between thermocouples E-T, when the cold end temperature is not 0 ℃, the actual thermoelectric potential E (T, t0) of the known circuit cannot be used to directly check the table to obtain the hot end temperature value; It is also impossible to use the actual thermoelectric potential E (T, t0) of the known circuit to directly check the temperature value obtained from the table, plus the temperature of the cold end to determine the measured temperature value of the hot end, which needs to be corrected according to the intermediate temperature law. Beginners often do not follow the law of intermediate temperature to correct

4, reference electrode law

this law is only studied and concerned by professionals. Generally, people in the production and use links do not know it very well. The simple explanation is: use high-purity platinum wire as the standard electrode, assuming that the positive and negative electrodes of nickel chromium nickel silicon thermocouple are matched with the standard electrode respectively, and the sum of their values is equal to the value of this nickel chromium nickel silicon thermocouple

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