A, thermocouple,
Thermocouple is one of the industry commonly used temperature detecting element. Its advantage is:
1) high precision measurement. Because of the thermocouple and the contact object directly, don't have to suffer the effect of intermediate medium.(2) the measurement range. Commonly used thermocouple from - 50 ~ + 1600 ℃ can renew has edge measurement, some special thermocouple minimum measurable to - 269 ℃ (e.g., gold iron nickel chrome), the highest temperature can be up to + 2800 ℃ (such as tungsten rhenium).
3. Simple structure, easy to use. Thermocouple is usually composed of two different kinds of metal wire, therefore not limited by the size and the beginning of, outside to protect casing, which are very convenient. 1. The basic principle of thermocouple temperature measurement
Will be two different welding material A conductor or semi-conductor A and B, form A closed loop, as shown in figure 2-1-1. When the conductor A and B two persistent point temperature difference between 1 and 2, occurs between the electromotive force and so on
The size of a current loop, and this kind of phenomenon called the thermoelectric effect. Thermocouple is in using this effect to work.
2, thermocouple type and structure formation
(1) the types of thermocouple
Commonly used thermocouple could be divided into two types: standard thermocouple and non-standard thermocouple. Invokes standard thermocouple is refers to the country
Standard specifies its thermoelectric potential and the relationship between temperature, permissible error, and a unified standard score table of thermocouple, it
Are available in the matched display instrument selection. Non-standard thermocouple on the using range or orders of magnitude less than standard
Thermocouple, generally no unified indexing table, mainly used for the measurement of some special occasions. Standardized thermocouple since January 1, 1988 in our country, the thermocouple and thermal resistance were manufactured according to IEC international standard, and specify the S, B, E, K, R, J, T seven standardization thermocouple type thermocouple for our country unified design.
(1) the structure of thermocouple in order to keep the thermocouple is reliable, steady work, the structure of its requirement is as follows:
(1) of the two thermocouple thermal electrode welding must be strong;
(2) two hot electrode should be well insulated between each other, in case of short circuit;
(3) compensation conductor should be connected to the free cod of a thermocouple to be convenient and reliable;
(4) protect casing thermal electrodes should be able to make sufficient isolation and harmful medium.
3, the thermocouple cold end temperature compensation
Due to the thermocouple materials are generally more expensive (especially when using precious metals), and the temperature measurement points are far away the distance to the instrument, in order to save thermocouple materials, reduce cost, usually adopt the compensating conductor) (the free end of the cold junction of the thermocouple to the steady control of indoor temperature, connected to the meter terminals. It must be pointed out that the role of the thermocouple compensation wire only extend thermal electrode, make moves to the control room of the cold junction of the thermocouple meter terminal, it itself does not eliminate the cold end temperature change on the influence of temperature, no compensation. So, still need other correction method is used to compensate the cold end temperature t0 indicates influence on measuring temperature 0 ℃.
Must pay attention to when using thermocouple compensating conductor model match, cannot be wrong polarity, compensation conductor should be connected to the thermocouple temperature should not exceed 100 ℃.
Second, the thermal resistance
Three, thermal resistance is low temperature zone in one of the most commonly used temperature detector. Its main characteristic is high accuracy, stable performance. Among them is platinum thermal resistance measurement accuracy is the highest, it is not only widely used in industrial temperature measurement, and is made into standard benchmark instrument.
1, the thermal resistance temperature measurement principle and material
Thermal resistance temperature measurement is based on the metal conductor resistance value increases with the increase of the temperature of the properties for temperature measurement.
Thermal resistance is mostly made of pure metal materials, the application at present most is platinum and copper, in addition, now has started using survey, nickel, manganese and rhodium materials such as thermal resistance.
2, the structure of the thermal resistance
(1) master type thermal resistance industry commonly used thermal resistance temperature sensing element (resistance) of structure and characteristics are shown in table 2-1-11. In temperature measurement principle of thermal resistance, measured the change in temperature is directly measured by the change of the thermal resistance tolerance, therefore, the wires of a thermal resistance and so on all kinds of wire resistance change will affect temperature measurement. As to eliminate the influence of the lead resistance with the three wire or four wire system, the specific content see this chapter 3 in the first quarter.
(2) armoring armored thermal resistance heat resistance is by thermal element (resistance), wire, insulation materials, stainless steel casing combination solid body, as shown in figure 2-1-7, its diameter is generally from 2 to 8 mm, minimum of phi mm.
Compared with the normal thermal resistance, it has the following advantages:
(1) small volume, no internal air gap, thermal inertia, small measuring hysteresis;
(2) good mechanical properties, resistance to vibration and shock.
(3) can be bent, and ease of installation
4. Long service life.
(3) face the end resistance heat resistance temperature sensing element resistance wire winding by special processing, close to the face in the thermometer, its structure as shown in figure 2-1-8. Compared with general axial thermal resistance, it can reflect more correctly and quickly end face of the actual temperature being measured, is suitable for measuring temperature bearing and other parts of the face.
(4) type flameproof thermal cut off detonation heat resistance through the special structure of the junction box, its shell internal explosive mixed gas by affected by sparks or electric arc explosion limit within the junction box, that wouldn't make production field explosion. Flameproof thermal resistance can be used in the Bla ~ B3c zone with explosion danger place temperature measurement.
3. The composition of thermal resistance temperature measurement system
Thermal resistance temperature measurement system in general by the thermal resistance, connecting wires and display instrument, etc. Must pay attention to the following two points:
(1) thermal resistance and display instrument index number must be consistent
(2) in order to eliminate the influence of the connection wire resistance change, must use three wire connection. Specific content see this chapter 3.
(2) armoring armored thermal resistance heat resistance is by thermal element (resistance), wire, insulation materials, stainless steel casing combination solid body, as shown in figure 2-1-7, its diameter is generally from 2 to 8 mm, minimum of phi mm.
Compared with the normal thermal resistance, it has the following advantages:
(1) small volume, no internal air gap, thermal inertia, small measuring hysteresis;
(2) good mechanical properties, resistance to vibration and shock.
(3) can be bent, and ease of installation
4. Long service life.
(3) face the end resistance heat resistance temperature sensing element resistance wire winding by special processing, close to the face in the thermometer, its structure as shown in figure 2-1-8. Compared with general axial thermal resistance, it can reflect more correctly and quickly end face of the actual temperature being measured, is suitable for measuring temperature bearing and other parts of the face.
(4) type flameproof thermal cut off detonation heat resistance through the special structure of the junction box, its shell internal explosive mixed gas by a spark or arc
Repair in resistance of the circuit is bound to change the length of the wire resistance and impact resistance, resistance to replace the new body is good, if repair welding, after welding and calibrated before use.