Temperature is the most common and very important physical parameter in chemical production. Since many physical and chemical properties of the object are related to temperature, many production processes must be carried out at the appropriate temperature, so it is very important to accurately measure and control the temperature. First of all, temperature can not be measured directly, only by heat exchange!
Bimetallic thermometer
First, the working principle of bimetal thermometer
The working principle of the bimetal thermometer is to use two different temperature expansion coefficient of the metal, in order to improve the temperature sensitivity, usually the metal sheet is made into a spiral coil shape, when the temperature of the multi-layer metal sheet changes, the expansion or contraction of each layer of metal varies, so that the spiral roll up or loose. Since one end of the spiral coil is fixed and the other end is connected to a freely rotating pointer, when the bimetal feels the temperature change, the pointer can indicate the temperature on a circular indexing scale.
The temperature range of this instrument is generally between -80 ° C and +500 ° C, and the allowable error is about 1.5% of the scale range.
Second, bimetal thermometer classification
Common bimetal thermometer, shock-resistant bimetal thermometer, electric joint bimetal thermometer. According to the connection direction of the bimetal thermometer dial and the protective tube, the bimetal thermometer can be divided into four types: axial type, radial type, 135° type and universal type. ① Axial bimetal thermometer: the dial is connected to the protective tube vertically.
(2) Radial bimetal thermometer: the dial is connected with the protective tube in parallel. (3) 135° directional bimetal thermometer: the dial is connected to the protection tube at 135°. ④ Universal bimetal thermometer: the Angle of connection between the dial and the protective tube can be adjusted arbitrarily.
3. Selection and use
In the selection of bimetal temperature timing to fully consider the actual application environment and requirements, such as dial diameter, accuracy level, installation and fixing mode, the type of measured media and environmental risk. In addition, we should pay attention to factors such as cost performance and maintenance workload.
In addition, the following points should be noted during the use of bimetallic thermometers:
A. The length of the bimetal thermometer protection tube immersed in the measured medium must be greater than the length of the temperature sensing element, generally the immersion length is greater than 100mm, and the immersion length of the 0-50℃ range is greater than 150mm to ensure the accuracy of the measurement.
B, all kinds of bimetal thermometer should not be used to measure the temperature of the medium in the open container, live contact thermometer should not be used in the control loop of the occasion of large vibration.
C, bimetal thermometer in storage, use, installation and transportation, should avoid collision with the protective tube, do not bend the protective tube and use the table as a wrench.
D, the thermometer should be regularly tested under normal use. Generally every six months is appropriate. The electric contact thermometer is not allowed to work under strong vibration, so as not to affect the reliability of the contact.
E, the instrument often works at a temperature that is best in the scale range of 1/3 ~ 2/3.
Pressure type thermometer
First, the working principle of the pressure thermometer
The principle of the pressure thermometer is based on the relationship between the saturated vapor pressure and the temperature of the evaporated liquid in the closed temperature measuring system. When the temperature envelope feels the temperature change, the saturated vapor in the closed system produces the corresponding pressure, which causes the change of the curvature of the elastic element, so that the free end of the displacement, and then the displacement is changed to the indicating value by the gear amplifying mechanism. Two, pressure thermometer composition
The pressure thermometer is composed of a sensitive element temperature pack, a pressure transfer capillary tube and a spring tube pressure gauge. If the system is filled with gas, such as nitrogen, it is called an inflatable pressure thermometer, the upper limit of temperature measurement can reach 500 ° C, and the relationship between pressure and temperature is close to linear, but the temperature packet is large and the thermal inertia is large.
If filled with liquid, such as xylene, methanol, etc., the temperature packet is smaller, and the temperature range is -40℃ ~ 200℃ and -40℃ ~ 170℃, respectively.
If filled with a liquid with a low boiling point, its saturated vapor pressure should vary with the temperature being measured, such as acetone, for 50 ° C ~ 200 ° C. However, due to the nonlinear relationship between saturated vapor pressure and saturated vapor temperature, the thermometer scale is not uniform.
Third, the characteristics of pressure thermometer
The temperature packet must be fully immersed in the measured medium; The longest capillary shall not exceed 60 m; The instrument has low accuracy, but is easy to use and resistant to vibration.
Resistance thermometer
First, the working principle of the resistance thermometer
The principle of temperature measurement of thermal resistance is based on the property that the resistance value of a conductor or semiconductor changes with temperature to measure temperature or temperature-related parameters.
The resistance value of most metals varies with temperature, and the higher the temperature, the greater the resistance, that is, it has a positive resistance temperature coefficient. Most semiconductor materials have a negative temperature coefficient of resistance, that is, the higher the temperature, the smaller the resistance.
Second, the material requirements of the resistance thermometer
1. Stable chemical and physical properties within the temperature measurement range; 2. Good reproducibility;
3, the resistance temperature coefficient is large to obtain high sensitivity; 4, the resistivity is large, you can get small volume components; 5, resistance temperature characteristics as close to linear as possible; 6. Low price.
Third, commonly used thermal resistance components
Commonly used thermal resistance components are: platinum thermal resistance, copper thermal resistance, semiconductor thermistor. Platinum thermal resistance is made of high purity platinum wire, which has high temperature measurement accuracy and stable performance
Good reproducibility, antioxidant and other advantages, so it is widely used in the benchmark, laboratory and industry. However, it is easy to be polluted by reducing atmosphere at high temperature, which makes the platinum wire brittle and changes its resistance temperature characteristics, so it can be used with casing protection. The purity of platinum wire is the key to determine the accuracy of thermometer. The higher the purity of platinum wire, the higher the stability, the better the reproducibility and the higher the temperature measurement accuracy.
The resistance value of the copper thermal resistance is nearly linear with the temperature, the resistance temperature coefficient is also large, and the price is cheap, so in some cases where the measurement accuracy is not very high, the copper thermal resistance is often used. However, it is easily oxidized in an atmosphere higher than 100 ° C, so it is mostly used to measure the temperature range of -50 ~ 150 ° C.
Semiconductor thermistor advantages: the negative resistance temperature coefficient is large, so the sensitivity is high. With large resistivity, it can be made into a small volume and large resistance value, which makes it have small thermal inertia and measurable point temperature or dynamic temperature.
Disadvantages: The resistance temperature characteristics of the same kind of semiconductor thermistor are dispersed, non-linear, and unstable, so the interchangeability is poor and the accuracy is low.
Fourth, the wiring mode of the thermal resistance
Two-wire system: A wire is connected at each end of the thermal resistance to lead out the resistance signal
Wire system, this lead method is very simple, but because the connection wire must have lead resistance R, R size is related to the material and length of the wire, so this lead method is only suitable for occasions with low measurement accuracy
Three-wire system: Connect a lead wire at one end of the root of the thermal resistance, and connect two leads at the other end
The method is called the three-wire system, which is usually used with the bridge, can better eliminate the influence of the lead resistance, and is the most commonly used in industrial process control.
Four-wire system: The method of connecting two wires at each end of the root of the thermal resistance is called the four-wire system, where two leads provide constant current I to the thermal resistance, convert R into a voltage signal U, and then lead U to the secondary instrument through the other two leads. It can be seen that this lead mode can completely eliminate the resistance effect of the lead, which is mainly used for high-precision temperature detection.
Fifth, the installation requirements of the thermal resistance
The installation of thermal resistance should be conducive to accurate temperature measurement, safe and reliable, and convenient maintenance.
It does not affect equipment operation and production operations. When choosing the installation location and insertion depth of the thermal resistance, pay attention to the following points:
1, in order to make the measuring end of the thermal resistance and the measured medium have sufficient heat exchange, the location of the measuring point should be reasonably selected, and the thermal resistance should be installed near the dead Angle of the valve, elbow and pipeline and equipment as far as possible.
2, the thermal resistance with the protective sleeve has heat transfer and heat dissipation loss, in order to reduce the measurement error, the thermocouple and thermal resistance should have sufficient insertion depth:
1) For the thermal resistance measuring the fluid temperature in the center of the pipeline, the measuring end should generally be inserted into the center of the pipeline (vertical installation or inclined installation). If the pipe diameter of the measured body is 200 mm, the insertion depth of the thermal resistance should be selected to be 100 mm;
2) For the temperature measurement of high temperature and high pressure and high-speed fluids (such as the main steam temperature), in order to reduce the resistance of the protective sleeve to the fluid and prevent the fracture of the protective sleeve under the action of the fluid, the protection tube can be inserted shallow or the hot sleeve thermal resistance can be used. Shallow plug type thermal resistance protection sleeve, the depth of the main steam pipe inserted should not be less than 75mm; The standard insertion depth of hot-jacketed resistors is 100mm.
3) If it is necessary to measure the temperature of the flue gas in the flue, although the diameter of the flue is 4m, the insertion depth of the thermal resistance can be 1m.
4) When the insertion depth of the measured element exceeds 1m, it should be installed vertically as far as possible, or the support frame and protective sleeve should be installed.
3) If it is necessary to measure the temperature of the flue gas in the flue, although the diameter of the flue is 4m, the insertion depth of the thermal resistance can be 1m.
4) When the insertion depth of the measured element exceeds 1m, it should be installed vertically as far as possible, or the support frame and protective sleeve should be installed.
Thermocouple thermometer
Using the phenomenon of "thermoelectric effect" between different conductors, it has the advantages of simple structure, convenient production, wide measuring range, wide application range, high accuracy, low thermal inertia and so on. And can directly output electrical signals, easy to signal transmission, automatic recording and automatic control.
First, the working principle of thermocouples
Two different conductors or semiconductor materials A and B form A closed loop, if the temperature at the two junction points of the loop composed of a and B is not the same, there is a current in the loop, indicating that there is an electromotive force in the loop, this phenomenon is called the thermoelectric effect. Also known as Seebeck effect. The electromotive force generated by this effect is often called the thermoelectric potential.
The thermoelectric potential is composed of two parts, namely the contact potential and the thermoelectric potential.
Contact potential when two conductors or semiconductor materials with different properties come into contact with each other due to internal
The electron density is different, for example, the electron density of material A is greater than that of material B, then some electrons will diffuse from A to B, causing A to lose electrons and have A positive potential, and B to gain electrons and have a negative potential, and finally forming an electrostatic field from A to B. The electrostatic field prevents further diffusion of electrons from A to B.
When the diffusion force and the electric field force are in balance, A fixed electromotive force is established between material A and B.
The phenomenon of the formation of an electromotive force at the contact between two materials due to their different free electron densities is called the Peltier effect. The electromotive force is called the Perpa potential or contact potential.
Theoretically, it has been proved that the size and direction of the contact potential mainly depend on the properties of the two materials and the temperature of the contact surface. The relationship is as follows:
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Conclusion: The contact potential is only related to the contact temperature and the electron density of conductors A and B. The higher the temperature, the larger the contact potential, the larger the electron density ratio between the two materials, the larger the contact potential.
Because the temperature at both ends of the material is different, the energy of the electrons at both ends is different, the electrons at the higher temperature end have higher energy, and the electrons will move to the lower temperature end, so an electrostatic field from the high temperature end to the low temperature end is formed between the two ends of the material, this electric field will attract electrons from the low temperature end to the high temperature end, and finally achieve dynamic balance.
The phenomenon of electromotive force due to the difference in temperature between the two ends of the same conductor or semiconductor material is called the Thomson effect. The resulting electromotive force is called the Thomson electromotive force or thermoelectric potential. The direction of the thermoelectric potential is from the low temperature end to the high temperature end, and its size is related to the temperature at both ends of the material and the material properties.
The size of the thermocouple loop thermoelectric potential is only related to the temperature of the material composing the thermocouple and the connection point at both ends of the material, and has nothing to do with the diameter and length of the thermocouple wire and the temperature distribution along the path.
A thermocouple can only be formed with two materials of different properties, and a closed loop of the same material does not generate a thermoelectric potential.
After the two materials of the thermocouple are determined, the size of the thermoelectric potential is only related to the temperature of the contacts at both ends of the thermocouple. If T0 is known and constant, then f(T0) is constant. The total thermoelectric potential of the loop EAB(T,T0) is only a single-valued function of temperature.