Level includes liquid level and material level. The liquid level includes two levels of liquid level signal and continuous level measurement. The liquid level signal meter measures the liquid level of several fixed positions and uses the upper and lower limit alarms of the liquid level. Continuous level measurement is a continuous measurement of liquid level. It is widely used in petroleum, chemical, food processing and many other fields, and has very important significance. In this paper, more than 20 kinds of continuous level measurement methods were compared and analyzed.
1. Glass tube method, glass plate method, two-color water level method, and manual inspection method. Glass tube method: This method uses the principle of a connected device, as shown in Figure 1-1 [1]. In the drawing, 1- measured container; 2-glass tube; 3-indicating scale; 4, 5-valve; 6, 7-connected tube. The level reads directly from the indicator scale.
Glass plate method: The glass plate can be installed through the connector, or it can be installed in the wall of the container, and several glass plates can be connected in series to increase the measuring range. The level value is read directly from the glass scale.
Two-color water level gauge method: This method uses the principle of optics to make the water show green, and make the water vapor show red, thus indicating the water level [2].
Manual inspection method: This method is used to measure the tank level. During measurement, the surveyor puts the dipstick into the oil and lifts the dipstick when the caliper comes in contact with the bottom of the tank. According to the trace of oil on the dipstick, read the oil level; determine the height of the water cushion according to the change of the color of the end of the dipstick to determine the oil height and the water height [3].
The above four methods are manual measurement methods, which have the advantages of simple measurement, high reliability, intuitiveness, and low cost.
2, blowing method, differential pressure method, HTG method blowing method: The working principle of the method shown in Figure 2-1 [4]. In the drawing, 1-filter; 2-pressure reducing valve; 3-throttle element; 4-rotor flowmeter; 5-transmitter. Since the pressure in the air blow pipe is approximately equal to the static pressure of the liquid column, P = ÏgH
In the formula, Ï-liquid density; H-level. Therefore, the level H can be measured by the static pressure P. The air blowing method is suitable for measuring liquids with strong corrosiveness and suspended substances, and is mainly used in occasions where the measurement accuracy is not required.
Differential pressure method: The working principle of this method is shown in Figure 2-2 [4]. In the figure, 1, 2-valve; 3- differential pressure transmitter. For open or atmospheric vessels, valve 1 and gas phase pressure piping can be omitted. The relationship between pressure difference and liquid level is ΔP=P2-P1=ÏgH
In the formula: ΔP-transmitter pressure difference between positive and negative pressure chamber; P2, P1-pressure tube pressure; H-level. The differential pressure transmitter converts the pressure difference into a 4-20 mA DC signal. If the corresponding output signal is greater than or less than 4 mA when the pressure is at the lower limit of the measurement range, zero migration techniques such as adjusting the migration spring are required to equal 4 mA.
HTG method: This method is applied to differential pressure level measurement of oil tanks, as shown in Figure 2-3. In the picture: P1, P2, P3- high precision pressure sensor; RTD- temperature detection element; HIU-interface unit. P1 is located in the shell near the bottom of the tank, P2 is 8 feet higher than P1, and P3 is located in the shell near the top of the tank. For atmospheric tanks, the pressure sensor P3 can be omitted. Let the pressures measured by the pressure sensors P1, P2, and P3 be p1, p2, and p3, respectively, where: G-oil weight; Sav-tank average cross-sectional area; Ïav-between pressure sensors P1, P2 The average density of the product; g is the acceleration of gravity; H is the distance between the pressure sensors P1, P2; h is the height of the oil; h0 is the height of the pressure sensor P1. RTD is used to measure the oil temperature to compensate the measured value. The HTG measurement system has a lower price, but the accuracy of the liquid level measurement is low, and the installation must be open in the tank wall.
The above three methods all use liquid pressure difference to measure the liquid level.
3. Float method, pontoon method, float ball method, servo method, and sinking method float method: This method uses a float as a liquid level measuring element, and drives a display device such as a coded disc or a coded tape, or connects an electronic transmitter so far. Distance transmission measurement signal. Float method: This method uses a magnetic buoy with a hole in the middle as a liquid level sensor, as shown in Figure 3-1. The stainless steel sleeve passes through the middle hole of the buoy and is fixed between the top of the tank and the bottom of the tank. The change of liquid level drives the hollow magnetic buoy (contained permanent magnet) to move up and down along the casing, and attract the magnet in the casing to move up and down along the inner wall of the casing. The secondary instrument calculates the liquid level according to the movement of the magnet. Float method: This method uses the principle of leverage, as shown in Figure 3-2 [4]. In the figure: 1-Float; 2-link; 3-spindle; 4-balance weight; 5-lever. The floating ball rotates around the rotation axis following the change of the liquid level, drives the pointer on the rotation shaft and balances the balance with the other end of the lever, and at the same time indicates the liquid level value on the dial. Float method has two kinds of inner float and outer float, as shown in Figure 3-2. The float ball method is mainly used to measure the liquid level with high temperature and high viscosity, but the range is small.
Servo Method: This method uses a fluctuating integrator circuit to eliminate jitter, extend life, and improve liquid level measurement accuracy. The modern servo liquid level meter has high measurement accuracy, and has reached the accuracy of less than 1 mm in a 40m range, and generally has a function of measuring the density distribution and the average density.
Sinking method: The position of the cylinder changes with the change of the liquid level, but the change amount is not equal to the change of the liquid level. In Figure 3-3a [4], the relationship between the liquid level and the float position is as follows:
In the above formula: ΔH-change of the liquid level; elasticity coefficient of the C-spring; A-cross-sectional area of ​​the cylinder; Ï liquid density; Under normal circumstances, the buoy position change ΔX is far less than the level change ΔH. Figure 3 - 3b is the principle of the torsion tube type cylinder sinking method [4], in which: 1 - sinker; 2 - lever; 3 - torsion tube; 4 - mandrel; The cylinder position changes with the change of the liquid level. Under the action of the lever, the torsion angle of the torsion core axis changes, and the secondary instrument calculates the liquid level according to the change of the torsion angle.
The above five methods all use the buoyancy principle to work.
4. Capacitance, Resistance, and Inductance Capacitance: The principle of the capacitance method for measuring non-conductive liquids is shown in Figure 4-1 [4]. In Figure 4-1, the capacitor consists of two concentric cylindrical surface plates, the capacitance CH is in the above formula: ε1-relative permittivity of the liquid to be measured; ε2-relative permittivity of the gas-phase medium; H- The depth of the capacitive sensor immersed in the liquid (m); l - the vertical height of the capacitive sensor (m); R - the radius of the bottom surface of the inner plate (m); r - the radius of the bottom surface of the outer plate (m). Since R, r, and l are fixed values, the liquid level H can be calculated by using ε1, ε2, and CH. Figure 4-2 is the principle of the capacitance method for measuring conductive liquids [4]. Capacitance type liquid level meter is low in price, easy to install, and can be applied to high temperature and high pressure occasions. However, the measurement accuracy of the capacitance liquid level meter is low, requiring regular maintenance and recalibration, and the working life is not very long.
Resistance method: This method [5] is particularly suitable for the measurement of conductive liquids. Sensitive devices have resistance characteristics. The resistance value changes with the change of liquid level, so the resistance change value is transmitted to the secondary circuit to obtain the liquid level. The probe type uses the tracking measurement method to measure the liquid level. The principle of the liquid level measurement is explained by taking the liquid level rise as an example. When the liquid level rises, the probe is lifted off the liquid completely, and then the probe is slowly lowered to find the liquid level. The position of the probe just before the liquid is in contact with the liquid level. The probe type features high measurement accuracy and a complex control circuit.
Inductance method: This method [5] is suitable for liquid level measurement of conductive liquids, especially liquid metals. The principle of the inductive method is that the change of the liquid level causes the self-inductance, mutual inductance, or permeability of the inductive element to change, so the change amount is sent to the secondary circuit to obtain the corresponding liquid level value. The most widely used inductive method is the high frequency liquid level meter. The measurement principle of the liquid level meter is that the frequency modulation signal is coupled to the transmission line sensor resonant circuit through a radio frequency cable, the output voltage of the resonance circuit is transmitted to the low-pass filter through a detection circuit and a radio frequency cable, and then is controlled according to the output voltage of the low-pass filter. Tuning the circuit generates a new oscillation frequency until the sensor resonant circuit is in a fully resonant state. At this time, the oscillation frequency corresponds to the inductance of the sensor and thus corresponds to the liquid level.
The above three methods measure the liquid level using the method of generating changes in the electrical parameters of the liquid level sensor.
5. Magnetostrictive method, ultrasonic method, modulated optical method, and microwave magnetostrictive method: The principle of this method for measuring the liquid level of an oil tank is shown in Fig. 5-1 [6]. In Figure 5-1, there are two floats, which are used to detect the oil and gas interface and the oil-water interface. Each float contains a set of permanent magnets to generate a fixed magnetic field. During measurement, the head of the level gauge emits a low-current “interrogation†pulse, which produces a magnetic field that travels down the waveguide. When the current magnetic field meets the float magnetic field, a "return" pulse (also called a "waveguide twist" pulse) is generated. The time difference between the interrogation pulse and the return pulse corresponds to the height of the oil-water interface and the oil-gas interface. The magnetostrictive level gauge is easy to install and has high measurement accuracy. However, the variation of liquid density and temperature change will bring about measurement error [7]. The float moves up and down along the guard conduit outside the waveguide, and is easily stuck.
Ultrasonic method: The transducer converts the electric power pulse into an ultrasonic wave and shoots it to the liquid surface. After being reflected by the liquid surface, the transducer converts the ultrasonic wave into an electric signal. Ultrasonic wave is a mechanical wave, with small propagation attenuation, strong interface reflection signal, and simple emission and receiving circuit, so it is widely used. However, ultrasonic wave propagation speed is influenced by factors such as density, concentration, temperature and pressure of the medium, and its measurement accuracy is low.
Microwave method: Microwaves are radiated through antennas (mostly aperture antennas and planar antennas). After being reflected by the liquid surface, they are received by the antenna. Then the secondary circuit calculates the time difference between the transmitted signal and the received signal. The principle of CW radar liquid level meter is shown in Figure 5-2. The liquid level meter adopts triangle wave frequency modulation and analyzes the difference signal obtained by mixing the transmitted signal and the received signal to obtain the microwave transmission time and calculate Liquid level. The microwave velocity is affected little by the propagation medium, temperature, pressure, and permittivity of the liquid, but the fluctuation of the liquid interface, the foam on the liquid surface, and the dielectric constant of the liquid medium have a great influence on the strength of the microwave reflected signal. When the pressure exceeds the specified value, pressure will have a significant effect on the level measurement accuracy. For liquids whose dielectric constant is less than the specified value, most radar level gauges need to use a waveguide, but the rust, bending and tilt of the waveguide will affect the measurement accuracy. For example, when the space height h is 20m and the inclination angle α of the waveguide and the vertical direction exceeds 0.573°, the resulting liquid level error Δh will exceed 1mm, thereby demonstrating that when the inclination angle α (in degrees) is small Δh satisfies:
Radar level instruments are particularly suitable for high-pollution or high-viscosity products such as asphalt. The radar liquid level meter has a high repeatability measurement, and does not require regular maintenance and recalibration. The measurement accuracy is also high, but the price is high, and it is difficult to measure the oil-water interface.
The modulation optical method is similar to the microwave method except that phase or frequency modulated optical signals are used instead of microwave signals. Figure 5-3 is a schematic diagram of a laser radar liquid level gauge [8]. However, the optical signal is greatly affected by water vapor and oil vapor, and it is very sensitive to fluctuations in the liquid level, and optical lenses that are subject to contamination must be used.
The above three methods determine the liquid level by detecting the time of signal propagation. Let the time difference between the transmitted signal and the received signal be t, then the space height h=vt/2, v is the propagation speed of the wave.
6, magnetic flap method, vibration method, nuclear radiation method, optical fiber sensor method magnetic flap method principle shown in Figure 6-1a [1], 1- flap indicating assembly; 2-float; 3-connected pipe assembly; 4- Adjust the screw; 5- drain plug. The float is equipped with a set of permanent magnets, which moves up and down with the change of the liquid level, and drives the magnetic flap assembly to reverse by magnetic coupling. When the liquid level rises, the red side of the magnetic flap faces outward; when the liquid level falls, the white side faces outward. Therefore, the liquid level can be determined according to the color of the magnetic flap. The magnetic structure of the inner magnet of the float and the magnetic flap is shown in Figure 6-1b [5]. The distance between each flap is 10mm. Using several magnetic flap devices in series can increase the range.
The principle of the vibration method is shown in Figure 6-2 [9]. The vibration level gauge consists of a guide rail, a test stand, a hammer, a vibration sensor, and a servo mechanism. The servo mechanism controls the vibrator to crawl up and down and vibrate. The free vibration after excitation is detected by the vibration sensor. After the detection signal is converted by the FET, the frequency at the maximum power is obtained. Finally, the liquid is obtained from the natural frequency/liquid level relationship of the empty can. Bit. This liquid level measurement method requires mechanical moving parts such as a hammer and a servo, and its working life is not long. It requires regular maintenance and recalibration, and the installation is also complicated.
Radiation method: Radioisotope will radiate radiation in the process of decay. Common rays have alpha, beta, and gamma rays. Among them, γ-rays have a strong penetrating power and a long range, so they are widely used in nuclear radiation level measurement. Experiments have shown that the gamma ray intensity changes before and after passing through the material and satisfies the following relationship[5]
In the above formula: J0 - strength before passing through the substance; J - strength after penetrating the substance; μ - attenuation characteristic of the substance to γ ​​rays; d - thickness of the substance. The nuclear radiation type liquid level instrument consists of a radiation source, a detector and a processing circuit. Most of the radioactive sources use cobalt-60 or cesium-137. The detector has several kinds of ionization chambers, counting tubes, scintillation counters, etc. Its role is to detect the intensity of the radiation penetrating the substance. The nuclear radiation level meter uses non-contact installation, as shown in Figure 6-3. Figure 6-3a uses point type radioactive sources and detectors with a small measuring range; Figure 6-3b uses point type radioactive sources and linear detectors with a large measuring range; Figure 6-3c uses linear radioactive sources and detectors. The maximum measurement range. In addition to gamma rays, neutron rays can also be used to measure liquid levels. The penetration of neutron beams is extremely strong, more than 10 times stronger than gamma rays, and can penetrate steel containers with a wall thickness of 9 inches [10]. The ray liquid level meter is easy to install and the measurement accuracy can meet the needs of the large tank measurement and has certain application occasions.
Optical fiber sensing method: In [11], an optical fiber liquid level sensor is proposed. When the liquid level changes, the sensitive elastic diaphragm of the pressure sensor generates a displacement and drives the reflective film to move, so that the light intensity perceived by the probe changes, and the calculation is performed. Liquid level. In [12], another optical fiber liquid level sensor was proposed. According to the difference of light intensity perceived by the probe in the gas phase and the liquid phase medium, the position of the probe is judged, and the change of the liquid level of the probe is controlled to obtain the liquid level value.
7. Conclusion In this paper, more than 20 kinds of liquid level measurement methods were analyzed and compared. In practical applications, the type of liquid level meter should be reasonably selected based on factors such as price, measurement accuracy, and characteristics of the measured medium.
1. Glass tube method, glass plate method, two-color water level method, and manual inspection method. Glass tube method: This method uses the principle of a connected device, as shown in Figure 1-1 [1]. In the drawing, 1- measured container; 2-glass tube; 3-indicating scale; 4, 5-valve; 6, 7-connected tube. The level reads directly from the indicator scale.
Glass plate method: The glass plate can be installed through the connector, or it can be installed in the wall of the container, and several glass plates can be connected in series to increase the measuring range. The level value is read directly from the glass scale.
Two-color water level gauge method: This method uses the principle of optics to make the water show green, and make the water vapor show red, thus indicating the water level [2].
Manual inspection method: This method is used to measure the tank level. During measurement, the surveyor puts the dipstick into the oil and lifts the dipstick when the caliper comes in contact with the bottom of the tank. According to the trace of oil on the dipstick, read the oil level; determine the height of the water cushion according to the change of the color of the end of the dipstick to determine the oil height and the water height [3].
The above four methods are manual measurement methods, which have the advantages of simple measurement, high reliability, intuitiveness, and low cost.
2, blowing method, differential pressure method, HTG method blowing method: The working principle of the method shown in Figure 2-1 [4]. In the drawing, 1-filter; 2-pressure reducing valve; 3-throttle element; 4-rotor flowmeter; 5-transmitter. Since the pressure in the air blow pipe is approximately equal to the static pressure of the liquid column, P = ÏgH
In the formula, Ï-liquid density; H-level. Therefore, the level H can be measured by the static pressure P. The air blowing method is suitable for measuring liquids with strong corrosiveness and suspended substances, and is mainly used in occasions where the measurement accuracy is not required.
Differential pressure method: The working principle of this method is shown in Figure 2-2 [4]. In the figure, 1, 2-valve; 3- differential pressure transmitter. For open or atmospheric vessels, valve 1 and gas phase pressure piping can be omitted. The relationship between pressure difference and liquid level is ΔP=P2-P1=ÏgH
In the formula: ΔP-transmitter pressure difference between positive and negative pressure chamber; P2, P1-pressure tube pressure; H-level. The differential pressure transmitter converts the pressure difference into a 4-20 mA DC signal. If the corresponding output signal is greater than or less than 4 mA when the pressure is at the lower limit of the measurement range, zero migration techniques such as adjusting the migration spring are required to equal 4 mA.
HTG method: This method is applied to differential pressure level measurement of oil tanks, as shown in Figure 2-3. In the picture: P1, P2, P3- high precision pressure sensor; RTD- temperature detection element; HIU-interface unit. P1 is located in the shell near the bottom of the tank, P2 is 8 feet higher than P1, and P3 is located in the shell near the top of the tank. For atmospheric tanks, the pressure sensor P3 can be omitted. Let the pressures measured by the pressure sensors P1, P2, and P3 be p1, p2, and p3, respectively, where: G-oil weight; Sav-tank average cross-sectional area; Ïav-between pressure sensors P1, P2 The average density of the product; g is the acceleration of gravity; H is the distance between the pressure sensors P1, P2; h is the height of the oil; h0 is the height of the pressure sensor P1. RTD is used to measure the oil temperature to compensate the measured value. The HTG measurement system has a lower price, but the accuracy of the liquid level measurement is low, and the installation must be open in the tank wall.
The above three methods all use liquid pressure difference to measure the liquid level.
3. Float method, pontoon method, float ball method, servo method, and sinking method float method: This method uses a float as a liquid level measuring element, and drives a display device such as a coded disc or a coded tape, or connects an electronic transmitter so far. Distance transmission measurement signal. Float method: This method uses a magnetic buoy with a hole in the middle as a liquid level sensor, as shown in Figure 3-1. The stainless steel sleeve passes through the middle hole of the buoy and is fixed between the top of the tank and the bottom of the tank. The change of liquid level drives the hollow magnetic buoy (contained permanent magnet) to move up and down along the casing, and attract the magnet in the casing to move up and down along the inner wall of the casing. The secondary instrument calculates the liquid level according to the movement of the magnet. Float method: This method uses the principle of leverage, as shown in Figure 3-2 [4]. In the figure: 1-Float; 2-link; 3-spindle; 4-balance weight; 5-lever. The floating ball rotates around the rotation axis following the change of the liquid level, drives the pointer on the rotation shaft and balances the balance with the other end of the lever, and at the same time indicates the liquid level value on the dial. Float method has two kinds of inner float and outer float, as shown in Figure 3-2. The float ball method is mainly used to measure the liquid level with high temperature and high viscosity, but the range is small.
Servo Method: This method uses a fluctuating integrator circuit to eliminate jitter, extend life, and improve liquid level measurement accuracy. The modern servo liquid level meter has high measurement accuracy, and has reached the accuracy of less than 1 mm in a 40m range, and generally has a function of measuring the density distribution and the average density.
Sinking method: The position of the cylinder changes with the change of the liquid level, but the change amount is not equal to the change of the liquid level. In Figure 3-3a [4], the relationship between the liquid level and the float position is as follows:
In the above formula: ΔH-change of the liquid level; elasticity coefficient of the C-spring; A-cross-sectional area of ​​the cylinder; Ï liquid density; Under normal circumstances, the buoy position change ΔX is far less than the level change ΔH. Figure 3 - 3b is the principle of the torsion tube type cylinder sinking method [4], in which: 1 - sinker; 2 - lever; 3 - torsion tube; 4 - mandrel; The cylinder position changes with the change of the liquid level. Under the action of the lever, the torsion angle of the torsion core axis changes, and the secondary instrument calculates the liquid level according to the change of the torsion angle.
The above five methods all use the buoyancy principle to work.
4. Capacitance, Resistance, and Inductance Capacitance: The principle of the capacitance method for measuring non-conductive liquids is shown in Figure 4-1 [4]. In Figure 4-1, the capacitor consists of two concentric cylindrical surface plates, the capacitance CH is in the above formula: ε1-relative permittivity of the liquid to be measured; ε2-relative permittivity of the gas-phase medium; H- The depth of the capacitive sensor immersed in the liquid (m); l - the vertical height of the capacitive sensor (m); R - the radius of the bottom surface of the inner plate (m); r - the radius of the bottom surface of the outer plate (m). Since R, r, and l are fixed values, the liquid level H can be calculated by using ε1, ε2, and CH. Figure 4-2 is the principle of the capacitance method for measuring conductive liquids [4]. Capacitance type liquid level meter is low in price, easy to install, and can be applied to high temperature and high pressure occasions. However, the measurement accuracy of the capacitance liquid level meter is low, requiring regular maintenance and recalibration, and the working life is not very long.
Resistance method: This method [5] is particularly suitable for the measurement of conductive liquids. Sensitive devices have resistance characteristics. The resistance value changes with the change of liquid level, so the resistance change value is transmitted to the secondary circuit to obtain the liquid level. The probe type uses the tracking measurement method to measure the liquid level. The principle of the liquid level measurement is explained by taking the liquid level rise as an example. When the liquid level rises, the probe is lifted off the liquid completely, and then the probe is slowly lowered to find the liquid level. The position of the probe just before the liquid is in contact with the liquid level. The probe type features high measurement accuracy and a complex control circuit.
Inductance method: This method [5] is suitable for liquid level measurement of conductive liquids, especially liquid metals. The principle of the inductive method is that the change of the liquid level causes the self-inductance, mutual inductance, or permeability of the inductive element to change, so the change amount is sent to the secondary circuit to obtain the corresponding liquid level value. The most widely used inductive method is the high frequency liquid level meter. The measurement principle of the liquid level meter is that the frequency modulation signal is coupled to the transmission line sensor resonant circuit through a radio frequency cable, the output voltage of the resonance circuit is transmitted to the low-pass filter through a detection circuit and a radio frequency cable, and then is controlled according to the output voltage of the low-pass filter. Tuning the circuit generates a new oscillation frequency until the sensor resonant circuit is in a fully resonant state. At this time, the oscillation frequency corresponds to the inductance of the sensor and thus corresponds to the liquid level.
The above three methods measure the liquid level using the method of generating changes in the electrical parameters of the liquid level sensor.
5. Magnetostrictive method, ultrasonic method, modulated optical method, and microwave magnetostrictive method: The principle of this method for measuring the liquid level of an oil tank is shown in Fig. 5-1 [6]. In Figure 5-1, there are two floats, which are used to detect the oil and gas interface and the oil-water interface. Each float contains a set of permanent magnets to generate a fixed magnetic field. During measurement, the head of the level gauge emits a low-current “interrogation†pulse, which produces a magnetic field that travels down the waveguide. When the current magnetic field meets the float magnetic field, a "return" pulse (also called a "waveguide twist" pulse) is generated. The time difference between the interrogation pulse and the return pulse corresponds to the height of the oil-water interface and the oil-gas interface. The magnetostrictive level gauge is easy to install and has high measurement accuracy. However, the variation of liquid density and temperature change will bring about measurement error [7]. The float moves up and down along the guard conduit outside the waveguide, and is easily stuck.
Ultrasonic method: The transducer converts the electric power pulse into an ultrasonic wave and shoots it to the liquid surface. After being reflected by the liquid surface, the transducer converts the ultrasonic wave into an electric signal. Ultrasonic wave is a mechanical wave, with small propagation attenuation, strong interface reflection signal, and simple emission and receiving circuit, so it is widely used. However, ultrasonic wave propagation speed is influenced by factors such as density, concentration, temperature and pressure of the medium, and its measurement accuracy is low.
Microwave method: Microwaves are radiated through antennas (mostly aperture antennas and planar antennas). After being reflected by the liquid surface, they are received by the antenna. Then the secondary circuit calculates the time difference between the transmitted signal and the received signal. The principle of CW radar liquid level meter is shown in Figure 5-2. The liquid level meter adopts triangle wave frequency modulation and analyzes the difference signal obtained by mixing the transmitted signal and the received signal to obtain the microwave transmission time and calculate Liquid level. The microwave velocity is affected little by the propagation medium, temperature, pressure, and permittivity of the liquid, but the fluctuation of the liquid interface, the foam on the liquid surface, and the dielectric constant of the liquid medium have a great influence on the strength of the microwave reflected signal. When the pressure exceeds the specified value, pressure will have a significant effect on the level measurement accuracy. For liquids whose dielectric constant is less than the specified value, most radar level gauges need to use a waveguide, but the rust, bending and tilt of the waveguide will affect the measurement accuracy. For example, when the space height h is 20m and the inclination angle α of the waveguide and the vertical direction exceeds 0.573°, the resulting liquid level error Δh will exceed 1mm, thereby demonstrating that when the inclination angle α (in degrees) is small Δh satisfies:
Radar level instruments are particularly suitable for high-pollution or high-viscosity products such as asphalt. The radar liquid level meter has a high repeatability measurement, and does not require regular maintenance and recalibration. The measurement accuracy is also high, but the price is high, and it is difficult to measure the oil-water interface.
The modulation optical method is similar to the microwave method except that phase or frequency modulated optical signals are used instead of microwave signals. Figure 5-3 is a schematic diagram of a laser radar liquid level gauge [8]. However, the optical signal is greatly affected by water vapor and oil vapor, and it is very sensitive to fluctuations in the liquid level, and optical lenses that are subject to contamination must be used.
The above three methods determine the liquid level by detecting the time of signal propagation. Let the time difference between the transmitted signal and the received signal be t, then the space height h=vt/2, v is the propagation speed of the wave.
6, magnetic flap method, vibration method, nuclear radiation method, optical fiber sensor method magnetic flap method principle shown in Figure 6-1a [1], 1- flap indicating assembly; 2-float; 3-connected pipe assembly; 4- Adjust the screw; 5- drain plug. The float is equipped with a set of permanent magnets, which moves up and down with the change of the liquid level, and drives the magnetic flap assembly to reverse by magnetic coupling. When the liquid level rises, the red side of the magnetic flap faces outward; when the liquid level falls, the white side faces outward. Therefore, the liquid level can be determined according to the color of the magnetic flap. The magnetic structure of the inner magnet of the float and the magnetic flap is shown in Figure 6-1b [5]. The distance between each flap is 10mm. Using several magnetic flap devices in series can increase the range.
The principle of the vibration method is shown in Figure 6-2 [9]. The vibration level gauge consists of a guide rail, a test stand, a hammer, a vibration sensor, and a servo mechanism. The servo mechanism controls the vibrator to crawl up and down and vibrate. The free vibration after excitation is detected by the vibration sensor. After the detection signal is converted by the FET, the frequency at the maximum power is obtained. Finally, the liquid is obtained from the natural frequency/liquid level relationship of the empty can. Bit. This liquid level measurement method requires mechanical moving parts such as a hammer and a servo, and its working life is not long. It requires regular maintenance and recalibration, and the installation is also complicated.
Radiation method: Radioisotope will radiate radiation in the process of decay. Common rays have alpha, beta, and gamma rays. Among them, γ-rays have a strong penetrating power and a long range, so they are widely used in nuclear radiation level measurement. Experiments have shown that the gamma ray intensity changes before and after passing through the material and satisfies the following relationship[5]
In the above formula: J0 - strength before passing through the substance; J - strength after penetrating the substance; μ - attenuation characteristic of the substance to γ ​​rays; d - thickness of the substance. The nuclear radiation type liquid level instrument consists of a radiation source, a detector and a processing circuit. Most of the radioactive sources use cobalt-60 or cesium-137. The detector has several kinds of ionization chambers, counting tubes, scintillation counters, etc. Its role is to detect the intensity of the radiation penetrating the substance. The nuclear radiation level meter uses non-contact installation, as shown in Figure 6-3. Figure 6-3a uses point type radioactive sources and detectors with a small measuring range; Figure 6-3b uses point type radioactive sources and linear detectors with a large measuring range; Figure 6-3c uses linear radioactive sources and detectors. The maximum measurement range. In addition to gamma rays, neutron rays can also be used to measure liquid levels. The penetration of neutron beams is extremely strong, more than 10 times stronger than gamma rays, and can penetrate steel containers with a wall thickness of 9 inches [10]. The ray liquid level meter is easy to install and the measurement accuracy can meet the needs of the large tank measurement and has certain application occasions.
Optical fiber sensing method: In [11], an optical fiber liquid level sensor is proposed. When the liquid level changes, the sensitive elastic diaphragm of the pressure sensor generates a displacement and drives the reflective film to move, so that the light intensity perceived by the probe changes, and the calculation is performed. Liquid level. In [12], another optical fiber liquid level sensor was proposed. According to the difference of light intensity perceived by the probe in the gas phase and the liquid phase medium, the position of the probe is judged, and the change of the liquid level of the probe is controlled to obtain the liquid level value.
7. Conclusion In this paper, more than 20 kinds of liquid level measurement methods were analyzed and compared. In practical applications, the type of liquid level meter should be reasonably selected based on factors such as price, measurement accuracy, and characteristics of the measured medium.
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