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Several methods of level measurement and their advantages and disadvantages

by:Sure     2021-08-19
In level measurement, there are many methods, but they all have their own scope of application: 1. Contact measurement Contact measurement starts with a steel belt float level gauge, and has evolved to various modernizations by accurately measuring the distance of the float in various ways. Instruments such as servo type, magnetostrictive type, etc. Steel belt float type: the earliest liquid level gauges are now facing a new generation. Working principle. The float floats in the level measurement by buoyancy. There are many methods, but they all have their own applications. Scope: 1. Contact measurement The contact measurement starts with the steel belt float level gauge, and has evolved to various modern instruments such as servo type, magnetostrictive type, etc. steel belt float type by accurately measuring the distance of the float in various ways : The earliest liquid level gauges are now facing a new generation of working principles. The float floats on the surface of the medium under buoyancy, and reads the liquid level value on the scaled steel belt through the variable speed gear. After the liquid level rises or falls, the force balance is destroyed. , The float also rises and falls to drive the steel belt to run. The theoretical accuracy is about 2-3mm. The installation is complicated and the reliability is low. Due to the large number of mechanical parts, it is easy to cause the steel belt to get stuck. The optical fiber type realizes the digitization of the steel strip liquid level through the optical code disk. 2. Magnetostrictive magnetostrictive working principle The upper electronic component of the probe generates a low-voltage current pulse and starts timing. The generated magnetic field travels down the magnetostrictive line. The float moves up and down along the measuring rod as the liquid level changes. With a magnet, a magnetic field is also generated. When two magnetic fields meet, the magnetostrictive wire is twisted to form a torsional stress wave pulse. The pulse speed is known. The calculation of the pulse propagation time corresponds to the precise change of the liquid level. (The current runs at the speed of light, so its propagation time is negligible compared with the force wave time.) The accuracy can reach up to 1mm. Advantages and disadvantages analysis. Magnetostrictive liquid level has higher accuracy and can measure the oil-water interface. However, due to its contact measurement method and comparison High installation and maintenance requirements have led to a lack of widespread market penetration. 3. Servo-type liquid level gauge The servo-type liquid level gauge is a relatively successful new type of liquid level gauge, which is mainly used in the high-precision measurement of light oil products. It forms a relatively strong competition with radar level gauges. The basic principle is the same as the steel belt type liquid level gauge, but it has an accurate force sensor and a servo system to form a closed-loop adjustment system. By considering the gravity of the steel belt, the height of the float is accurately adjusted to balance the buoyancy and gravity to obtain an accurate current liquid level. To the height of the tank top to get the level value. The accuracy is high, which can reach 1mm, which meets the requirements of measurement level. Used in calm, light and non-corrosive liquids. Installation and debugging are more troublesome, and there are also various disadvantages of contact level gauges and the price is high. 4. Static pressure level gauge The hydrostatic level gauge is special. It uses the proportional relationship between the pressure of the uniform liquid and the height to convert the liquid level height by measuring the pressure at the bottom of the liquid. Pu003dρgh (P pressure) Because it is greatly affected by the density and temperature of the medium, the accuracy is often poor. In order to eliminate these effects, many other test instruments are needed. As a result, building a complete static pressure measurement system is expensive. 5. Non-contact measurement Non-contact measurement usually adopts the form of transmitting a wave that can be reflected by the measured medium to measure. Using the known wave propagation speed, the liquid level and measuring instrument can be obtained by directly or indirectly measuring the wave propagation time. The distance between the two, and then get the level value. According to the type of emitted wave, there are light wave laser level gauge, ultrasonic ultrasonic level gauge, electromagnetic wave radar level gauge. 6. Radar measurement Radar measurement adopts the form of emitted electromagnetic wave. Because the dielectric constant of the measured medium is greater than 1 of air and vacuum, due to the discontinuity of the medium, reflection will occur at the interface of air and liquid. Electromagnetic waves are in the air. The propagation speed of the medium is basically not affected by temperature, so by measuring the time between the electromagnetic wave being emitted to the reflection being received, the height of the liquid level gauge from the liquid surface can be measured, and then the liquid level value can be obtained. Radar level gauges are divided into two categories, and their specific measurement principles are not the same. Radar level gauges are classified as pulsed and frequency modulated continuous wave (FMCW). 7. Pulse radar measurement The principle of pulse radar is basically the same as that of ultrasonic. The radar emits short microwave pulses, which are reflected at the liquid surface, and the reflected echoes received by the radar are processed by the signal to obtain the target distance. Ru003dc*(t1-t0)/2 The general low-priced radar level gauges on the market are all pulsed, representing KROHNE, Siemens, E+H, VEGA, etc. Accuracy: ±5~10mm 8. Continuous frequency modulation Principle of wave mode (FMCW): linear frequency sweep, frequency measurement is equivalent to time measurement, the electromagnetic wave propagation time is obtained, and then the distance is obtained. The advantages of FM continuous wave radar, high accuracy, up to ± 0.5mm, strong anti-interference ability, wide application range, can be used for corrosive, high temperature and high pressure, unstable liquids, no moving parts, maintenance-free, high reliability, easy installation, flexible configuration, different antennas and radars The head combination is suitable for different measurement environments. The high-precision frequency modulated continuous wave radar of foreign mines is typical of the saab-pro series and saab-rex series of Sweden's SAAB company in the world. The pro series are control-level level gauges with an accuracy of ±3~10mm. The Rex series is a ±0.5mm accuracy level gauge with custody transfer certification. It is also the most accurate radar level gauge today. All high-end products of the level gauge adopt the frequency modulation continuous wave method, and the price is relatively expensive.
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