A brief analysis of how to classify common liquid level meters based on different measurement methods

In modern industrial production, various liquid raw materials and finished products are stored, transported and used, such as oil and various solvents of petrochemical enterprises, various slurries and pulps used in the production of pharmaceutical and papermaking enterprises, and various emulsions, beverages and juices produced and stored by food enterprises. All of them need to be stored safely and perfectly. Therefore, the measurement of the liquid level of these liquids is an important task in production. As an important basic data in industrial production, liquid level data is a basic requirement to ensure the normal and stable production of enterprises.
 
         At present, the measurement principle of liquid level meters commonly used in the industrial field involves mechanics, thermodynamics, electricity, optics and other aspects. If the difference in the appearance of liquid level meter products is added, there are even hundreds of types. Of course, each type of liquid level meter has more or less differences from other types. No liquid level meter product can be competent for all measurement environments. Different types of liquid level meters mean that their production methods, usage methods and calibration methods are different. How to classify them effectively and clearly has become a common problem faced by manufacturers, users and measurement departments. This article is written from the perspective of measurement methods, and classifies more than ten types of liquid level gauges currently on the market, in order to obtain a simple and clear classification result. To help users make appropriate choices in product selection.
　　　
1. Types of liquid level gauges that measure by buoyancy
　　The buoyancy type of liquid level gauges all have a float (float or float), which is in a balanced state on the static liquid surface. When the liquid level changes, the float moves with the liquid surface. The electronic components obtain the displacement of the float in a certain way and then convert it into a change in the liquid level.
　　Due to the different ways in which electronic components receive the displacement of the float, the buoyancy type liquid level gauge is divided into magnetic flap liquid level gauge, float liquid level gauge, magnetostrictive liquid level gauge, etc.
　　1.1  Magnetic flap liquid level gauge . The magnetic flap liquid level gauge consists of a magnetic float, a cylindrical container, a scale and a transmitter. The float moves with the liquid level in the container, and the magnetic field of the float acts on the sensor. The sensor is a bakelite strip sealed in a stainless steel tube with the same length as the measured liquid level. There are many reed switches welded on it at intervals of 1 cm or less. When the reed switch and the float are at the same height, they are closed, otherwise they are open. The transmitter receives the amount of resistance connected to the reed switch to generate a current signal, which is converted into the liquid level. The movement of the float is displayed on the scale as the flip of the red and white beads, indicating the liquid level. 1.2 Float level gauge. The float level gauge consists of a float, a spring, a magnetic steel chamber and an indicator. It is designed according to Archimedes' law and the principle of magnetic coupling. The displacement of the float causes the magnetic field of the magnetic steel chamber to change. The magnetic sensor in the indicator accurately measures the changes in the surrounding magnetic field, which corresponds to the liquid level and reflects the changes in the liquid level. 　　1.3 Magnetostrictive level gauge. The magnetostrictive level gauge is mainly composed of a float, a waveguide and a transmitter. The electronic components of the transmitter generate low-voltage current pulses, start timing, generate a magnetic field that propagates downward along the magnetostrictive line, and the float moves along the measuring rod as the liquid level changes. The magnet inside the float also generates a magnetic field. The two magnetic fields meet, and the magnetostrictive line twists to form a torsional stress wave pulse. The pulse speed is known, and the pulse propagation time is calculated to correspond to the precise change of the liquid level. 2. Types of liquid level gauges that measure by pressure 　　The pressure level gauge is a liquid level gauge that reflects the liquid level by measuring the liquid pressure through its pressure measuring element. It is generally composed of a transmitter, a sensor, and a pressure probe. The sensor is a special metal rod or cable with a built-in capillary hose. The probe is a stainless steel core with a diaphragm at the bottom. The probe is immersed in the liquid to test the difference between the static pressure of the liquid and the actual atmospheric pressure. The pressure difference changes with the liquid level, and the electronic component receives the pressure difference signal to reflect the change of the liquid level. According to the different materials of the sensor, it is divided into rod type and cable type. 　　2.1 Rod type pressure level gauge. The sensor of the rod type pressure level gauge is a metal rod. Its general technical parameters are: the measuring range is generally 0~5m, the output signal is: (4~20)mA, and the measuring accuracy is generally ±0.5%. 　　2.2 Cable pressure level gauge. Since the sensor is a cable, it can be bent, which is convenient for handling and loading and unloading, so the measuring range of the cable pressure level gauge can be very large, 100m or even 300m, and its measuring accuracy is generally ±1.0%. 　　2.3 Differential pressure level gauge. The differential pressure level gauge also measures the liquid level by detecting the pressure difference. The level gauge has two pressure ports, gas phase and liquid phase. The pressure at the gas phase pressure point is the gas phase pressure in the equipment; the pressure at the liquid phase pressure point is affected by the gas phase pressure and the static pressure of the liquid column. The difference between the liquid phase and gas phase pressures is the static pressure generated by the liquid column, which is converted into the liquid level height. 3. Types of level gauges that measure by reflection 　　Currently, there are mainly two types of reflection level gauges: radar level gauges and ultrasonic level gauges. Their principles are basically the same, and they all measure the liquid level by transmitting, reflecting, and receiving ultrasonic or electromagnetic waves (radar waves). 　　4.1 Radar level meter. The antenna of the radar level meter emits electromagnetic waves, which are reflected by the surface of the object being measured and then received by the antenna. The time from the emission to the reception of the electromagnetic wave is proportional to the distance to the liquid surface. The radar level meter records the time experienced by the pulse wave, and the transmission speed of the electromagnetic wave is a constant. The distance from the liquid surface to the radar antenna can be calculated, and thus the liquid level of the liquid surface can be calculated. 　　4.2 Ultrasonic level meter. The ultrasonic level meter is a digital level meter controlled by a microprocessor. In the measurement, the pulse ultrasonic wave is emitted by the sensor (transducer), and the sound wave is reflected by the liquid surface and received by the same sensor and converted into an electrical signal. The distance from the sensor to the measured liquid is calculated by the time between the emission and reception of the sound wave. 4. Types of level meters that measure by conductive method The 　　conductive level meter measures the liquid level by measuring capacitance or resistance based on electrical principles. This type of level meter mainly includes capacitance level meter, radio frequency admittance level meter, and resistance level meter. 　　3.1 Capacitance level meter. The capacitance level meter measures the height of the liquid level by measuring the change in capacitance. A metal rod is inserted into the liquid container, the metal rod serves as one pole of the capacitor, and the container wall serves as the other pole of the capacitor. The medium between the two electrodes is the liquid and the gas above it. Since the dielectric constant of the liquid is different from the dielectric constant of the gas on the liquid surface, the height of the liquid level is measured by the change in capacitance between the two electrodes. 　　3.2 Radio frequency admittance level meter. The radio frequency admittance level meter is an upgraded version of the capacitance level meter. It can be understood as measuring admittance with high-frequency radio waves. When the instrument is working, the sensor of the instrument forms an admittance value with the container wall and the measured medium. When the level changes, the admittance value changes accordingly. The circuit unit converts the measured admittance value into a level signal output to achieve liquid level measurement. 　　3.3 Resistance level meter. The resistance level meter contacts the conductive liquid through the electrodes, and applies a low-frequency voltage between the electrodes. When the electrodes change from the disconnected state to the series liquid resistor, a conductive path is formed, that is, the liquid level is measured by the conductivity of the liquid. The water level can be continuously measured by measuring resistors at multiple fixed points. V. Conclusion of this article 　　Based on the different measurement methods, the level meter is divided into the above four categories, which enables readers to have a complete and clear understanding of the level meter, and also provides a certain reference value for the purchase of the majority of users. This classification method is an exploration of the author based on the knowledge learned and practical applications. If there are any deficiencies, please criticize and correct them.