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4 common classification methods for measuring liquid level. In modern industrial production, various liquid raw materials and finished products are stored, transported, and used – such as oils and various solvents in petrochemical industries, various slurries and pastes used in pharmaceutical and papermaking industries, as well as various emulsions, beverages, and juices produced and stored in food industries. It is necessary to ensure the safe and proper storage of these liquids; therefore, data on their liquid levels is an important fundamental parameter in industrial production, and it is essential for ensuring that companies can operate smoothly and stably. The measurement principles of level gauges commonly used in the industrial sector involve various fields such as mechanics, thermodynamics, electricity, and optics, with hundreds of different product models available. Of course, different types of level gauges imply different manufacturing methods, usage methods, and calibration methods, and they cannot be suitable for all measurement environments. Below, from the perspective of measurement methods, more than a dozen types of level gauges currently available on the market will be classified to assist users in making appropriate choices when selecting products. 1. Level gauges that use buoyancy for measurement: All level gauges of the buoyancy type have a float (a buoy or float tube) which remains in equilibrium on the surface of the stationary liquid. When the liquid level changes, the float moves along with it; electronic components detect this movement of the float and convert it into an indication of the change in liquid level. Depending on the way in which electronic components detect the displacement of the float, buoyancy-based level gauges are classified into magnetic flap level gauges, float level gauges, magnetostrictive level gauges, etc. (1) Magnetic flap level gauge. A magnetic flap level gauge consists of a magnetic float, a cylindrical container, a scale, and a transmitter. The float moves within the container along with the liquid level; the magnetic field generated by the float acts on a sensor. This sensor is a bakelite strip enclosed in a stainless steel tube and having a length equal to that of the liquid level being measured. Numerous reed switches are welded onto this strip at intervals of 1 cm or less. The reed switches close when they are at the same height as the float, and remain open otherwise. The transmitter receives the electrical current resulting from the resistance values associated with these reed switches, and converts this into a value representing the liquid level height. The movement of the float is displayed on the scale as the flipping of red and white beads, indicating the liquid level. (2) Float level gauge. A float level gauge consists of a float, a spring, a magnet chamber, and an indicator, and is designed based on Archimedes’ principle and magnetic coupling. The displacement of the float causes a change in the magnetic field within the magnet chamber; the magnetic sensor inside the indicator accurately measures these changes in the surrounding magnetic field, which corresponds to the liquid level and thus reflects any changes in the liquid level. (3) Magnetostrictive level gauge. A 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 to start timing; a magnetic field is generated and propagates downward along the magnetostrictive wire. The float moves along the measuring rod as the liquid level changes, and the magnet inside the float also generates a magnetic field. When these two magnetic fields meet, the magnetostrictive wire twists, creating torsional stress wave pulses. Since the speed of these pulses is known, by calculating the time it takes for the pulses to travel, the precise change in liquid level can be determined. 2. Types of level gauges that measure using pressure. Pressure-type level gauges are those that determine the liquid level height by measuring the pressure of the liquid through their pressure-sensing elements. It generally consists of a transmitter, sensors, and pressure probes. A sensor is a special metal rod or cable with built-in capillary tubes; its probe consists of a stainless steel core with a diaphragm at the bottom. The probe is inserted into the liquid to measure the difference between the hydrostatic pressure exerted by the liquid and the actual atmospheric pressure. This pressure difference changes as the liquid level changes, and electronic components receive these pressure difference signals to reflect the changes in liquid level. They are divided into rod type and cable type depending on the material of the sensor. (1) Rod-type pressure level gauge. The sensor of a rod-type pressure level gauge is a metal rod. Its general technical parameters are as follows: the measurement range is typically 0–5 m, the output signal is (4–20) mA, and the measurement accuracy is generally ± ； 0.5%. (2) Cable-type pressure level gauge. Since the sensor is a cable that can be bent, making it easy to transport and install, cable-type pressure level gauges can have a very large measurement range – 100 meters, or even 300 meters. Their measurement accuracy is generally ± ； 1.0%. (3) Differential pressure level gauge. A differential pressure level gauge also measures the liquid level by detecting pressure differences. The level gauge has pressure tapping points for both the gas phase and the liquid phase. The pressure at the gas-phase pressure sampling point is the pressure of the gas phase inside the equipment ； At the pressure measurement point in the liquid phase, the pressure is influenced not only by the gas-phase pressure but also by the hydrostatic pressure of the liquid column. The difference between the pressures in the liquid and gas phases represents the hydrostatic pressure generated by the liquid column, which is then converted into a height corresponding to the liquid level. 3. Types of level gauges that use reflection for measurement. Currently, the main level gauges that employ reflection are radar level gauges and ultrasonic level gauges. Their principles are basically the same; they all measure the liquid level by emitting, reflecting, and receiving ultrasonic waves or electromagnetic waves (radar waves). (1) Radar level gauge. The antenna of a radar level gauge emits electromagnetic waves, which are reflected by the surface of the substance being measured before being detected by the antenna. The time it takes for these electromagnetic waves to travel from emission to reception is proportional to the distance to the liquid surface. By recording the time taken by the pulse waves, and since the speed of propagation of electromagnetic waves is constant, it is possible to determine the distance from the liquid surface to the radar antenna, and thus calculate the liquid level. (2) Ultrasonic level gauge. An ultrasonic level gauge is a digital level measuring instrument controlled by a microprocessor. During measurement, pulsed ultrasonic waves are emitted by the sensor (transducer); the sound waves are reflected off the surface of the liquid and then received by the same sensor, where they are converted into electrical signals. The distance from the sensor to the liquid being measured is calculated based on the time between the emission and reception of sound waves. 4. Level gauge types that use electrical conductivity for measurement. Level gauges based on electrical conductivity measure the liquid level by detecting capacitance or resistance. Such level gauges mainly include capacitive level gauges, radio frequency admittance level gauges, and resistive level gauges. (1) Capacitive level gauge. A capacitive level gauge measures the height of the liquid level by detecting changes in capacitance. A metal rod is inserted into a liquid container; the metal rod serves as one pole of the capacitor, while the container wall serves as the other pole. The dielectric between the two electrodes is the liquid and the gas above it. Since the dielectric constant of the liquid differs from that of the gas above the liquid surface, the level of the liquid is measured by changes in the capacitance between the two electrodes. (2) Radio frequency admittance level gauge. The radio frequency admittance level gauge is an upgraded version of the capacitive level gauge; it can be understood as a device that measures admittance using high-frequency radio waves. When the instrument is in operation, its sensors form an admittance value with the wall of the tank and the medium being measured. As the liquid level changes, this admittance value changes accordingly; the circuit unit converts this measured admittance value into a liquid level signal, thereby enabling the measurement of the liquid level. (3) Resistive level gauge. A resistive level gauge operates by having electrodes in contact with a conductive liquid; a low-frequency voltage is applied between these electrodes. When the electrodes come into contact with the resistance of the liquid, a conductive path is formed. In this way, the level of the liquid is measured based on its electrical conductivity. Continuous measurement of the water level can be achieved by using resistance values at multiple fixed points.