Level measurement includes two categories: liquid level and material level. Liquid level measurement further includes two types: liquid level signal device and continuous liquid level measurement. Liquid level signal device measurement measures the liquid level at several fixed positions and is used for upper and lower limit alarms, etc.; continuous liquid level measurement is a linear output measurement of liquid level changes.
Liquid level measurement is widely used in many fields such as petroleum, chemical, and food processing, and is of great significance. There are numerous methods for liquid level measurement, each with its own characteristics and application scope:
1. Contact-type liquid level measurement
Contact measurement started with steel strip float level gauges and has gradually evolved into a variety of measurement methods based on different mechanical and electrical principles, such as guided wave radar, magnetostrictive, servo , tuning fork, and photoelectric types.
Steel Belt Float Type
The earliest level gauges used a float that floated on the surface of the medium. The level was read by a graduated steel belt via a gear train. When the level rose or fell, disrupting the force balance, the float rose or fell accordingly, driving the steel belt. These gauges were complex to install, had low reliability, and were prone to jamming due to the numerous mechanical parts. They are now facing replacement.
Guided wave radar level gauge
A commonly used liquid level measuring instrument in industry is based on the time domain reflection (TDR) principle. The electromagnetic pulse of the radar level gauge propagates at the speed of light along the steel cable or probe. When it encounters the surface of the measured medium, part of the radar level gauge's pulse is reflected to form an echo and returns to the pulse transmitting device along the same path. The distance between the transmitting device and the surface of the measured medium is proportional to the propagation time of the pulse. The liquid level height can be calculated from this distance.
LFP series guided wave radar level gauge
Magnetostrictive
The electronic components at the upper end of the probe generate a low-voltage current pulse, which starts timing. This generates a magnetic field that propagates downwards along the magnetostrictive lines. The float moves up and down along the measuring rod as the liquid level changes. The float contains a magnet, which also generates a magnetic field. When the two magnetic fields meet, the magnetostrictive lines twist to form a torsional stress wave pulse. The pulse velocity is known, and the pulse propagation time corresponds to the precise change in liquid level.
Magnetostrictive liquid level measurement has high accuracy and can measure the oil-water interface, but its contact measurement method and high installation and maintenance requirements have limited its market adoption.
hydrostatic level gauge
Hydrostatic level gauges are unique in that they utilize the proportional relationship between the pressure and height of a uniform liquid to calculate the liquid level by measuring the pressure at the bottom of the liquid. P = ρgh (P = pressure). Because their accuracy is greatly affected by the density and temperature of the medium, they are often relatively poor. To eliminate these effects, many other testing instruments are needed.
It is mainly used in industrial sites as an input device, and the signal is brought out by cable. It is mainly used for some deep water measurement systems, such as hydropower stations, river dams, and deep water measurement in ports.
LFH series hydrostatic level gauge
Servo level gauge
The basic principle is the same as that of the steel belt level gauge, but it has a precise force sensor and servo system to form a closed-loop control system. By taking into account the weight of the steel belt itself, the height of the float is precisely adjusted to balance buoyancy and gravity, so as to obtain the accurate height of the current liquid level from the top of the tank and thus obtain the liquid level value.
Suitable for calm, light, non-corrosive liquids. Installation and commissioning are relatively complicated, and it shares the same disadvantages as contact level gauges, but is also expensive.
Tuning fork level switch
A liquid level switch tool designed using the tuning fork principle works by inducing vibrations in a piezoelectric crystal. When damped by material, the amplitude decreases sharply, and the frequency and phase change significantly. These changes are detected by internal electronic circuitry, processed, and converted into a switching signal output. This product can monitor, control, and alarm on the high and low levels of a tank, and is suitable for various liquids, powders, and granular solids. It is practical, simple to use, reliable in operation, highly adaptable, and essentially maintenance-free.
LFV200 series level switches
Photoelectric liquid level switch
Photoelectric level switches use infrared detection, utilizing the principles of light refraction and reflection. Light will be reflected or refracted at the interface between two different media. When the measured liquid is at a high level, an interface is formed between the measured liquid and the photoelectric switch; when the measured liquid is at a low level, another interface is formed between the air and the photoelectric switch. These two interfaces cause the intensity of reflected light received by the light-receiving crystal inside the photoelectric switch to differ, corresponding to two different switching states.
MH15 Series Photoelectric Liquid Level Switch
2. Non-contact liquid level measurement
Non-contact measurement typically uses waves that can be reflected by the measured medium. Utilizing the known wave propagation speed, the distance between the liquid surface and the measuring instrument is determined by directly or indirectly measuring the wave's propagation time, thus yielding the liquid level value. Based on the type of emitted wave, there are ultrasonic level gauges and electromagnetic radar level gauges.
Ultrasonic level gauge
An ultrasonic level gauge is a digital level instrument controlled by a microprocessor.
In this measurement, ultrasonic pulses are emitted by a sensor (transducer). The sound waves are reflected by the liquid surface and then received by the same sensor or an ultrasonic receiver. These pulses are converted into electrical signals by a piezoelectric crystal or magnetostrictive device. The distance from the sensor to the surface of the liquid being measured is calculated from the time between the emission and reception of the sound waves. Because this is a non-contact measurement method, the measured medium is virtually unlimited, making it widely applicable for measuring the height of various liquids and solids.
UP56 Ultrasonic Level Gauge
