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Liquid level sensor classification and working principle

source:HEYYEN Electronics (Shanghai) Co., Ltd.time:2019-08-28views:Loading...

  The level sensor (static pressure level gauge / level transmitter / level sensor / water level sensor) is a pressure sensor for measuring the liquid level. The static pressure input type liquid level sensor (level gauge) is based on the principle that the measured hydrostatic pressure is proportional to the height of the liquid, and the static pressure conversion is performed by using an advanced foreign type of diffused silicon sensitive element or ceramic capacitor pressure sensitive sensor. The electrical signal is converted into a standard electrical signal (typically 4-20 mA / 1 ~ 5 VDC) after temperature compensation and linear correction.

  1. Liquid level sensor classification

  Divided into two categories:

  The first type is contact type, including single flange static pressure / double flange differential pressure level sensor, float ball level sensor, magnetic level sensor, input level sensor, electric inner float level sensor, electric Float level sensor, capacitive level sensor, magnetostrictive level sensor, servo level sensor, etc.

  The second type is non-contact type, which is divided into ultrasonic liquid level sensor and radar liquid level sensor.

  The static pressure input type liquid level sensor (level gauge) is suitable for liquid level measurement of various media in petroleum, metallurgy, electric power, pharmaceutical, water supply and drainage, environmental protection and other systems and industries. The compact structure, simple adjustment and flexible installation make it easy for users to use. Standard signal output methods such as 4 to 20 mA, 0 to 5 v, and 0 to 10 mA are optional by the user as needed. The use of hydrostatic principles to measure liquid level is an important application of pressure sensors. The special intermediate cable with ventilation duct and special sealing technology not only ensure the watertightness of the sensor, but also make the reference pressure chamber communicate with the ambient pressure, thus ensuring high precision and high stability of the measurement.

  2. Comparison of different liquid level sensors

  1. Float type liquid level sensor The float type liquid level transmitter changes the magnetic float ball to the float tube. The liquid level sensor is designed according to the Archimedes buoyancy principle. The float type liquid level transmitter uses a small metal film strain sensing technology to measure the liquid level, boundary or density of the liquid. It can be used for routine setting operations by field buttons during operation.

  2. Float type liquid level sensor Float type liquid level transmitter consists of magnetic float ball, measuring tube, signal unit, electronic unit, junction box and mounting parts. Generally, the magnetic float ball has a specific gravity of less than 0.5 and can float in liquid. Above the surface and moving up and down along the measuring tube, the measuring tube is equipped with a measuring component, which can convert the measured liquid level signal into a resistance signal proportional to the liquid level change under the action of external magnetic field, and convert the electronic unit into 4-20 mA or other Standard signal output. The liquid level sensor is a module circuit, which has the advantages of acid resistance, moisture resistance, shock resistance, corrosion resistance, etc. The circuit contains a constant current feedback circuit and an internal protection circuit, so that the maximum output current does not exceed 28 mA, thus reliably protecting the power supply and making the power The meter is not damaged.

  3. Static pressure liquid level sensor The transmitter uses the measurement principle of liquid static pressure. It generally uses a silicon pressure pressure sensor to convert the measured pressure into an electrical signal, and then the amplifier circuit amplifies and compensates the circuit for compensation. Output in 4 to 20 mA or 0 to 10 mA current.

  3. The working principle of the liquid level sensor

  The principle of static pressure measurement: When the liquid level sensor is put into a certain depth in the liquid to be tested, the pressure formula of the liquid surface of the sensor is: Ρ = ρ .gH + Po in the formula: P: the surface of the transmitter Pressure ρ: measured liquid density g: local gravity acceleration Po: liquid level atmospheric pressure H: the depth of the transmitter into the liquid at the same time, the pressure of the liquid is introduced into the positive pressure chamber of the sensor through the gas-conducting stainless steel, and then the liquid level The upper atmospheric pressure Po is connected to the negative pressure chamber of the sensor to offset the Po on the back of the sensor, so that the measured pressure of the sensor is: ρ .gH. Obviously, by measuring the pressure P, the liquid level depth can be obtained.


  ◆Good stability, long-term stability of fullness and zero can reach 0.1%FS/year. In the compensation temperature range of 0 to 70 °C, the temperature drift is less than 0.1% FS and is less than 0.3% FS over the entire allowable operating temperature range.

  ◆With reverse protection and current limiting protection circuit, the positive and negative poles will not damage the transmitter during installation, and the abnormal time transmitter will automatically limit the flow within 35MA.

  ◆ Solid structure, no moving parts, high reliability and long service life.

  ◆ Easy installation, simple structure, economical and durable.

  4. The difference between the liquid level switch liquid level sensor

  The liquid level switch is a controller that keeps the water level constant according to the signal output of the liquid level sensor to open the valve for discharging water or water. It can also be said that the liquid level switch outputs a kind of switching signal. The liquid level switch first determines the height of the liquid level, and outputs the switching quantity signal according to the height. The liquid level sensor outputs a form in which the height of the liquid level is converted into an electrical signal. We can process the electrical signal, such as the plc, data collector or professional display to output the level of the liquid level. There is also the same principle of the liquid level switch and the liquid level sensor. However, the liquid level switch is a switch control circuit, and the liquid level sensor is a circuit element equivalent to a transformer and a converter.

  5. Photoelectric level sensor

  The photoelectric liquid level sensor is a new contact point liquid level measurement and control device developed by utilizing the principle of light reflection and refraction at the interface of two different media. The utility model has the advantages of simple structure, high positioning precision, no mechanical parts, no need for debugging, high sensitivity and corrosion resistance, low power consumption, small volume and the like, and is gradually recognized by the market.

  1. Since the output of the liquid level is only related to whether the photoelectric probe is in contact with the liquid surface, and is independent of other characteristics of the medium, such as temperature, pressure, density, electricity, etc., the photoelectric liquid level sensor has accurate detection and high repeatability; the response speed is fast. The liquid level control is very precise and can be installed directly without adjustment.

  2. Since the photoelectric liquid level sensor probe is relatively small in size, it can be installed separately in a small space for use in a special tank or container. In addition, a plurality of photoelectric probes can be mounted on one measuring body to form a multi-point liquid level sensor and a variable controller.

  3. Since all the components inside the sensor are resin-sealed, there is no mechanical moving parts inside the sensor, so the photoelectric liquid level sensor has high reliability, long life and maintenance-free.

  6. Current status of liquid level sensor industry

  The sensor industry is a technology-intensive industry. Sensors are characterized by technological diversity, marginality, comprehensiveness and technicality in the R&D and manufacturing process. They are a collection of high-tech technologies. At the same time, the sensor industry is also an investment-intensive industry, which is determined by its technology-intensive nature, and requires large investments in engineering and large-scale production processes. It is the above-mentioned industrial characteristics of the sensor that makes Xiqiao countries have great advantages in developing the sensor industry. Still in the current sensor market, the world's top ten sensor manufacturers are being taken over by foreign companies. At present, the major foreign sensor companies include Honeywell, STMicroelectronics and Freescale. Domestically, due to the huge prospects of sensors, China's sensor companies are also increasing, mainly including Huagong Technology, Suzhou Gushu, and Da Li Technology. In terms of sensor technology, Chinese companies have basically mastered the technology of low-end and mid-range sensor development, and are gradually expanding into high-end fields. The gap with foreign countries has provided space for our growth, and also pointed out the direction. Moreover, China has achieved rapid development in electronic information technology in recent years, so it is stimulated by huge market demand and in many local enterprises. With the participation, Chinese sensor companies are expected to achieve impressive results.

  With the advancement of technologies such as electronic technology, material technology, physical technology, and chemical technology, liquid level sensors are still developing in the direction of new types. The new type of liquid level sensor is mainly characterized by miniaturization, intelligence, digitization, multi-function, systemization and networking. These characteristics of the new liquid level sensor are the result of the evolution of the sensing technology and the requirements of the new era of intelligent sensing technology.

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