A liquid level sensor ( static pressure level gauge/level transmitter/level sensor/water level sensor) is a pressure sensor that measures liquid level. The static pressure submersible level sensor (level gauge) is based on the principle that the static pressure of the measured liquid is proportional to the height of the liquid. It uses advanced foreign isolated diffused silicon sensitive elements or ceramic capacitive pressure sensitive sensors to convert static pressure into an electrical signal. After temperature compensation and linear correction, it is converted into a standard electrical signal (typically 4–20mA/1–5VDC).
Classification of liquid level sensors
Divided into two categories:
The first category is contact type, including single-flange static pressure/double-flange differential pressure level sensors, float level sensors, magnetic level sensors, submersible level sensors, electric internal float level sensors, electric float level sensors, capacitive level sensors, magnetostrictive level sensors, servo level sensors, etc.
The second category is non-contact type, which includes ultrasonic level sensors, radar level sensors, etc.
The hydrostatic submersible level sensor (level gauge) is suitable for measuring the level of various media in systems and industries such as petrochemicals, metallurgy, power, pharmaceuticals, water supply and drainage, and environmental protection. Its compact structure, simple calibration, and flexible installation methods provide users with easy operation. Standard signal output options such as 4-20mA, 0-5V, and 0-10mA can be selected by the user according to their needs. Measuring liquid level using the principle of hydrostatics is an important application of pressure sensors. The use of a special cable with an intermediate vent and specialized sealing technology ensures both the watertightness of the sensor and communication between the reference pressure chamber and the ambient pressure, thus guaranteeing high accuracy and stability in the measurement.
Comparison of different liquid level sensors
1. Float-type liquid level sensor: The float-type liquid level transmitter replaces the magnetic float with a float. The liquid level sensor is designed based on Archimedes' principle of buoyancy. The float-type liquid level transmitter uses tiny metal film strain sensing technology to measure the liquid level, interface level, or density. During operation, routine settings can be performed via on-site buttons.
2. Float-type liquid level sensor: The float-type liquid level transmitter consists of a magnetic float, measuring guide tube, signal unit, electronic unit, junction box, and mounting components. Generally, the specific gravity of the magnetic float is less than 0.5 , allowing it to float on the liquid surface and move up and down along the measuring guide tube. The measuring element is installed inside the guide tube. Under the action of an external magnet, it converts the measured liquid level signal into a resistance signal proportional to the liquid level change, and the electronic unit converts this into a 4-20mA or other standard signal output. The liquid level sensor is a modular circuit, possessing advantages such as acid resistance, moisture resistance, shock resistance, and corrosion resistance. The internal circuitry includes a constant current feedback circuit and an internal protection circuit, ensuring that the maximum output current does not exceed 28mA, thus reliably protecting the power supply and preventing damage to secondary instruments.
3. Static pressure level sensor: This transmitter works on the principle of measuring the static pressure of liquid. It generally uses a silicon pressure sensor to convert the measured pressure into an electrical signal, which is then amplified by an amplifier circuit and compensated by a compensation circuit, and finally output as a current of 4-20mA or 0-10mA.
Working principle of liquid level sensor
The hydrostatic measurement principle applies: When the level sensor is immersed to a certain depth in the liquid being measured, the pressure on the sensor's liquid-facing surface is calculated as follows: P = ρ * g * H + Po Where: P: Pressure on the transmitter's liquid-facing surface; ρ: Density of the liquid being measured; g: Local gravitational acceleration; Po: Atmospheric pressure above the liquid surface; H: Depth of the transmitter immersed in the liquid. Simultaneously, the liquid pressure is introduced into the sensor's positive pressure chamber via a stainless steel guide tube. The atmospheric pressure Po above the liquid surface is then connected to the sensor's negative pressure chamber to counteract the pressure Po at the back of the sensor, resulting in a pressure of ρ * g * H. Clearly, the liquid level depth can be obtained by measuring the pressure P. Features: ◆ Good stability; long-term stability at full scale and zero point can reach 0.1 %FS/year. Temperature drift is less than 0.1 %FS within the compensation temperature range of 0～70℃, and less than 0.3 %FS across the entire allowable operating temperature range.
◆ It has reverse protection and current limiting protection circuits. Reversing the positive and negative terminals during installation will not damage the transmitter. In case of abnormality, the transmitter will automatically limit the current to within 35mA.
◆Solid-state structure, no moving parts, high reliability, and long service life.
◆ Easy to install, simple structure, economical and durable.
Difference between liquid level switch and liquid level sensor
A level switch is a controller that opens a valve to release or release water, maintaining a constant water level, based on the signal output of a level sensor. In other words, a level switch outputs a switching signal; it first determines the liquid level and then outputs a switching signal accordingly. A level sensor, on the other hand, converts the liquid level into an electrical signal for output. This electrical signal can be processed, for example, and connected to a PLC, data acquisition unit, or professional display to output the liquid level. Although the principles of level switches and level sensors are similar, a level switch is a switching control circuit, while a level sensor is essentially a circuit component used for voltage and current conversion.
Photoelectric liquid level sensor
Photoelectric level sensors are a new type of contact-type point level measurement and control device developed based on the principle of light reflection and refraction at the interface of two different media. They have gained increasing market acceptance due to their numerous advantages, including simple structure, high positioning accuracy, no mechanical parts, no need for adjustment, high sensitivity and corrosion resistance, low power consumption, and small size. 1. Because the level output depends only on 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, and electrical parameters, photoelectric level sensors offer accurate detection, high repeatability, fast response, and very precise liquid level control. Furthermore, they require no calibration and can be directly installed and used.
2. Due to the relatively small size of the photoelectric level sensor probe, it can be installed separately in confined spaces, making it suitable for use in special tanks or containers. Furthermore, multiple photoelectric probes can be mounted on a single measuring body to create a multi-point level sensor or controller.
3. Because all components inside the sensor are encapsulated with resin , there are no moving mechanical parts inside the sensor . Therefore, the photoelectric liquid level sensor has high reliability, long life and is maintenance-free.
Current Status of the Liquid Level Sensor Industry
The sensor industry is a technology-intensive industry. Sensors exhibit significant technological diversity, marginality, comprehensiveness, and technical sophistication in their research and manufacturing, representing a fusion of multiple high technologies. Simultaneously, the sensor industry is also investment-intensive, a characteristic determined by its technology-intensive nature, requiring substantial investment in engineering and large-scale production. These characteristics give Western countries a significant advantage in developing their sensor industries. Currently, the top ten global sensor manufacturers are all foreign companies, including Honeywell, STMicroelectronics, and Freescale. Domestically, influenced by the enormous potential of the sensor market, the number of Chinese sensor companies is steadily increasing, including companies like HGTECH, Suzhou Goodix, and Dali Technology. In terms of sensor technology, Chinese companies have largely mastered the R&D of low-to-mid-range sensors and are gradually expanding into high-end fields. The gap with foreign companies provides space for growth and points the way forward. Furthermore, China has achieved rapid development in electronic information technology in recent years. Therefore, stimulated by huge market demand and with the participation of numerous domestic enterprises, Chinese sensor companies are expected to achieve remarkable results.
With advancements in electronic, materials, physical, and chemical technologies, liquid level sensors are evolving from traditional to novel designs. These novelties are primarily characterized by miniaturization, intelligence, digitalization, multifunctionality, systematization, and networking. These features of new liquid level sensors are a result of the evolution of sensing technology and also reflect the requirements of intelligent sensing technology in the new era.
