What Is Liquid Level Sensor?
A Liquid Level Sensor is a measuring device used to determine the height of the liquid level in a tank or container, helping to assess the remaining amount of liquid. Some sensors simply detect the presence or absence of liquid, while others can calculate the percentage of liquid remaining by continuously measuring the level. They are also sometimes referred to as level sensors. Some liquid sensors can be diverted to particulate solids such as sand.
Uses of Liquid Level Sensors
In the industrial field, materials or cleaning solvents are often stored in sealed, opaque containers. Liquid Level Sensors are often used to monitor tank volume in large facilities, including oil refining plants, water treatment plants, and beverage and food manufacturing plants, as well as in pulp and paper production sites. They are also used not only to monitor residual volume but also to check the deterioration and quality of liquids.
Principle of Liquid Level Sensor
There are various types of Liquid Level Sensors, each operating on a different principle. The following four types are representative:
1. Float Type
A tube with a built-in wire is attached to the top and bottom of the tank, and a magnetized float, which rises and falls in tandem with the liquid level along the tube, floats on the surface of the liquid. The liquid level is measured based on the distance between the float and the wire inside the tube.
2. Ultrasonic Type
Ultrasonic waves are transmitted to the Liquid Level Sensor, and the distance to the liquid level is determined by measuring the time it takes for the waves to return.
3. Capacitance Type
The sensor is placed a short distance away from the tank wall and monitors the capacitance between the wall and the sensor. If there is a liquid between the sensor and the wall, the capacitance will be large; if the wall is empty, the capacitance will be small.
Here, it is assumed that the tank walls are made of metal. If the liquid is nonconductive, the contribution of the liquid-derived capacitive component increases with the height of the liquid surface.
In general, since the relative dielectric constant of a nonconductive liquid is greater than that of air, the capacitance value monitored will be larger at higher liquid surface heights and smaller at lower liquid surface heights. If this relationship is obtained in advance as a known function, the height of the Liquid Level Sensor can be determined from the actual measured values of the Sensor output.
On the other hand, if the liquid is conductive, there is no liquid-derived capacitance component, so the Liquid Level Sensor electrode can be covered with an insulator (dielectric with a constant relative permittivity) and configured to effectively detect fluctuations in the air-derived capacitance component based on changes in the liquid surface height to determine the liquid surface height in a similar manner.
4. Optical Type
The optical system comprises an infrared LED and a light-receiving part. It’s designed such that infrared light reaches the light-receiving part when there is no liquid. Immersion in liquid prevents the light from reaching the light-receiving section due to refraction, allowing the detection of a rise in liquid level.
Non-contact Liquid Level Sensor
Non-contact Liquid Level Sensors are known as ultrasonic, radio wave, laser, radiation, gravimetric, and direct-viewing types.
The ultrasonic type measures the time it takes for ultrasonic waves to be reflected back from the liquid surface. While this type of measurement is independent of the type of liquid, it is vulnerable to condensation and obstacles.
The radio wave type measures the time it takes for electromagnetic waves to be reflected back from the liquid surface. While it is characterized by high environmental resistance, it is expensive and has the disadvantage of being large and heavy.
The laser type uses a semiconductor laser to measure the reflection of light from the liquid surface. The spot diameter is small, so it is not affected by obstacles in the tank. On the other hand, it is expensive and requires safety control of the laser.
The radiation type uses the transmission and absorption of gamma rays to make measurements. While it can be used in toxic, hot, and corrosive environments, it also poses a potential risk to human health and requires safety control.
The gravimetric method measures the liquid level by weighing the entire tank. While it has the advantage of not being affected by the inside of the tank, it is vulnerable to changes in the specific gravity of the liquid, such as bubbles.
The direct visual method is to check the level visually. It is the least expensive method but has the disadvantages of requiring cleaning when dirty and being difficult to automate.
Electrode Type Liquid Level Sensor
The electrode-type Liquid Level Sensor (Level Switch) is an electrical level sensor with no moving parts. It is widely used for general-purpose liquid level control in various industries such as steel, food, chemical, pharmaceutical, and semiconductor industries, as well as in agricultural water, water treatment plants, and sewage treatment.
When measuring, an alternating voltage is applied between the ground electrode and the detection electrode. If the electrodes are not in contact with the liquid, no current flows, but if the electrodes are in contact with the liquid, a current flows. Using this principle, only conductive liquids can be detected.
The device consists of an electrode holder, process connection, electrode rod, and relay unit. Other than the relay unit, there are no electronic components and no moving parts, making it a simple configuration.