Classification of liquid level switches and precautions

Liquid level switch is a switch used to control liquid level, which is mainly divided into contact type and non-contact type in form. The common non-contact switches are capacitive liquid level switches, and the contact float level switches are widely used. Liquid level switches are often used in purification projects. As computer automatic measurement and control management systems are widely used in the automation field of water purification plants, liquid level switches, as a part of automatic measurement and control systems, also play an important role. The automatic instrument can continuously detect various process parameters, and perform manual or automatic control according to the data of these parameters, so as to coordinate the relationship between supply and demand, various components of the system, and various water treatment processes, so that various equipment and facilities can be fully and reasonably used. In modern water purification plants, liquid level switches of automatic instruments are commonly used instruments. What problems should be paid attention to when selecting them? The following are two reference standards:
(1) Measuring objects, such as the physical and chemical properties of the measured medium, working pressure and temperature, installation conditions, liquid level change speed, etc;
(2) Measurement and control requirements, such as measurement range, measurement (or control) degree, display mode, on-site indication, remote indication, interface with computer, safety corrosion prevention, reliability and construction convenience.
The commonly used liquid level meters in water purification projects and the key points of their selection are as follows:

Float level switch
Put a hollow floating ball in the liquid. When the liquid level changes, the floating ball will have the same displacement as the liquid level change. The displacement of the floating ball can be measured mechanically or electrically, and its degree is ± (1~2)%. This liquid level switch is not applicable to high viscosity liquid, and its output end has switch control and continuous output.
In the design of water purification plant, this type of liquid level switch is often used for the liquid level measurement of the sump to control the automatic start and stop of the drainage pump. At present, the foam float type liquid level switch is widely used. It is popular because of its simple installation, low price and long service life.
Static pressure (or differential pressure) level switch
Since the static pressure of the liquid column is proportional to the liquid level, the liquid level can be measured by measuring the static pressure of the liquid column on the reference plane with a pressure gauge. Calculate the pressure or differential pressure range according to the density of the measured medium and the liquid measurement range, and then select the pressure gauge or differential pressure gauge with appropriate range, degree and other performance. The degree of this level switch is ± (0.5~2)%

Capacitive liquid level switch
Insert an electrode into the container. When the liquid level changes, the medium inside the electrode changes, and the capacitance between electrodes (or between the electrode and the container wall) also changes. The capacitance change is converted into a standardized DC electrical signal. Its degree is ± (0.5~1.5)%
Capacitive liquid level switch has the following advantages: no mechanical movable part of the sensor, simple and reliable structure; High degree; The detection terminal consumes less power and has fast dynamic response; Easy maintenance and long service life. The disadvantage is that the instability of the dielectric constant of the measured liquid will cause errors. Capacitive liquid level switch is generally used for liquid level measurement of regulating tank, clean water tank, etc.
When the measuring range does not exceed 2m, rod, plate and coaxial electrodes shall be used; When it exceeds 2m, cable electrode shall be used. When the measured medium is water, electrodes with insulating layer (polyethylene can be used) shall be used.

Ultrasonic level switch
The sensor of ultrasonic level gauge is composed of a pair of transmitting and receiving transducers. The transmitting transducer sends ultrasonic pulse to the liquid surface, and the ultrasonic pulse is reflected back from the liquid surface and received by the receiving transducer. The distance between the sensor and the liquid level can be determined according to the time from transmission to reception, which can be converted into liquid level. Its degree is ± 0.5%
The liquid level switch has no mechanical movable part, high reliability, simple and convenient installation, belongs to non-contact measurement, and is not affected by liquid viscosity, density, etc., so it is mostly used for liquid level measurement of medicine tank, medicine tank, sludge drainage tank, etc. However, this method has some blind spots and is expensive.


Post time: 29-11-22