With the automation degree of complete sets of equipment in fine chemical industry becoming higher and higher, more and more devices are applying liquid level sensors. When using transmitters, the first step is to select the correct type, which not only makes our measurement more accurate, but also makes the service life of our liquid level sensors longer.

This paper briefly introduces the working principle of several common liquid level gauges.

1. Magnetic tilting plate level gauge

Magnetic tilting plate liquid level gauge: also known as maglev liquid level gauge, magnetic tilting column liquid level gauge.

Principle: principle of communicating vessels, according to the principle of buoyancy and magnetic coupling, when the measured liquid level rise or drop, the permanent magnets by magnetic coupling transmitting to a magnetic column indicator panel, turns red and white by column180 °.  when the liquid level rises, turning column changes from white to red;  when the liquid level drops, it changes from red to white,  interface of red and white on the panel is the actual liquid level inside the container, so as to realize the liquid level display.

2. Float ball level gauge

The structure of float ball level gauge is mainly designed and produced based on the principle of buoyancy and static magnetic field. The position of a float with a magnet (referred to as a float) in the medium under test is affected by buoyancy: changes in the liquid level lead to changes in the position of the magnetic float. The magnet in the float and the sensor (reed switch) act to change the number of components (such as constant resistance) that are connected to the circuit, thus changing the electrical quantity of the instrument circuit system. That is, a change in the position of the magnetic float causes a change in electrical quantities. The change of electrical quantity is detected to reflect the situation of liquid level in the container.

3. Radar level gauge

A radar level gauge is a measuring instrument based on the time-travel principle. Radar waves travel at the speed of light, and the time of operation can be converted into object level signals by electronic components. The probe sends a high-frequency pulse and propagates along the cable probe. When the pulse encounters the material surface, it is reflected back and received by the receiver in the instrument, and the distance signal is converted into the object level signal.

4. Magnetostrictive liquid level meter

When the sensor of the magnetostrictive level gauge works, the circuit part of the sensor will excite a pulse current on the waveguide wire, which will generate a pulse current magnetic field around the waveguide wire as it travels along the wire. A float is arranged outside the sensor measuring rod of the magnetostrictive liquid level meter, which can move up and down along the measuring rod as the liquid level changes. Inside the float there is a permanent set of magnetic rings. When the pulsed current magnetic field meets the magnetic ring field generated by the float, the magnetic field around the float changes so that the waveguide wire made of magnetostrictive material produces a torsional pulse at the float's position, which is transmitted back along the waveguide wire at a fixed speed and detected by the detector. By measuring the time difference between the pulse current and the torsion wave, the position of the float, that is, the liquid level, can be accurately determined.

5. Rf admittance level gauge

The rf admittance level gauge consists of a sensor and a control instrument. The sensor can be installed on the top of with probing type as rod, coaxial or cable. The pulse card in the sensor can convert the change of object level into the pulse signal and send it to the control instrument, which can be converted into the quantity display after the operation of the control instrument, thus realizing the continuous measurement of object level.

6. Tuning fork level meter

The principle of the tuning fork object level controller is to make the tuning fork vibrate at a certain resonant frequency through a pair of piezoelectric crystals mounted on the base of the tuning fork. When the fork is in contact with the medium under test, the frequency and amplitude of the fork will change. These changes are

7. Glass plate level gauge (glass tube level gauge)

The glass plate level gauge is connected with the container through the flange to form a connector. The height of the liquid level in the container can be read directly through the glass plate.

8. Pressure level transmitter

Pressure type level gauge on the principle of static pressure measurement. When the liquid level transmitter into the liquid being measured a certain depth, the sensor toward the liquid under pressure at the same time, through the gas leading stainless steel to introduce to positive pressure cavity of the liquid pressure sensor , and the atmospheric pressure on the surface of the liquid Po and sensors connected to the negative pressure cavity, to offset the Po on the reverse of the sensor, the sensor measured pressure is ρ.g.H, through the measurement of pressure P, you can get the depth of the liquid level.

Capacitive level gauge measures the level of liquid by measuring the change in capacitance. It is a metal rod inserted into the container, the metal rod as one pole of the capacitor, the container wall as the other pole of the capacitor.

The medium between the two electrodes is liquid and the gas above. Since the permittivity ε1 of the liquid is different from the permittivity ε2 of the liquid surface, for example, ε1> ε2, the total permittivity value between the two electrodes of the capacitive liquid level gauge increases when the liquid level rises, so the capacitance increases. On the contrary, when the liquid level drops, the value decreases and the capacitance also decreases. Therefore, capacitive level gauge can measure the level of liquid by the change of capacitance between two electrodes.

10. Hydrostatic level gauge

Static pressure type liquid level transmitter encapsulates the diffused silicon oil-filled core body in the stainless steel shell. The front protective cap protects the sensor diaphragm and enables the liquid to contact the diaphragm smoothly. The waterproof wire is connected to the shell sealed, the snorkelling tube is connected to the outside in the cable, and the internal structure is designed to prevent condensation.