Static pressure liquid level gauge is based on the principle that the measured liquid static pressure is proportional to the liquid height. Through the pressure sensor, the static pressure is converted into electrical signal, and then through temperature compensation and linear correction, it is converted into standard electrical signal to detect the liquid level. It is mainly divided into pressure type and differential pressure type liquid level sensors.

(1) Working principle of pressure type liquid level sensor

For open vessels, there are three kinds of pressure type liquid level meters for measuring liquid level height.

The pressure gauge type liquid level gauge is shown in figure (a). It uses the pressure pipe to introduce the pressure change value into the high sensitivity pressure gauge for measurement. The height of the pressure gauge is equal to that of the bottom of the container, and the reading in the pressure gauge directly reflects the height of the liquid level.

If the height of the pressure gauge is not as high as the bottom of the container, when the liquid level in the container is zero, the reading in the gauge is not zero, which is the pressure difference between the bottom of the container and the pressure gauge. This difference is called zero point migration. At this time, positive or negative migration should be carried out to eliminate the influence. The pressure gauge type liquid level gauge is widely used, but the medium should be clean and the viscosity should not be too large to avoid blocking the pressure pipe.

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The flange type liquid level transmitter is shown in figure (b). The transmitter is installed on the flange at the bottom of the vessel. The metal film and pressure pipe as the sensitive element are connected with the measurement room of the pressure transmitter. Silicon crystal with high boiling point and small expansion coefficient is sealed in the pressure pipe to isolate the measured medium from the measurement system.

Flange type liquid level transmitter converts liquid level signal into electrical signal or pneumatic signal for liquid level display or control adjustment. Because of the use of flange connection, and the medium does not have to flow through the pressure pipe. Therefore, it can detect corrosive, easy to crystallize, high viscosity or colored media.

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As shown in figure (c), a blowing pipe is inserted into the bottom of the measured liquid (zero liquid level), so that a certain amount of gas is injected into the blowing pipe. The pressure in the blowing pipe is equal to the static pressure of the liquid column at the pipe orifice. The liquid level can be measured by measuring the pressure at the upper end of the blowing pipe with a pressure gauge.

Because the pressure detection circuit of the blow type liquid level gauge is moved to the top, its use and maintenance are very convenient, and it is very suitable for underground storage, deep wells and other occasions.

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(2) Differential pressure liquid level sensor

When the height of liquid level is measured in a closed container, the pressure on the surface of the measured liquid is not necessarily the same as the atmospheric pressure, so the pressure type liquid level gauge can not be used. Because the liquid pressure in the lower part of the container is related to the liquid level height as well as the gas medium pressure on the liquid level. In this case, the method of measuring differential pressure can be used to measure the liquid level.

As shown in figure (a), it is the principle of differential pressure liquid level meter. This measurement method needs to eliminate the influence of liquid level fluctuation on the indication in the measurement process. The differential pressure liquid level meter adopts differential pressure transmitter, which is introduced into the positive pressure chamber of transformer by reflecting the liquid level height at the bottom of the container and the gas pressure on the liquid level; the gas pressure on the container is introduced into the negative pressure chamber of transmitter.

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In order to prevent the gas in the pressure pipe from condensing into liquid due to the temperature difference between the inside and outside, the low pressure pipe is usually filled with isolation liquid. The differential pressure measured by differential pressure transmitter is proportional to the liquid level.

Using this method, the differential pressure transmitter outputs the differential pressure and converts the pressure into electrical signal by other means. The liquid level can be detected by the output electrical signal. When the differential pressure transmitter is not parallel to the bottom of the vessel, as shown in figure (b), zero shift must be adopted [when the input is at the lower limit of the range, the change of output value caused by some influence quantity]. When the lower limit value is not zero, it is also called starting point migration (migration) to overcome the influence of fixed differential pressure.

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If the measured medium is corrosive, isolation tanks should be installed between the positive and negative pressure chambers of the differential pressure transmitter and the pressure tap to prevent the corrosive medium from directly contacting with the transmitter, as shown in figure (c).

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Editor in charge: CC

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