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(1) Measurement objects , such as the physical and chemical properties of the measured medium, working pressure and temperature, installation conditions, speed of liquid level change, etc.;
(2) Measurement and control requirements , such as measurement range, measurement (or control) accuracy, display mode, on-site instructions, remote indication, interface with the computer, safety and corrosion protection, reliability, and construction convenience.
The main points of level gauges and selections commonly used in water supply engineering are as follows:
a. Float level gauge
Put a hollow ball in the liquid. When the liquid level changes, the float will produce the same displacement as the liquid level change. The displacement of the float ball can be measured mechanically or electrically. The accuracy is Â± (1~2)%. This level gauge is not suitable for high-viscosity liquids, and its output has switch control and continuous output.
In the design of a water purification plant, such level gauges are often used to measure the level of a collecting well to control the automatic start and stop of the drainage pump.
b. Static pressure (or differential pressure) level gauge
Since the static pressure of the liquid column is proportional to the liquid level, the liquid level can be measured by using a pressure gauge to measure the static pressure of the liquid column on the reference surface. Calculate the pressure or pressure difference range according to the density of the measured medium and the measurement range of the liquid, and then select the suitable pressure gauge or differential pressure gauge for range and accuracy. This level gauge has an accuracy of Â± (0.5 to 2)%.
c. RF capacitance level gauge
Insert the electrode in the container, when the liquid level changes, the medium inside the electrode changes, the capacitance between the electrodes (or between the electrode and the container wall) also changes, and the change of the capacitance is converted into a standardized DC signal. Its accuracy is Â±(0.5~1.5)%.
Capacitance type liquid level meter has the following advantages: the sensor has no mechanically movable part, and the structure is simple and reliable; the precision is high; the detection end consumes small electric energy, and the dynamic response is fast; the maintenance is convenient and the service life is long. The disadvantage is that the dielectric constant of the fluid being measured can cause errors. Capacitive level gauges are generally used to adjust the level measurement of tanks, clear water tanks, etc.
When the measuring range does not exceed 2m, rod-shaped, plate-shaped, coaxial electrodes are used; when it exceeds 2m, cable-type electrodes are used. When the measured medium is water, an electrode with an insulating layer (polyethylene available) is used.
d. Ultrasonic level gauge
The ultrasonic level gauge sensor consists of a pair of transmitter and receiver transducers. The transmitting transducer faces the liquid surface to emit ultrasonic pulses, and the ultrasonic pulses are reflected back from the liquid surface and received by the receiving transducer. According to the time from launch to reception, the distance between the sensor and the liquid level can be determined, which can be converted into liquid level. Its accuracy is Â±0.5%.
This level gauge has no mechanical movable part, high reliability, simple and convenient installation, is non-contact measurement, and is not affected by the viscosity, density, etc. of the liquid, so it is often used in drug pools, medicine tanks, sludge tanks, etc. Level measurement. However, this method has a certain blind area, and the price is more expensive.
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