Commonly used in the measurement of liquid level gauge with a communication type, bubble-blowing, differential pressure, capacitance, etc., the level measurement of ultrasonic level meter and radioactive level meter. The principle and characteristics are as follows: a. Communicator is the most common glass level gauge, which is characterized by simple structure, low cost, intuitive, suitable for on-site use, but easy to damage, contamination of the inner surface, resulting in reading difficulties , Inconvenient to remote and adjust. b. Buoyancy level gauge including constant buoyancy and variable buoyancy two types. (1) Constant buoyancy level gauge Constant buoyancy level gauge depends on buoy or float floating in the liquid with the liquid level changes and movements, it is characterized by simple structure, low price, suitable for a variety of tank measurement ; (2) variable buoyancy level variable buoyancy also known as Shen cylinder type liquid level gauge, when the liquid level is different, immersed in the liquid in the cylinder volume is different, and thus subject to buoyancy and displacement, through the mechanical transmission Convert to angular displacement to measure level. Such instruments can achieve remote and automatic adjustment. c. Bubble level gauge is the application of static pressure principle open container level measurement. Compressed air through the filter valve, and then by a certain value of the device output pressure, the throttle components after two: (1) all the way into the container installed in the catheter, blown out from the bottom of the container; (2) the other All the way into the pressure gauge instructions. When the liquid level is lowest, the bubbles are blown out without resistance, the back pressure is zero, and the pressure gauge means zero. When the liquid level is increased, the bubble is blown out to overcome the static pressure of the liquid column, the back pressure is increased and the pressure indication is increased. Therefore, the back pressure that the pressure gauge indicates the size of the pressure, it reflects the level of the liquid. Bubble level gauge simple, inexpensive, suitable for measuring corrosive, viscous and open container containing suspended particles level, but the accuracy is low. d. differential pressure gauge with gas and liquid two pressure port. Gas pressure at the point of pressure for the gas pressure within the device; liquid pressure at the point of pressure in addition to the role of gas pressure, but also by the hydrostatic liquid column pressure, the difference between liquid and gas phase pressure is generated by the liquid column static pressure. Therefore, the liquid level is measured by the following formula: