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Positive displacement flowmeters use mechanical measurement files to continuously divide the liquid into a single known volume portion, and repeatedly fill and discharge the volume portion and the fluid according to the number of times the metering chamber does not measure the total volume of the liquid.

It is the class with the highest accuracy in the flow meter. The advantages of positive displacement flowmeter are as follows: high degree, the basic error is generally ±0.5%, and the special one can reach ±0.2%; there is no requirement for the straight pipe section in front, which is of great significance in field use; it can be used in high viscosity The measurement of fluids has a wide range, generally 10:1 to 5:1. It is a layer direct-reading instrument that requires no external energy and is easy to operate.

Any instrument has its advantages as well as its limitations. The disadvantages of positive displacement flowmeters are mainly manifested in: complex structure, large volume and heavy weight, so they are generally only suitable for small and medium calibers; Large, narrow range of application; poor safety, such as the detection of moving parts stuck, the fluid cannot pass; some forms of positive displacement flowmeters will bring pulsation to the flow during the measurement process. Due to the high precision, when metering expensive media in industrial sectors such as petrochemical, pharmaceutical, food and energy. However, due to the need for regular maintenance, it is not suitable for maintenance sites such as radioactive toxic fluids that do not allow access to people. At present, the positive displacement flowmeter is used as a trade settlement storage and transportation handover instrument.

(3) Float flowmeter

The float flow meter is a volume flow meter in which the float rises and falls with the flow change in the vertical conical tube, and the flow area between them is changed to measure. Also called rotameter.

Rotameters are suitable for small pipe diameters and low flow rates. Commonly used instruments have a diameter of 40-50mm or less, and the smallest diameter is 1.5-4mm. Float flowmeters do not have high requirements for straight pipe sections and have a wide flow range. At present, it is widely used in electric power, petrochemical, metallurgy and other process industries and public utilities such as sewage treatment. It is mainly used as a visual flow indication or a field indicating instrument that does not require high measurement accuracy.

Except for these there types, there are also many other flowmeters like electromagnetic flowmeter, vortex flowmeter, turbine flowmeter, ultrasonic flow meter, orifice flowmeter, metal tube rotameter, etc. And various of device is used for measuring as well, for example there are pressure transmitters and differential pressure transmitters, ultrasonic level gauge and radar level gauge and flow totalizer.