According to the structural principle of flowmeter, according to the actual situation of flowmeter products at present, according to the structural principle of flowmeter, it can be roughly summarized as follows: 1. The positive displacement flowmeter is equivalent to a container with standard volume, which continuously measures the flowing medium. The greater the flow rate, the more measurement times and the higher the output frequency. The principle of positive displacement flowmeter is relatively simple, and it is suitable for measuring fluids with high viscosity and low Reynolds number. According to the different shapes of rotating bodies, the products currently produced are divided into: elliptical gear flowmeter, waist wheel flowmeter (Roots flowmeter), rotary piston flowmeter and scraper flowmeter, which are suitable for measuring liquid flow; Servo displacement flowmeter, diaphragm flowmeter and simple flowmeter are suitable for measuring gas flow. 2. The working principle of the impeller flowmeter is that the impeller is placed in the measured fluid and rotated by the impact of the fluid flow, and the flow rate is reflected by the rotating speed of the impeller. Typical impeller flowmeters include water meters and turbine flowmeters, and their structures can be mechanical transmission output or electric pulse output. Generally, the accuracy of water meter output by mechanical transmission is low, with an error of about 2%, but it has simple structure and low cost. It has been mass-produced, standardized, generalized and serialized in China. The accuracy of turbine flowmeter with electric pulse signal output is high, and the general error is 0.2%-0.5%. 3. Differential pressure flowmeter (pressure drop flowmeter) The differential pressure flowmeter consists of a primary device and a secondary device. The primary equipment is called a flow measuring element, which is installed in the pipeline of the measured fluid and produces a pressure difference proportional to the flow rate (flow rate) for the secondary equipment to display the flow rate. Secondary equipment is called display instrument. It receives the differential pressure signal generated by the measuring element and converts it into the corresponding flow for display. The main device of differential pressure flowmeter is usually throttling device or dynamic pressure measuring device (pitot tube, pressure equalizing tube, etc.). ). The secondary equipment is all kinds of mechanical, electronic and combined differential pressure meters with flow display instruments. The differential pressure sensitive elements of differential pressure gauge are mostly elastic elements. Because of the square root relationship between pressure difference and flow, the flow display instrument is equipped with a square root device to linearize the flow scale. Most meters are also equipped with a flow totalizer to display the accumulated flow of economic accounting. This method of measuring flow by pressure difference has a long history and is relatively mature. It is generally used in more important occasions around the world, accounting for about 70% of all kinds of flow measurement methods. This instrument is used to measure the flow of main steam, feed water and condensate in power plant.