Throttle flowmeter usually consists of throttle device which converts fluid flow into differential pressure signal and differential pressure meter which measures differential pressure and displays flow. The throttling device installed in the circulating pipeline is also called "primary device", which includes throttling element, pressure taking device and front and rear straight pipe sections. The display device, also known as the "secondary device", includes instruments required for differential pressure signal pipeline measurement.

Many countries have done a lot of research work on throttling devices. . AGA (American Gas Association) and ASME (American Society of Mechanical Engineers) have been experimenting with throttling devices since the beginning of this century. The research results were published in 1969 and 197 1 respectively. . DIN (German Industrial Standard) has stipulated the throttling device. On the basis of summarizing the research results of various countries, the International Organization for Standardization (ISO) published ISO/R54 1 and ISO/R78 1 in 1968 respectively as the international standards of throttling devices, namely ISO 5 167. Suitable for orifice plates, nozzles and venturis .48666.666666666666 China has also issued the national standard GB 2624 for flow measurement throttling device in 198 1, which specifies the standard orifice plate and the standard nozzles for angle pressure and flange pressure in detail.

When using the standard throttling device, the nature and state of the fluid must meet the following conditions:

(1) fluid must be filled with pipeline and throttling device, and continue to flow through the pipeline.

② The fluid must be a Newtonian fluid, that is, it is homogeneous and single-phase in physics and thermodynamics, or it can be considered as a single-phase fluid, including mixed gas, solution and colloid with dispersed particles less than 0. 1 m, and the uniformly dispersed solid particles in the gas do not exceed 2% (mass), or the uniformly dispersed bubbles in the liquid do not exceed 5% (volume), or it can be considered as a single-phase fluid. however

③ The fluid does not intersect when it flows through the throttle valve.

④ The fluid flow rate does not change with time or changes slowly.

⑤ Before the fluid flows through the throttle valve, the flow is parallel to the axis of the pipeline and there is no vortex.

Standard throttling device is not suitable for flow measurement of dynamic flow and critical flow.

Although the accuracy is poor, it is still the most widely used flowmeter because of its simple structure and convenient manufacture.