The fluid filled with pipeline, when they flow through the throttling device in the pipeline, the flow beam will be in the throttling device at the throttling part of the local contraction, so that the flow rate increases, static pressure is low, so before and after the throttling part will produce a pressure drop, that is, the pressure difference, the greater the flow of the medium flow, the greater the pressure difference before and after the throttling part. So orifice flowmeter can measure the fluid flow by measuring the pressure difference.

In order to ensure that the flow of the fluid forms a fully developed turbulent velocity distribution 1D in front of the throttling part, and make the distribution into a uniform axissymmetric shape, the straight pipe section must be round, and the roundness of the first 2D range of the throttling part is very strict, and there are certain roundness indicators.

If the upstream side of the throttling part is an open space or a large container with diameter ≥2D, the length of the straight pipe between the open space or large container and the throttling part shall not be less than 30D(15D). If there are other local resistance elements between the throttling element and the open space or large container, the total length of the straight pipe section from the open space to the throttling element shall not be less than 30D(15D), in addition to the minimum straight pipe section length specified between the throttling element and the local resistance element.