PD meter is a positive displacement flow meter. orifice meter is a differential pressure flow meter. First, they work differently. When the fluid passes through the PD meter, a certain pressure difference will be generated between the inlet and outlet of the flow meter. The rotating part of the flow meter (referred to as the rotor) rotates under the action of this pressure difference and discharges the fluid from the inlet to the outlet. In this process, the fluid fills the "metering space" of the flow meter again and again. Then they are sent to the exit continuously. Under given flow meter conditions, the volume of this metering space is determined. Just measure the number of rotations of the rotor. The cumulative value of the fluid volume passing through the flow meter can be obtained. The working principle of the orifice plate meter: When the fluid fills the pipe and flows through the throttling device in the pipe, the flow stream will shrink locally, thereby increasing the flow rate and reducing the static pressure. As a result, a pressure drop occurs before and after the throttling device, that is, the pressure Difference. The greater the flow rate of the medium flowing, the greater the pressure difference generated before and after the throttling member. Therefore, the orifice flowmeter can measure the fluid flow rate by measuring the pressure difference. This measurement method is based on the law of energy conservation and the law of flow continuity. Secondly, the main reason is that their measurable media are different. The orifice flowmeter has a wide range of applications. It can measure all single-phase flows (gas, liquid, steam) and some mixed-phase flows. PD meter is mainly used to measure liquids. Oil measurement applications are mostly used. Can be used for measuring high viscosity liquids.