An orifice plate is a very simple device installed in a straight run of pipe. The orifice plate contains a hole smaller than the pipe diameter. The flow constricts, experiences a pressure drop, and then the differential pressure can be related to a flow.

Figure 1: Orifice Plate Arrangement

For a discussion of how pressure drop is related to liquid flow for concentric orifices,
visit LMNO Engineering. They have a very good explanation on their website.

It is also important to note that relating differential pressure to flow across an orifice depends on the location of the pressure taps in relation to the orifice. In Figure 2 below, the pressure taps are designated as P1 and P2. "D" is the diameter of the pipe and "d" is the diameter of the orifice.

Figure 2: Various Tap Positions for Orifice Plates

The principle of differential pressure measurement as an inferred measurement of rate of flow is well established and widely used in many specific forms.

The use of more sophisticated electronic readout equipment has enabled the effect of other process variables to be taken into account in computing rate of flow.

Probably the most widely used differential pressure device is the orifice plate , the characteristics of which as a flow measurement device have been researched for over a 100 years , leading to the publication of many national and international standards. A whole measurement cult has emerged around orifice plate technology.

Orifice Plate


Available in an infinite number of sizes to suit an almost infinite number of application conditions. Can be readily made from a wide variety of materials to fulfill costs of applications requirement criteria. More widely used for gases where it is utilized as a primary measurement element.

In larger sizes it is available as a removable assembly which can be interchanged under line pressure. For custody transfer applications in gas it is common to see such large orifice plate metering set ups with higher accuracies referred to as senior orifice plate meters.

Flow Nozzles


This is used as a primary measuring element for air and gas flow, for a number of industrial applications.

The flow nozzle is also available cheaply manufactured from plastics with simple manometer readout as the basis for an effective cheap measurement system where rangeability requirements are small and where limited accuracy is acceptable



Traditionally used for water measurement and water based chemicals including corrosive fluids

Annubars and Pitot Tubes



The use of the pitot tube is restricted to a point velocity measurement but these are occasionally employed to traverse pipe velocity profiles for the purpose of inferring volumetric flow rate. However the Annubar , or multi-orifice probe is widely used as a flow measurement device. By virtue of the disposition of the dynamic pressure measurement holes across the velocity profile , an averaging effect is obtained. The position of the sensing holes on the velocity profile is thus critical to the accuracy of the device, as is the shape of the profile itself.