Many types of differential pressure (d/p) flowmeters are available. Some of the commonly used types (including orifice plates, flow nozzles, venturi tubes, flow tubes, pitot tubes and pitot venturi tubes) are described in API RP 551.
Outside the U.S.A., ISO 5167 Standard is widely used for orifice meters. It also covers other types of d/p flow meters.
This section gives information about these flow meters and describes three additional d/p flow meters: annubar, elbow, and wedge.
When selecting d/p flow meters, note that the rangeability of the differential pressure transmitter is limited to approximately 10:1, while the flow turndown is usually limited to 3.5:1 due to the nature of a squared output. Differential pressure transmitters with different calibrated ranges can be “stacked” or installed in parallel across the orifice plate to achieve flow turndowns of 10:1 or better, when the orifice plate is operated within its operating constraints. However, care must be taken in measuring fractional inches of water column in developed differential.
Piping requirements (meter runs) are very important for the accuracy and repeatability of differential pressure flow meters.