An orifice meter is a differential-pressure flow meter that measures flow by placing a precisely machined plate with a hole in it across the pipe and reading the pressure drop the restriction creates. It is the workhorse of natural gas measurement - inexpensive, well understood, and standardized - and for gas custody transfer it is governed by AGA Report No. 3.
Orifice Meter in one line: An orifice meter measures flow by reading the differential pressure across a machined orifice plate in the pipe; higher flow creates a larger pressure drop, which a flow computer converts to flow rate using the AGA 3 equations.
When fluid is forced through the smaller hole in the orifice plate, it speeds up and its static pressure falls, creating a measurable pressure difference between the upstream and downstream sides. That differential pressure rises with the square of the flow rate, so a differential-pressure (DP) transmitter reading the two taps, combined with the line's static pressure and temperature, lets the meter infer mass or volume flow.
Because the relationship is square-root, an orifice meter has a limited rangeability - it loses accuracy at very low flows where the DP is tiny. Its accuracy also depends heavily on installation: the plate must be square-edged and clean, and it needs adequate straight run of pipe upstream and downstream (often with flow conditioners) so the velocity profile is fully developed. The dimensionless beta ratio - orifice bore divided by pipe bore - is chosen to balance measurable DP against pressure loss.
For natural gas, the orifice meter is the reference case. AGA Report No. 3 (also API MPMS Chapter 14.3) defines the plate and meter-tube geometry, the tap locations, the discharge-coefficient equations, and the full calculation, while AGA 8 supplies the gas compressibility factor. A flow computer takes live differential pressure, static pressure, and temperature, applies these equations and the gas composition, and outputs a corrected volume at standard conditions.
A typical custody orifice meter run comprises a senior or junior orifice fitting holding the plate, upstream and downstream meter tube with flow conditioning, a multivariable DP/static/temperature transmitter, and a flow computer that logs the AGA 3 calculation and audit trail per API 21.1. That flow computer almost always speaks Modbus, so a cloud SCADA platform can poll the corrected flow rate, totals, DP, static pressure, and temperature to trend production and catch a plugged tap or a plate that has worn or come loose.
The differential pressure across the plate rises with the square of the flow rate, so flow is proportional to the square root of the measured DP. This is why orifice meters have limited turndown: at low flow the DP becomes very small and hard to measure accurately, degrading precision at the bottom of the range.
Beta ratio is the orifice bore diameter divided by the internal pipe diameter. A larger beta gives less permanent pressure loss but a smaller, harder-to-measure differential; a smaller beta gives a strong signal but more pressure loss. The value is selected to produce a measurable DP across the expected flow range while limiting energy loss.
AGA Report No. 3, equivalent to API MPMS Chapter 14.3, defines the plate and tube geometry, tap placement, discharge coefficient, and flow calculation for orifice gas measurement. It is used with AGA 8 for compressibility and API 21.1 for the flow computer's calculation and audit-log requirements.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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