AGA 3 is the industry standard document that defines how a concentric orifice plate is used to measure natural gas flow, from the exact geometry of the plate and meter tube to the flow equation a computer solves to turn a pressure drop into a rate. Published jointly by the American Gas Association and the American Petroleum Institute, it is the reference that makes an orifice measurement auditable and comparable between a buyer and a seller. This page explains what AGA 3 covers as a document, how it is organized, and why it underpins so much custody-transfer gas measurement.
AGA 3 Orifice Metering Standard in one line: AGA Report No. 3, published jointly with the API as API Manual of Petroleum Measurement Standards Chapter 14.3, is the standard that specifies orifice metering of natural gas: the construction and tolerances of the orifice plate and meter tube, the installation and straight-run requirements, and the differential-pressure flow equation used to compute rate. It is issued in parts covering the equation, the physical hardware, and application guidance, so that an orifice measurement made anywhere to AGA 3 can be reproduced and verified by any party.
AGA 3 exists to standardize a single, widely used way of measuring gas: the concentric, square-edged orifice plate. When gas flows through a precisely machined hole in a plate mounted across the pipe, it speeds up and its pressure drops, and the size of that differential pressure relates to the flow rate. AGA 3 pins down every detail that determines how faithfully that pressure drop represents flow - the plate bore, edge sharpness, thickness, and flatness; the meter tube's bore, roundness, and internal condition; and the location of the pressure taps. By fixing these, the standard makes the measurement repeatable regardless of who built the meter.
The heart of the standard is the flow equation itself, which ties the measured differential pressure and static pressure, the gas density, and the geometry together into a computed rate. Central to that equation is the discharge coefficient, an empirically derived factor that accounts for the real behavior of flow through the orifice as opposed to ideal theory. AGA 3 provides the accepted formulation for that coefficient as a function of the meter geometry and flow conditions, which is what allows every flow computer running the standard to arrive at the same rate from the same inputs.
Just as important as the equation is the specification of the physical installation. AGA 3 sets the straight lengths of pipe required upstream and downstream of the plate, the use of flow conditioners, and the tolerances on the meter tube, because the flow arriving at the orifice must be a well-behaved, fully developed profile for the coefficient to apply. A meter that violates these installation rules can read accurately-looking numbers that are simply wrong, which is why the hardware requirements are as much a part of the standard as the math.
AGA 3 is not a single monolithic text but a set of parts, each addressing a different aspect of orifice measurement, so that users can reference the piece relevant to their role. One part is devoted to the flow equation and the underlying theory - the mathematics of relating differential pressure to flow and the treatment of the discharge coefficient. This is the part a flow-computer manufacturer or a measurement engineer works from when implementing or auditing the calculation.
Separate parts cover the physical specifications of the orifice plate and meter tube and the practical application of the standard. The specification part is what a manufacturer and an inspector use to confirm that a plate's bore, edge, and thickness and a tube's bore and finish meet tolerance, since a plate that is worn, nicked, or eccentric no longer matches the geometry the equation assumes. The application-oriented material addresses how to actually field, operate, and maintain an orifice installation so that it keeps meeting the standard over time.
This division into parts is deliberate. Custody-transfer measurement involves several parties - the meter builder, the operator, the flow-computer vendor, and the accountants who reconcile volumes - and each needs authoritative rules for their piece without wading through the others. Because everyone references the same document, a dispute over a measured volume can be resolved by checking each element against AGA 3: was the plate to spec, was the tube installed with the required straight run, and was the equation applied correctly with the right inputs.
In practice AGA 3 lives inside the flow computer at an orifice meter run. The computer continuously reads differential pressure, static pressure, and temperature, applies the AGA 3 equation with the configured plate and tube geometry, and produces a flow rate and accumulated volume. The quality of that number depends entirely on the computer being configured to match the physical meter - the correct bore, pipe diameter, and tap arrangement - because the standard's accuracy assumes the inputs describe the real hardware. A transposed diameter or a stale plate size makes the computer compute a precise but incorrect volume.
A cloud SCADA system such as Merobix carries those AGA 3 outputs off the flow computer and makes them visible and trendable, along with the live differential pressure, static pressure, and temperature that drive them. That visibility is where measurement problems surface. A differential pressure that has drifted to the low end of the plate's usable range, a static-pressure reading that looks off, or a volume that does not track the underlying differential all point at issues the standard's accuracy depends on, and seeing them continuously lets an operator catch a degrading measurement rather than discovering it at the next audit.
Because AGA 3 makes the measurement auditable, continuous SCADA history strengthens that auditability. The trended inputs and outputs form a record that can be reconciled against the plate change-outs, calibrations, and configuration entries that the standard governs. On remote gas measurement points that are rarely visited, that combination - a standardized, reproducible AGA 3 calculation in the flow computer and a continuous, reviewable record in the cloud - is what lets both parties to a custody transfer trust the volumes without someone standing at the meter.
They are the same standard published jointly. AGA Report No. 3 and API Manual of Petroleum Measurement Standards Chapter 14.3 are the American Gas Association and American Petroleum Institute designations for one document that governs orifice metering of natural gas. Referring to a meter as being AGA 3 or API 14.3 compliant means the same thing - it is built and calculated to that shared standard.
Because the orifice flow equation and its discharge coefficient assume a well-behaved, fully developed flow profile arriving at the plate. If the meter tube is too short, poorly conditioned, or disturbed by fittings just upstream, the flow is distorted and the coefficient no longer applies, so the computed rate is wrong even though the math is correct. AGA 3 therefore specifies straight lengths, flow conditioning, and tube tolerances as part of the standard, since the hardware and the equation only work together.
An orifice meter is the general device - a plate that creates a pressure drop related to flow. AGA 3 is the specific standard document that defines exactly how that device must be built, installed, and calculated for natural gas measurement, down to plate tolerances, tap locations, and the discharge-coefficient equation. The standard is what makes an orifice measurement reproducible and auditable between parties, rather than just a working principle.
Primary references from the standards bodies and regulators that define this topic:
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