ISO 5167 vs. AGA Report No. 3
An interactive guide to the world's leading standards for differential pressure flow measurement.
The Philosophical Divide
The core differences between ISO 5167 and AGA 3 stem from their origins. One was designed for universal engineering versatility, the other for specialized fiscal accuracy in a single industry.
ISO 5167: The Universal Framework
Developed by a cross-industrial, international body, ISO 5167 provides a scientifically robust and versatile framework for a vast range of fluids and devices. Its goal is consistent and reliable measurement for broad engineering applications like process control, efficiency monitoring, and energy allocation.
Fluids: Any single-phase Newtonian fluid (liquids, gases, steam).
Devices: Orifice Plates, Nozzles, Venturi Tubes, Cone & Wedge Meters.
AGA Report No. 3: The Fiscal Specialist
Born from the high-stakes commercial needs of the U.S. natural gas industry, AGA 3 is a highly prescriptive and detailed standard. Its primary driver is absolute accuracy and contractual defensibility for the custody transfer of a high-value commodity.
Fluids: Primarily natural gas and related hydrocarbon fluids.
Devices: Almost exclusively the concentric, flange-tapped orifice meter.
Technical Deep Dive
Orifice Plate Specifications
Both standards demand precision, but AGA 3's focus on fiscal metering leads to significantly tighter tolerances in key areas. This chart compares relative stringency (higher score = tighter tolerance). For a detailed breakdown, view the specification table.
| Parameter | ISO 5167-2 Specification | AGA 3 Part 2 Specification |
|---|---|---|
| Edge Sharpness | Edge radius r < 0.0004d | Square and sharp; free from defects visible to the naked eye |
| Plate Flatness | Deviation < 1% of dam height | Deviation < 0.010 in/in of dam height |
| Eccentricity | Must be concentric | Extremely tight tolerances (e.g., 0.006" for 2" pipe) |
| Beta Ratio (β) | 0.10 to 0.75 | 0.10 to 0.75 (recommends 0.2 to 0.6 for min. uncertainty) |
| Reynolds No. (ReD) | ≥ 5000 | ≥ 4000 |
Practical Application
The choice is not about which standard is "better," but which is correct for the specific application, fluid, location, and, most importantly, the commercial contract.
Decision Flowchart
(North America)
(International / Contract Dependent)