ISO 3966:2025

Overview

ISO 3966:2025 - Measurement of fluid flow in closed conduits - specifies the velocity–area method using Pitot static tubes for determining the volumetric flow rate in closed conduits. The standard applies to steady, full-pipe flows of fluids with substantially constant density (or Mach number ≤ 0.25) and substantially uniform stagnation temperature across the measuring section. It covers the design, use, maintenance and performance requirements for Pitot static tubes, computation of local velocities from differential pressure measurements, and integration methods to compute flow rate.

Key topics and technical requirements

• Measurement principle: velocity–area method based on local velocity measurements from Pitot static tubes and integration across the cross-section.
• Scope constraints: closed conduit, full flow, steady conditions, low compressibility (Mach ≤ 0.25), nearly constant density and stagnation temperature.
• Pitot tube design & maintenance: requirements for Pitot static tube geometry, inspection and maintenance procedures to ensure accurate differential-pressure sensing.
• Traverse and positioning: guidance on selection and location of measuring cross-section, number and placement of measuring points for circular and rectangular ducts, and stem blockage considerations.
• Velocity computation & integration methods: graphical, numerical and arithmetical approaches (including log-linear and Log‑Chebyshev methods) for integrating measured velocities to obtain discharge.
• Flow conditioning: recommendations for anti-swirl devices, profile developers, turning vanes, static mixers and provisional guiding installations to achieve representative velocity profiles.
• Corrections & uncertainty: corrections for stem blockage, transverse velocity gradients, turbulence effects and head loss; procedures for estimating measurement error, standard uncertainty and expanded uncertainty.
• Annexes: normative and informative annexes addressing Pitot tube specifications, turbulence correction, compressible-fluid measurements, damping of pressure gauges, and worked uncertainty examples.

Applications and practical value

ISO 3966:2025 is used where accurate volumetric flow measurement in pipes is required:

• Industrial process flow monitoring and control (chemical, petrochemical, power)
• Water and wastewater discharge metering
• HVAC and building services commissioning and diagnostics
• Calibration and flow metrology laboratories Benefits include standardized methods for achieving reproducible, traceable flow measurements, improved measurement uncertainty estimation, and guidance on avoiding common errors (swirl, asymmetry, turbulence).

Who should use this standard

• Flow measurement engineers and instrumentation specialists
• Test & calibration laboratories and metrologists
• Plant operations and process control engineers
• Manufacturers of Pitot static tubes and flow conditioning devices
• Third‑party auditors and compliance officers assessing flow-measurement systems

Related standards

Consult other ISO standards and national standards on flow measurement and instrumentation for complementary methods (e.g., differential‑pressure meters, ultrasonic, electromagnetic) and for broader metrological and calibration requirements.