ISO 3966:2020

Overview - ISO 3966:2020 (Velocity area method using Pitot static tubes)

ISO 3966:2020 specifies a standardized method to determine the volume flow rate of fluid flowing in closed conduits using the velocity–area method with Pitot static tubes. It applies when flow is steady, the conduit is running full, the fluid has substantially constant density (or Mach number ≤ 0.25), and the stagnation temperature across the measuring cross‑section is essentially uniform. The standard covers the technology, calibration, positioning and maintenance of Pitot static tubes, conversion of measured differential pressures to local velocities, and integration procedures to compute the total flow rate.

Key topics and technical requirements

• Principle and measurement methods: graphical, numerical and arithmetic integration of the velocity area to obtain discharge velocity (see Clauses 4, 9–11).
• Pitot static tube design and criteria: recommended geometries and performance requirements (Clause 5 and Annex A).
• Traverse strategy and measuring points: selection, number and location of measurement points for circular and rectangular cross‑sections (Clauses 4.4, 9–11).
• Differential pressure measurement: instrumentation, damping and handling of fluctuations (Clauses 6.4, Annex D).
• Corrections and limitations: stem blockage, transverse velocity gradients, turbulence, compressibility effects and head‑loss corrections (Clause 12 and Annexes B, C, E).
• Performance, inspection and maintenance: procedures for Pitot tube care and verification (Clauses 6.5, 7).
• Uncertainty and error analysis: definitions, random and systematic error sources, and methods for calculating standard deviation and measurement tolerance (Clause 13 and Annex G).

Practical applications and typical users

ISO 3966:2020 is used where in‑line, point‑based velocity profiling is practical:

• Water and wastewater utilities measuring pipe flow where insertion meters are suitable.
• HVAC and building services for duct airflow diagnostics and commissioning.
• Process industries (chemical, food, pharmaceuticals) for flow verification in closed pipelines.
• Oil & gas and utilities for custody transfer checks, leak detection and verification when conditions meet the standard’s limits.
• Calibration laboratories, instrumentation manufacturers and flow metrologists implementing standardized Pitot tube procedures and uncertainty budgets.

Benefits include repeatable flow measurements, documented correction methods (turbulence, blockage, compressibility), and a clear route to quantify measurement uncertainty.

Related standards and guidance

• Other ISO flow measurement documents (for example, differential‑pressure device standards such as the ISO 5167 series) and metrology guidance on measurement uncertainty complement ISO 3966:2020.

Keywords: ISO 3966:2020, Pitot static tubes, velocity area method, flow measurement, closed conduit flow, differential pressure, velocity integration, flow uncertainty.