Measurement of fluid flow in closed conduits -- Velocity-area methods of flow measurement in swirling or asymmetric flow conditions in circular ducts by means of current-meters or Pitot static tubes
ISO 7194:2008 specifies the measurements required, the precautions to be taken, the corrections to apply, and describes the additional uncertainties which are introduced when a measurement in asymmetric or swirling flow has to be made.
Only flows with a negligible radial component are considered, however. Furthermore, it is not possible to make a measurement in accordance with ISO 7194:2008 if, at any point in the measuring cross-section, the local velocity makes an angle of greater than 40° with the axis of the duct, or where the index of asymmetry Y (defined within ISO 7194:2008) is greater than 0,15.
ISO 7194:2008 deals only with instruments for measuring local velocity as defined in ISO 3354 and ISO 3966. If Pitot static tubes are used, ISO 7194:2008 applies only to flows where the Mach number corresponding to local velocities does not exceed 0,25.