ISO 3966:2008 specifies a method for the determination in a closed conduit of the volume rate of flow of a regular flow: a) of a fluid of substantially constant density or corresponding to a Mach number not exceeding 0,25; b) with substantially uniform stagnation temperature across the measuring cross-section; c) running full in the conduit; d) under steady flow conditions. In particular, ISO 3966:2008 deals with the technology and maintenance of Pitot static tubes, with the calculation of local velocities from measured differential pressures and with the computation of the flow rate by velocity integration.