Acoustic flow measurement method and measurement apparatus implementing the method

An acoustic method of flow measurement for the determination of fluid flow velocity (v), volumetric flow (Q) or mass flow rate (M) in a measurement pipe section have an axial direction by determining over an exactly determined length 2a the downstream propagation time T.sub.D and upstream propagation time T.sub.U for naturally occurring or artificially generated acoustic wave modes (15,16) propagating upstream and downstream in the measurement pipe section in the form of plane wavefronts of low frequency sound, comprising the steps of:

• mounting two pairs of opposed transducers (11,12;
• 13,14), separated in the axial direction by a certain interpair distance (2a) and a certain intrapair distance (2b) on the measurement pipe section;
• delaying, scaling, and summing signals from the transducers into four direction-discriminating filters (2a, 2b, 2c, 2d) of which two filters (2c, 2d) are tuned to eliminate signals related to downstream acoustic waves and two filters (2a, 2b) are tuned to eliminate signals related to the upstream acoustic waves in the measurement pipe section;
• comparing with each other the output signals of said two direction-discriminating filters by forming a symmetrical equilibrium function with respect to a delay variable about a true time shift between the said signals to be compared from (a) cross-correlation function, (b) a time integral function of squared difference, or (c) a time integral function of squared sum, of the output signals, whereby one of the alternative functions (a), (b), and (c) is computed for acoustic waves in both downstream and upstream directions, said equilibrium functions being maximally symmetrical with respect to their maxima or minima;
• and
• determining from any of said maximally correct symmetric equilibrium function (a,b,c), with the help of a certain predetermined algorithm, correct values of the acoustic wavefront propagation times T.sub.D and T.sub.U for use in the computation of both the measurement result given by the method and values of delay time variables to be employed in the direction-discriminating filters.