Transducers located on either side of the cavity emit and receive sound in
alternating directions. When gas flow is present in the tube, a pulse that travels against the
flow (traveling upstream) is slowed down and takes a longer time to reach the opposite
transducer. Conversely g ( g ) , a pulse traveling with the slow (traveling downstream) is sped up and
takes a shorter time to reach the opposite transducer. The transit time of each sound pulse is
precisely measured with a digital clock. The gas flow through the cavity is then calculated from
the upstream and downstream transit times.