If the pressure gradient  is not measured, it can still be obtained by measuring the velocity at the centre line. The measured velocity has only the real part of the full expression in the form of
Noting that , the full physical expression becomes
at the centre line. The measured velocity is compared with the full expression by applying some properties of complex number. For any product of complex numbers (), the amplitude and phase have the relations  and , respectively. Hence,
and
which finally yield