Looking at the reference provided by gaspode42 ....reply #8, above ....It seems like the fundamental quantity that is being sought after is the actual force of the subject's airflow, as it is exhaled.. In terms of basic physics, this is the product of mass flow, the mass here being air, and velocity.... This force quantity is sometimes referred to as the momentum flux of a fluid stream .... It derives from the Newtonian definition of force as the derivative of momentum .... F = d(p=mv)/dt = m dv/dt + v dm/dt .... where the former term is not relevant, but the second term is the fluid velocity times the mass flow rate of the fluid, as it crosses an arbitrary boundary .... If all you want is a relative figure of merit, then you can just employ the volumetric flow rate in the equation.... as mentioned above.
Consequently, it would appear that the force of the airstream produced by the subject would serve as a reasonable and practical measure of the quantity in question.
One possibility would be to have the test subject exhale through a tube, which allowed the airstream to impact a plate or surface area that would be attached to a cantilever beam, and a suitable strain gage. The objective would then be to record the maximum deflection of the strain gage during the test, as the airstream deflected the cantilever beam.