Variations in regenerative wheel flow channel pore sizes occur as a consequence of manufacturing tolerances. Although the determination of these pore size variations can be difficult using optical methods, a pressure probe method offers the opportunity to determine these variations by using a simple procedure that has at its foundation the equations for fully developed laminar flow through long tubes. This paper describes the details of the probe geometry and flow conditions that will give accurate data that is used to calculate the standard deviation in pore size relative to the mean hydraulic diameter. Knowledge of this ratio allows the analyst to then calculate the degradation of the important performance factors for regenerative wheels caused by these flow channel variations. Example tests on regenerative wheels show that the pressure probe method can be used to determine the flow channel variations and to predict their performance degradation and its uncertainty.