Flows approaching a Parshall Flume should be non-turbulent, sub-critical, and possessing a good velocity profile.  In some appliations it is possible to correct for less than ideal approach condtions.  Flow straighteners, screens, and tranquilizing racks have been developed to help condition flow.  

Here flow entering a 3-inch Parshall Flume is conditioned by means of a set of screens installed at the inlet of the flume.  The screens slide up and down in vertical guides to allow them to be cleaned and the upstream distribution box flushed as necessary.