A flow measuring arrangement for measuring flow through a measuring tube comprises at least one constriction located in the measuring tube, which effects a reduction of a volume through which a medium flows; at least one deflectable membrane applied to said constriction, wherein deflection of the membrane effects a change of a volume through which medium flows in the measuring tube; a pressure measuring system connected to the measuring tube for measuring pressure (Δp, p); measuring electronics, which determines flow based on the measured pressure (Δp, p); an apparatus for producing a time variable deflection of the membrane and a diagnostic system, which determines an affect of said deflections of the membrane on the measured pressure (Δp, p), and which diagnoses the flow measuring arrangement based on the deflections and determined effects thereof on the measured pressure (Δp, p).