A method of determining an inlet flow rate (Finlet) of a flowable material including passing an inlet stream of flowable material through a chamber (20) having an outlet aperture (100); measuring a first rate of change of quantity of material in the chamber (20) when the material is entering at said inlet flow rate; measuring a second rate of change of quantity of material in the chamber (20) when no material is entering the chamber (20); and calculating the inlet flow rate (Finlet) from said first and second rates, wherein both rate measurements are made while the whole of the outlet aperture (100) of the chamber (20) is occupied by the flowable material.