Apparatus for providing a mass flow measurement in respect of a fluid flowing in a conduit (11, 30), the apparatus comprising at least one non-intrusive device configured to obtain, in use, data representative of the total volumetric flow of said flowing fluid 32a,b, 20a,b), at least one device (16, 42) configured to obtain, in use, data representative of density of said flowing fluid, at least one device (38, 42, 44, 46, 43) configured to obtain, in use, data representative of the level of said fluid in said conduit, and a device (22, 40) configured to calculate, in use, a corrected mass flow of said flowing fluid using said total volumetric flow, corrected using said fluid level data, and said density data.