The invention relates to methods for verification of correct function of sampling equipment is disclosed, wherein said method comprises the steps of: a) providing a pump assembly (1) comprising an inlet (2) and an outlet (3), a flow channel (4) extending between said inlet (2) and outlet (3), a pump located along said flow channel (4) adapted to force an gas flow through said flow channel (4), a first mass flow sensor (6) located inside said flow channel (4), a first pressure sensor (7) located near said first mass flow sensor (6) adapted to measure a first pressure inside said flow channel (4), and a second pressure sensor (8) located outside said flow channel (4), said second pressure sensor (8) being adapted to measure a second pressure being the ambient atmospheric pressure, b) calculating the pressure difference between said first pressure and said second pressure c) calculating any error in an output signal from the mass flow meter by comparing said pressure difference with a value in a pre-calibrated table of mass flow output signal values as a function of said pressure difference, d) providing an error signal comprising a value of said calculated error if said value of said calculated error is above a predetermined threshold. The invention further relates to alternatives to said method.