A gas flow meter for determining the volume of a gas supplied from a gas source, comprising a linear measuring tube having an internal volume, with a gas inlet at a proximal end of the measuring tube and with a gas outlet at a distal end of the measuring tube, with a piston in the inner volume, the piston having at least one gas seal terminating with the measuring tube and being movable in a gas-tight manner at least between the gas inlet and the gas outlet by means of a force application, with at least one switching means for controlling the movement of the piston in the direction of the proximal end of the measuring tube and with at least one means for the time-repeated position detection of the piston in the measuring tube as well as an evaluation unit which is designed to establish and metrologically determine at least one relationship between a time course of the movement of the piston in the measuring tube and the quantity of gas supplied to the measuring tube, wherein a. the measuring tube is formed from a non-magnetic material, b. the piston has an undersize relative to the measuring tube which is at least so great that the pressure generated by the magnetic field gradient of the piston in the ferrofluid is sufficient to compensate to a large extent for the adhesive forces between the piston, ferrofluid and measuring tube, c. the piston has an undersize relative to the measuring tube which is so small that the tightness of the gas seal made of ferrofluid located between the measuring tube and piston is given for the designed pressure range of the gas to be measured, d. the at least one means for time-repeated position detection of the piston comprises at least one magnetic field sensor (8, 9), arranged at a predetermined point of the measuring tube, which indicates at least the presence of the piston at a predetermined point of the internal volume, and e. the gas seal terminating with the measuring tube is formed as a ferrofluid surrounding the piston, characterised in that the piston is permanent magnetic with a dipole moment along the axis of the measuring tube, in that, however, the gas flows through the seal at a gas pressure outside the designed pressure range, and that the pressure of the supplied gas is kept constant by a 3-way valve directing the pressure of a gas source to the proximal end of the measuring tube in one switching position to move the piston away from the proximal end, and that the other switching position connects gas inlet, gas outlet and overflow channel to each other whereby pressure equalisation occurs between the proximal end and distal end of the measuring tube, and thereby allowing the weight force of the piston to effect its movement towards the proximal end of the measuring tube, and the switchings can thus occur before the piston reaches its mechanical end stops. a. the measuring tube is formed from a non-magnetic material, b. the piston has an undersize relative to the measuring tube which is at least so great that the pressure generated by the magnetic field gradient of the piston in the ferrofluid is sufficient to compensate to a large extent for the adhesive forces between the piston, ferrofluid and measuring tube, c. the piston has an undersize relative to the measuring tube which is so small that the tightness of the gas seal made of ferrofluid located between the measuring tube and piston is given for the designed pressure range of the gas to be measured, d. the at least one means for time-repeated position detection of the piston comprises at least one magnetic field sensor (8, 9), arranged at a predetermined point of the measuring tube, which indicates at least the presence of the piston at a predetermined point of the internal volume, and e. the gas seal terminating with the measuring tube is formed as a ferrofluid surrounding the piston,