Abstract: A meter prover employs a piston arranged to slide in a bore within a proving cylinder which is coaxially disposed within a surrounding cylindrical shell. The effective area of the upstream face of the piston which can be acted on by fluid pressure is less than the effective area of the downstream face of the piston. In a proving run, fluid flow to the meter to be proved is diverted to an inlet in the shell upon closure of a bypass valve. The diverted fluid then flows through an annular space between the proving cylinder and the shell, enters the upstream end of the bore through openings in the cylinder, and drives the piston downstream along the cylinder in synchronism with the flow. Upon reopening of the bypass valve, equal fluid pressures act on the upstream and downstream faces of the piston and the difference in effective area of those faces causes the piston to return to the position it had at the start of the proving run.