Abstract: A meter prover comprises an upstream chamber (1) with a port (2) a downstream chamber (3) with a port (4) either port for connection to a meter (not shown) to be proved, a stationary seal (5) between the two chambers, a piston body (6) within and bridging the chambers, said piston body being in sealed sliding engagement with the stationary seal and having an outer diameter less than the minimum inside measurement across each chamber, an independent actuator (9) to initiate a proving run of the piston body (6) from one chamber to the other, a detector unit (10) for signalling the start and end of the proving run, and port openings (17) at at least one end of the piston body (6) providing fluid communication between the chamber prior to the onset of the proving run. The piston body (6) may be attached to a piston rod (7) which projects through a sealing means (8) at the end of at least one of the two chambers and is connected to the independent actuator (9) external of the chamber.