Abstract: A compact prover assembly (10) suitable for determining the accuracy of a flow meter. The assembly (10) includes a piston (20) and cylinder arrangement (12) with a cylinder (14) with an inlet (16) and an outlet (18) towards respective ends thereof. The arrangement (12) also includes a piston (20) movable by fluid flowing through the inlet (16) towards the outlet (18), and means for measuring the time taken for the piston (20) to move between predetermined points in the cylinder (14). The piston (20) includes a flow past arrangement, which includes an opening (28) and a cover (34). The cover (34) is selectively movable substantially parallel to the piston between a closed position closing the opening (28), and an open position clear of the opening (28) to permit fluid to pass through the opening (28) and hence through the piston (20) when the piston (20) is static or being moved backwards towards the inlet (16).