Piston accumulators are hydropneumatic accumulators with a free-moving piston as a separation element between a compressible gas cushion and the operating fluid. HYDAC piston accumulators are comprised of a cylinder with a finely processed interior, with screwed-in cover on the gas and fluid side as a rule and a piston with special sealing systems. Piston accumulators can be adjusted very flexibly to customer requirements (e.g. series SK280 with patented design). Selection of the volume is practically stepless. The piston position can be made visible and also be used for switching functions in the hydraulic system.