space 16 between the sleeve 13 and the vessel wall 10 in the present invention the forces exerted on the sleeve 13 as fluid flows around it in this restricted space will be smaller and more uniform in the prior art due to the compressibility of gas relative to liquid having gas reside on the outside of the sleeve increased the forces likely to act on the piston and sleeve assembly as gas flows through the interstitial space around the sleeve in response to rapid charging and discharging of the device again leading to the need to make the sleeve thicker and more rigid than would otherwise be required for piston sealing only this is avoided in the present invention a smaller and more uniform volumetric flow is generated in such conditions this reduces the required rigidity of the sleeve allowing consideration of previously unsuitable materials for the sleeve such as moldable polymers that are lower in cost easier to manufacture or provide superior sealing properties over the metal sleeves existing in the prior art the reduced stress acting on the sleeve 13 also alleviates the problem of retaining and centering the sleeve 13 within the vessel 10 previous designs rigidly attach the fixed inner end of the cylindrical sleeve to the inner surface of one end of the vessel thereby forming a cantilever style connection with the end surface this was done to provide sufficient retention and centering for the sleeve and also causes the surface of the inner end of the vessel to serve as one wall of the chamber that will contain working medium within the sleeve because the present invention reduces stress on the sleeve such a rigid connection is no longer necessary instead the sleeve 13 may be formed integrally.