A system for performing statistical process control. The jar member is releasably coupled to the piston by the release device such that, when the piston is in the first position in the fluid chamber, the jar member is coupled to the piston by the release device for movement therewith. The invention is more particularly applied to the detection of objects in \u201call-weather\u201d conditions. The system offers a locking system designed for sequential consistency with write-back caching, typical of distributed file systems.