A leak detector apparatus for a pressurized cylinder having a cylinder of a radius (r) with a slidable piston in the cylinder with a gas filled chamber positioned between a cylinder end and the piston face. The length (L) of the chamber is calculated according to load changes on the piston where the volume (Vc) of the chamber changes. The pressure and temperature of the chamber are measured as well as the length. These values are inputted to a processor to solve the ideal gas law equation PV=nRT where a change in n for a change in length indicates a leak.