A Compressor Case (also called a compression case) is a dive watch case that uses rising water pressure to press the caseback harder against its gasket, so the seal improves with depth.
A compressor case (sometimes called a compression case) is a dive watch case, developed by case maker Ervin Piquerez SA (EPSA) in the 1950s, designed so that increasing water pressure pushes the caseback tighter against its gasket, improving the seal the deeper you go. Popular in the 1960s and 1970s, it was used by brands including Longines, Jaeger-LeCoultre and Universal Genève. The best-known version, the Super Compressor, added a second crown to operate an internal rotating bezel, and that twin-crown look has been revived by modern brands seeking a vintage dive aesthetic.
A compressor case uses external water pressure to push the case back against the gasket, increasing the seal tightness as depth increases. Unlike traditional dive cases that rely on constant screw-down pressure, compressor cases become more water-resistant as you go deeper—the pressure that threatens the watch also protects it.
The Super Compressor, developed by EPSA in the late 1950s, added a second crown to operate an internal rotating bezel, so the bezel couldn't be knocked out of position. Well-known examples include the Jaeger-LeCoultre Polaris and the Longines diver that has since been revived as the Legend Diver.
Modern dive watches use screw-down crowns with multiple gaskets rather than compressor mechanisms, as advances in gasket materials and case engineering have surpassed the original compressor concept. However, several brands including Jaeger-LeCoultre have revived the dual-crown Super Compressor aesthetic in modern reissues, though without the original pressure-compression mechanism.