The Halcyon RB80 is a non-depth-compensated passive addition semi-closed circuit rebreather of similar external dimensions to a standard AL80 scuba cylinder (11-litre, 207-bar aluminium cylinder, 185 mm diameter and about 660 mm long). It was originally developed by Reinhard Buchaly (RB) in 1996 for the cave exploration dives conducted by the European Karst Plain Project (EKPP).[1]
About 1/10 of the respired volume of breathing gas in the circuit is discharged during each breathing cycle by a concentric bellows counterlung system, which reduces the loop volume and is replenished by internal valves, triggered by low loop volume, similar to the function of the demand valve of a scuba regulator.[1]
The Halcyon RB80 was introduced as a replacement for the much bulkier and more mechanically complex PVR-BASC, which was depth-compensated and used a ballasted bellows counterlung.[1]