Optimization of variable volume exhaust semi-closed underwater breathing apparatus

The performance of variable volume exhaust rebreathers are characterized by an exhaust volume ratio (EVR), the ratio of the small inner bellows volume relative to the main breathing bellows, and supply gas oxygen concentrations (XO₂). This paper details the development of the approach used to enhance the performance of these systems. An analytical model has recently been developed for predicting the circuit oxygen levels in variable volume exhaust semi-closed underwater breathing apparatus at various mission depths and diver activity levels. Unmanned testing in June 2000 with a commercially available system showed good agreement with this analytical model over a wide range of diver depths, orientations and activity levels. The analytical model showed that for given operational parameters of maximum and minimum depth there must be an optimum EVR and XO₂ which will maximize capability while ensuring allowable maximum and minimum partial pressures of oxygen (PO₂) in the breathing circuit. By specifying boundary conditions on the analytical model at maximum and minimum depth for the associated allowable minimum and maximum PO₂ an analytical relationship for EVR and XO₂ was developed and subsequently validated in unmanned testing at the Experimental Diving Unit in Panama City, Florida. Associated EVR and XO ₂ are presented for three missions of possible interest to the Navy; shallow depth (<60 FSW), deep depth (<190 FSW) and a switch over to closed circuit at very shallow depth (20 FSW)