Rebreather Lab .... Pelagian DCCCR
Andy Fritz started to modify Russian military rebreathers back in 1997 initially for personal use, but soon enough divers would travel to Thailand to get their rebreathers modernized and themselves trained to use them. Other popular rebreathers had their share of modification as well. The common theme was mechanical O2 addition through needle valves and after around 700 hours of virtually problem free diving the decision to manufacture a rebreather from scratch was made.
Mainly because it was difficult to improve the trim and buoyancy characteristics of the many rebreathers modified. One other reason was that exploring a particularly remote site in 90 meters of water called for a closed circuit rebreather that could be expanded with a large capacity scrubber. The final version Pelagian DCCCR prototype was extensively test dived for four months before production started.
The Pelagian Diver Controlled Closed Circuit Rebreather is exactly what the name suggest - diver controlled.
This means that oxygen is added mechanically through a fine metering needle valve at a constant flow set slightly lower than the divers metabolic rate. As the PO2 slowly drops the diver ad oxygen manually with a push button. Another similar approach used in other rebreathers is to use a non adjustable orifice instead of the needle valve.
The disadvantage with this system is that the regulators 1st stage need to be isolated from the water pressure in order to keep the flow constant, which means that when the water pressure exceed the set intermediate pressure of the 1st stage regulator - the flow stops. As the Pelagian DCCCR is equipped with a needle valve the 1st stage is kept responsive to the water pressure and will raise the mass flow of oxygen during deeper decents. The diver can balance this by turning down the needle valve while confirming the PO2 with the O2 displays.
This means you can dive deeper than with an orifice fed rebreather. How deep? Theoretically very deep. The practical limitation is the amount of bail out gas you can carry or stage and the scrubber life.