ALVBOV Exhale Spider 

The gas cross section of the web was chosen such that the sum of the entire rebreather loop with both flapper valves matches the mouthpiece of the largest oro-nasal mask opening that could be created using Comex oro-nasal masks. The end result is the breathing resistance of the system equates to the diver breathing through a 32mm diameter tube, 1cm long. The mushroom cross section is therefore mathematically optimal, reducing the Work Of Breathing and hence risk of CO2 retention to the lowest possible level.