dave t

twin counterlungs either front or back mounted offer some mechanical "assist" when moving gas around the loop, this effectively lowers WOB and lengthens dwell time which helps to improve scrubber efficiency. A single counterlung does not provide this and may not be as good at insulating the gas either. Increasing the diameter of the scrubber should slow the gas down some and help with WOB due to potential less resistance, but increases the profile (in-water) and maybe more chance of channeling although this will not happen with the cartridge style scrubber packs. Most Rebreather builders have opted for twin counterlungs for good reason, an efficient system, low profile and good WOB/scrubber duration. I see no reason for a single counterlung. a twin back mounted counterlung correctly designed and placed will perform as well if not better than OTS lungs and give a clean front and low in-water profile. but I know Kev Gurr very well and hes a smart guy who will do lots and lots of testing and experimenting and whatever his design ends up being I am sure it will work.