@rddvet the beauty of the needle valve is that you use an un-capped first stage.
Basically, with a normal CMF leaky valve, it is depth limited based on the IP of the first stage. In order to get more depth, you put in a stiffer spring. The end result, you've got a leaky valve that is non-adjustable for flow and is depth limited, and the resultant change in flow that comes from jacking the IP way up.
A needle valve doesn't have this limitation because you're essentially adjusting the size of the orifice on the fly to whatever you want. What's great about it is that you can tweak it to whatever you want, in relation to your metabolic rate. It really allows you to tune the O2 injection to wherever you want it. If I'm doing a long cave dive in no flow at a pretty static depth, I'll tweak it to almost perfectly match my metabolic rate. In doing so, I rarely have to manually inject O2 into the loop. If it's going to be a little more involved, I set it lower and just assume I will have to inject more, or I'll set it higher and remember that I've gotta turn it down once my metabolic rate evens out a little bit.
The only tricky thing is that when you radically change depth, you have to tweak it since it uses an uncapped first stage. However, that also means that when you deco out at 6m, you can whack it open and you're on a full O2 rebreather without having to do any intervention, as opposed to a CMF where you're still injecting manually, and since you're shallow and trying to maintain a high PO2, you're injecting pretty often.
I've heard from Charlie that the Fathom has a blocked first stage for "ease," but also that it has a needle valve, which sort of defeats the purpose for both, so I don't know what's going on..... pick one or the other. Personally I think a needle valve is much more versatile, so hopefully he just misspoke. Otherwise I'm really confused as to what's going on.
I think if I were interested in something similar and had issues with the Pelagian (understandable), I'd find a cheap COPIS Meg with the big radial, call up SubGravity or ATS and get some BMCL's, and get a needle valve MAV from one of several sources. Swap the handset for a Fischer cable and then you don't even need a battery in the head. That gets you a Fathom for much cheaper, and it's even simpler. Having dived both radial and axial scrubbers, I don't really see any appreciable difference, which means an inexpensive homebuilt is equally viable either way.
Another alternative would be to get an SF2 and swap the O2 MAV for a needle valve MAV. No depth limitation, and you can run the solenoid as a golden parachute. For squirrely cave, the streamlined profile and clutter free design is fantastic, even more so as a sidemount unit. You can still drive from offboard by shutting the dil MAV off, and if IP is higher in the offboard, it'll even run the ADV from offboard.
I just spent a few days with the SF2 in Florida, pretty much running it completely manual. After I'm super comfortable I may put a needle valve on it to turn it into a hybrid. Running a low set-point and keeping it up manually required a little more work than I wanted, but I still want to stay away from using the solenoid as much as I can. Just personal preference. That being said, the shearwater did a damn fine job of keeping me close to my setpoint when I did let it fly by itself.
