Actually it is quite a bit, I have witnessed it for myself. Remember, rebreathers become more efficient at greater depths, because of natural higher PO2's. It will give much more bottom time &/or cut deco times by 1/3- 1/2 off of standard OC 32% & O2. It is simple gas physics. The advantage comes from you can start your 1.6 PO at deeper levels as the ascent begins (as long as the work load is reduced). The beauty of the rebreather, is you are always diving the best mix for the depth to maintain the desired PO2, whereas on OC the PO2 changes according to depth. Now in my case, I am talking cave & while there are depth changes,... the changes are usually minimal during the bulk of the bottom time of dive.
Let me give you a real world example.... Actually, I'll give you two.
Peacock.... Average depth 50 feet. Best mix for depth at 50' is what? 56% at 1.4ppo2? Are you really going to go to Bill's and ask for 56%? Of course not. But, when I'm at 50, what's my rebreather feeding me? 48%. When I'm at 20', what's my rebreather feeding me? When I'm at 33', what's my rebreather feeding me? I'm at 1.2ppo2, the whole time. So, is my NDL longer? You betcha. Now, for most divers, it's not an issue, because most divers are not doing 3+ hours at Peacock anyway. But I am. Well, when I go to Peacock. I hate that place.
Let's take Madison Blue. In the back past Rocky Horror, there are sections that are 140' deep. So are you going to build back gas for that? Let's say you stay towards the top at 130' with a mix of 28%. That's great, except that all the way to Rocky Horror the depth is less than 60'. Are your stage bottles to Rocky Horror going to have 50% in them? (I actually know a guy staging to Rocky Horror with 50% which I don't agree with). I can tell you, when I do a 4 hour dive on my CCR, with my buddy on OC... my deco is WAY less. Because, he's diving 28/25 or 30% the whole dive. Where as I'm 1.2PPO2 the entire dive, whether I'm at 140' or 50'. There's a lot of savings in that. Only during a small portion of the dive is your back gas perfect for your depth. On a rebreather it's always perfect.
Where on a rebreather do we get caught?
Diving Ginnie Springs with my OC buddies, I always have less deco than they do, until the 20' stop. I'm ahead at 50', 40', 30, but then at 20' I'm at 1.2ppo2 and they are at 1.6PPO2 (breathing pure O2). This is where they'll sometimes catch up a little.
I did a 3.5 hour dive with a guy on this board at Ginnie a few years back. He was OC. I was CCR at 1.2ppo2. My deco was 30 minutes less than his deco. This was with no trickery, not jacking up my mix during the trip back.
It doesn't work for every dive. It works even less for square profiles.
