Andy wrote:chickdiver wrote:The idea that a constant setpoint buys you a whole bunch of time off deco is really a myth unless you are talking about extremely long dives. Running the profile with a constant PPO2 vs. OC buys you typically >5 minutes.
Sorry, you're going to have to run this one past me again....
What I think you're saying is that if you dived a perfectly square profile, where on OC you've got the same bottom ppO2 as on the breather - then you you've essentially got the same actual deco time on the two, give or take a few minutes.
The difference, I guess, comes in that if you're multilevelling the dive (like I can afford a VR3 right now...!), that the fO2 is increased to keep the ppO2 on the breather constant as you come shallower then it's making a huge difference on your NDL as the ppO2 is always 1.2-1.4, whereas on OC it's dropping as you ascend.
Well, that's enough for me... thanks for that. Scratch that one off the list.
THat's the basic idea. When we dive OC, we rarely dive a true square profile. It's always somewhat multi-level, and most often we spend times at varying depths- right? So... PO2 changes constantly during the dive. If you are aiming for a bottom PO2 of say... 1.2, and end up spending a significant portion of the dive 30 feet shallower, you have a PO2 of less than 1.2. With a rebreather, if I decide on a setpoint of 1.2, then my PO2 remains 1.2 regardless of depth fluctuation.
Theoretically this should buy me some credit in the decompression obligation. One of the arguements in favor of rebreathers has alwasy been the decompression advantage gained by constant PO2. In reality the benefit gained with regard to decompression is very slight, unless you start talking about VERY long dives. In fact, the time savings usually isn't worth running new tables for.
Does that help?