Genesis once bubbled...
with no deco required?
How's 'dat?
PO2 of 1.3 @ 130 feet is about 26% O2. The EAD is still 120' on that percentage..... which definitely doesn't give you much NDL time.... (like 10 minutes!)
When you get done with the first hour and a half you certainly do have a serious deco obligation if I'm doing the math right... Do you carry enough bail-out gas to meet that obligation if something goes wrong with the CCR at that point?
That's a heck of a lot of gas in an OC bailout, is it not, especially considering that you'd have deep stop obligations being down that long....
Assuming you hold a PO2 of 1.3 at 50 fsw, you'd be running about a 50% mix (actually 51.7%, but who's counting at that point)
Yes, you'd offgas a lot during that hour and a half, but do you really have no deco obligation remaining at the end of that 90 minutes?
I'm curious about the OC bailout requirements to do this safely...
(I'm not a rebreather diver, but am trying to understand them... I understand the gas consumption advantage, but am trying to get my arms around the elevated mix....)
Would it not be better to run the lower PO2 at the bottom, and then crank it up when you do your deco? That is, something adaptive to get you the offgassing advantage when shallow and doing your deco, but removing the O2 clock issue when deep?
The long 3 hr deep dives I discussed, I only do with another CCR diver.. Yes, after the deep part there is considerable deco reqired, but believe it or not you alter the deep stop requirement during the 100 foot leg..
I Plan these dives (and deeper) where the ccr TEAM has atleast 1.5 the gas required to get 1 diver home... in this case much more
If you look at the calculated profiles shown later in this thread, th 10 ft stop is equivalent to 100% oxygen, the 20ft stop is equivalent to 81%, oxygen.. You can run both of these off the 100% if you use 50% times are only slightly increased as does the deeper stops.. On most dives with this level of decompression switching from 50% to o2 doesn't take much time off of requirements, its usually only a few minutes difference for me one 40cf 50% bottle under most circumstances would suffice (besides I would have at least another 20cf of diluent available)..
We will start the dive with our diluent bottles filled (aprox 23 cf), and one diver will carry 50% and bottom mix the other will cary 50% and oxygen. The cylinders are usually 40cf each. 160cf of bailout gas is more than enough for these dives..
I have a very low RMV (I recently did an OC dive to 130 for 45 mins and used less than 100cf of bottom gas).
I always build bailout tables, and dive with 2 CCR capable dive computers.. My 1st choice would be to dive the computer in case of failure, but I still carry the tables just in case.. I dive an abysmal explorer and a VR3 as a backup on mix dives, or an Explorer and a cochran commander as backup on "air" dives.. The explorer and VR3 both can be switched from CCCR mode to OC underwater.
Run the profile on something like the GAP, If you use the default gradient factor you have a slight deco requirement, I reduce the GF a bit to give shorter deco times(in this case none - if you use an unmodified buhlman you were clear before the 1.5 hours), I find I don't need to be overly conservative when on my CCR.
The o2 clock issue really isn't a problem at 1.3 your limit is 3 hours for a normal exposure.. On longer dives you can take low PO2 breaks to simulate "air" breaks to extend the time.. The 1.3 is definately an advantage when doing multilevel dives since it increases o2 as you get shallower starting offgassing when OC divers are usually ongassing.
I also find that "personally" my o2 tolerance seems to increase as a week of diving progresses. On deep deco dives I'll do my 20 ft stop with 100% flushing the loop to keep it as close as possible, on the first few dives I get the usual congested feeling seen by divers using 100% oxygen, within a few days Its totally gone..