OP
Question re: minimum air volume when decompression diving
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Yeah, I know! I generate four schedules: the original, then another for a 10 min delay, then another for 10 fsw deeper, and then the last (which I expect to never have to use) for both a 10 min delay and 10 fsw deeper. Having to use schedule #2 or #3 doesn't seem extremely improbable.
rx7diver
J-Vo
Contributor
Ah if only it was so easy. There are no tec sidemount instructors within a 200 mile radius of where I currently reside, unfortunately (PNW). There was one, but he stopped instructing tec courses a year agoSo, if I want to learn ANDP up here, I need to go backmount.
Go somewhere and get good Sidemount training then come back and do deco training with an instructor that is in back mount. Your tech instructor doesn't have to be in Sidemount just because you are.
If you are already proficient with sidemount, I would recommend you take the class with that setup. In addition to being able to go with larger tanks, you will already be familiar with valve shutdowns, etc. I did my ANDP with a twinset and regretted every single dive.
IMO, too many people are getting pushed to CCR these days without a solid tech basis and/or need for a CCR to begin with. Not trying to open a can of worms, but OP gaining experience in their chosen OC configuration and working through bottle handling with multiple deco bottles will be of significantly more use. And of course building physical fitness along the way to be able to comfortably handle the gear and task loading.
If you are in doubles, you already have built in redundancy, so would not need additional "bailout" per se. If you are talking about a separate gas for accelerated deco or additional bottle(s) of the same gas as your doubles (bottom stage), that would not necessarily require additional strength if you don those tanks in the water (like you would with side mount). So with 2x50s on your back you could add an 80 or 40 of bottom gas and/or a 40 or 80 of deco gas by putting those tanks in (or by) the water first and then putting them on after you get in. This would give you 140, 180, or even 220 cf of gas for your dive and deco without any extra weight walking around out of the water beyond the double 50s.
Let's say "pretty low SAC" = 0.5 cfm. That's 2.5 cfm @130'. So 100 cf / 2.5 = 40 min @130'. You intend to spend 30 min @130' so that gives you only 25 cf of remaining gas for your deco (presumably all or mostly at 20', where you'll burn 0.8 cfm). So 25 cf / 0.8 = 30 min @ 20'. That should be plenty of time for deco from 30 min @ 130', but now you have no reserve of gas and no gas for sharing with a buddy on ascent if needed.
With double LP50s, there are 2 ways to solve that. First is the method I described above in which you take an additional AL80 stage, which nearly doubles your gas without making you unwieldy out of the water. Under this method you would breathe the stage first, and when it reaches (near) empty (80 / 2.5 = 32) right about the endo of your 30 min bottom time @130', you switch to the doubles and start your ascent and deco. At this point you still have nearly the full 100 cf in the doubles (minus the negligible amount you will have used in the wing and dry suit, so more than enough for you and a stressed buddy to share on a controlled ascent and full deco on only your doubles. Let's say the stress raises your and your buddy's SAC to 1.0 each. That's still only 3.2 cfm for 2 people at 20'. There's your half hour deco (more than you will actually have to do, btw). But if your buddy does not have an out of air disaster, you will only use about 5 or 10 cf for ascent and deco and will finish the dive with 80 or 90 cf left (about half what you started with).
The second way would be to get a "cave fill" in the double LP50s (i.e. fill 2640 psi tanks to 3600 - 4000 psi). This is, of course, controversial, and has been for the 30+ years people have been doing it, but it does give you up to 150 cf in a set of double 50s. Based on some of the calculations above, This will put you at about half your starting gas at the end of your 30 min @130' and you should finish normal deco with about 1/3 of your starting gas (or more). In the case of shared air deco under stress, you should have enough for 2 people to do up to 20 min deco @20'. This is cutting a bit closer than the other method, but there are no extra tanks to carry.
Rule of thirds is for overhead environments, mostly caves, where you must stay at depth for most of the exit (i.e. about as long as it took to get to the turn point. For OW diving, with or without deco, it is more appropriate to do the "rock bottom" gas calculations along the lines of the examples above.
Not excessive, as described above, but also not cumbersome if done in one of the ways described above.
And (disclaimer) for those other than the OP, bear in mind the above is a bunch of rough calculations using round numbers and broad assumptions for SAC rate, descent rates, ascent rates, square profiles, no deep stops, etc. Cut me some slack on the exact numbers, because, yes, I did not take everything into account.
My very first experience diving doubles involved PST HP80's (3,500 psig), the predecessor to the PST E7 80's (2,442 psig): Shortly after I returned from my Cavern and Basic Cave courses (in 1988, which used a Y-valve on a single), I purchased some used Al80's doubles bands from my LDS, doubled up my still-new HP80's using these bands, mounted them on my 3rd-gen Scubapro Stab Jacket (the orange, taco pocket model) as independent doubles, and went on my first post-cert cave dive (in Roubidoux Springs, MO).
Fun times! Plenty of gas, even diving sixths. But these HP80 doubles are quite negative for wetsuit diving in a 0.25" farmer John and shorty. And these cylinders are quite short (20 inches).
A few years later I purchased PST HP100's (3,500 pisg) and a doubles isolation manifold and BP/W as my first rig for Great Lakes deep diving in a drysuit. These 24" cylinders worked much better for me (6'2") for this type of diving. Even smallish divers seem to have no problem with these cylinders as doubles. And life is so much simpler on tech dives if buddy pairs are wearing the same type of cylinders.
rx7diver
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