Optimizing bottom time on repetitive deep dive.

[CuNanoTube]

Sounds like you know where the limits are, and want to push them...slightly.

For dives at 120 feet EAN32 is beyond of the 1.4 partial pressure safety zone (~106ft), so Nitrox is out of the question for the first dive - but OK for the other 3.

Since you brought it up... Are the dives part of the AOW class? Typically first AOW dive is just to 100 feet, to get your feet wet and focus on just the depth, and not combined with something like a wreck dive or maximum bottom time. Keep it simple, don't try to do too many things at once, work up to it.

It sounds like an awesome series of dives, but you might focus on simply doing a deep dive first, a few more to get comfortable, and then start looking at extending bottom time on some future dives.

Even if you were AOW cert you really can't extend your bottom time beyond your DC limts without wandering into the territory of mandatory decompression stops. You can, however, use a very gradual and slow ascent to keep you closer to the bottom and work up to extend overall dive time. But that takes practice.

- Dack

I am doing my AOW "Free-Style". We are a teaching club, so I just have to get the PADI advanced self learn text (My Basic was NAUI) and pay for the PADI fees and Martin (my "instructor", 4 of the club members are PADI / NAUI instructors) will insert the mandatory evaluations on a club dives over the season. It works out to being the cheapest way of doing it, and I can dive with my father the whole time (allowing me a good buddy, free transport, and the occasional "bonus" from the shop). I actually was going to do my AOW via NAUI, but the cost of gas alone getting to the shop for in classroom training changed my mind.

 

[TSandM]

We will both be nirtox trained by the dives (not that the math is any hard) however we will not be able to clean our tanks in time for the trip. I do however have an Al40 / an Al 19 at my disposal that could be filled with nitrox.

It sounds to me as though you are contemplating staged decompression using rich Nitrox mixes. Do not do this. There are big hazards involved that you don't understand at this point, including the lethal risk of oxygen toxicity, should you lose buoyancy control or otherwise exceed safe ppO2s on your "deco" gas.

The very simple fact is that repetitive dives in the depth range you are describing are really not very well done on air and with single tanks. As you have discovered while planning, bottom time at these depths is almost ridiculously short, and one might ask whether it is worth doing the dive at all.

Wall dives, that permit a multi-level profile, are a different story, but wrecks, with square profiles, are frustrating at these depths. This is a large part of the reason that people who really like wreck diving go on to get more advanced training, so they can "overstay their welcome" at depth, and do the appropriate decompression. But the minute you obligate yourself to decompression stops, many things change . . . You have to commit to solving all problems underwater, since the surface is not an option. You have to manage your gas differently. And, if you are going to accelerate decompression, you have to have a procedure for safe gas switching, and the control to execute it even under stress. The training for such dives is advanced and complex and difficult, as it should be.

If I were you, I wouldn't sign up for the second day of deep dives. If there is another, shallower option in the area, I'd do that.

 

[AfterDark]

It sounds to me as though you are contemplating staged decompression using rich Nitrox mixes. Do not do this. There are big hazards involved that you don't understand at this point, including the lethal risk of oxygen toxicity, should you lose buoyancy control or otherwise exceed safe ppO2s on your "deco" gas.

The very simple fact is that repetitive dives in the depth range you are describing are really not very well done on air and with single tanks. As you have discovered while planning, bottom time at these depths is almost ridiculously short, and one might ask whether it is worth doing the dive at all.

Wall dives, that permit a multi-level profile, are a different story, but wrecks, with square profiles, are frustrating at these depths. This is a large part of the reason that people who really like wreck diving go on to get more advanced training, so they can "overstay their welcome" at depth, and do the appropriate decompression. But the minute you obligate yourself to decompression stops, many things change . . . You have to commit to solving all problems underwater, since the surface is not an option. You have to manage your gas differently. And, if you are going to accelerate decompression, you have to have a procedure for safe gas switching, and the control to execute it even under stress. The training for such dives is advanced and complex and difficult, as it should be.

If I were you, I wouldn't sign up for the second day of deep dives. If there is another, shallower option in the area, I'd do that.

There it is right there everything you need to know about your question. Heed it. Don't wind up in the A&I section

 

[DaleC]

As this is in Advanced SCUBA this first part is just for discussion purposes and not meant to be advise to the OP.

The only way to reduce your nitrogen ongassing, and thus your penalty for a repetitive dive, is to either extend your SI or limit the initial uptake of nitrogen during the dive. Not withstanding the whole "trimix vs deep air" debate, you can still use nitrox at 120', it would be 30% at a PO2 of 1.4. Here's the formula: 1.4/(120/33+1). Taking longer stops at depth will reduce the risk of bubble formation (DCS) but it won't speed up nitrogen off loading. You would off gas fastest on the surface (all breathing gasses being equal). You could use richer mixes on ascent (deco gas) but as Lynne states, that comes with complications. You could also breath a rich mix on the surface during your SI to speed offgassing further but that also comes with complications both practical and theoretical.

So, in short, your available variables, given the parameters of: air dives, no deco are:

Decompression model used
Duration of dive(s)
Depth of dive(s)
Surface interval time

I personally don't like the profiles being described as NDL dives (120' followed by 90') not because they aren't doable in practice but because they don't seem to offer much, divewise, for the risk. According to the PADI tables, on air they give only 13 minutes BT (which includes descent time) for the first dive and 16 minutes BT for the second (again including descent time) after an hour SI . And that's pushing the NDL to the limit on both dives which is a risky proposition. I'm not sure how much the trip costs but you'll only get 29 minutes bottom time out of day one.

Of course, all tables and decompression models are just guestimates at what will happen; personal results may vary. Your general health, age, hydration, level of rest, temperature of water, ability to off gas, ascent rate etc... will determine whether or not you get bent. By pushing the limits on both dives you are betting that you will not come up short in any of those areas. At 25-50 dives you have to honestly ask yourself how much of a fudge factor you may require.

How much air will you need (for dive one)?
With a SAC of .75cuft/min (for discussion sake), 13 minutes BT will require 45cuft of air. Here's the formula: 120'/33'+1atm X .75cuft X 13min's.
For a minimum rock bottom reserve at a stressed SAC of 1.5cuft/min (double) you will need approximately 76cuft. Here's the formula:
2 min's problem solving at depth - 14cuft (120'/33'+1 X 1.5cuft X 2min's).
Ascent from 120-99' - 5cuft (5atm's X 1.5cuft X .66min's)
Ascent from 99-66' - 6cuft (4 X 1.5 X 1)
Ascent from 66-33' - 4.5cuft (3 X 1.5 X 1)
Ascent from 33-0' - 3cuft (2 X 1.5 X 1)
3 min. manditory SS - 6.5cuft (1.45atm's X 1.5cuft X 3min's)
Sub total - 39cuft (rounding up) X 2 (you and buddy)
Total rock bottom reserve - 78cuft.
BT + rock bottom reserve required: 123cuft.
Someone could check my math as it is past my nappy nap time
That of course, is the minimum air required at those SAC rates and leaves little room for unanticipated issues.


As far as doing these dives without AOW but with an AOW instructor. That's great for him, at least he will be prepared. You of course, will be putting your safety in another persons hands and hoping for the best. something to think about...

 

[dumpsterDiver]

Stay shallow, use the richest mix that is safe and if you stay out of deco on the first dive, you will have more time on the second.

Learning how to descend as fast as possible will increase your time on the bottom.

Not much else you can do to get more time out of your computer without adding things that would constitute technical diving.

 

[Blackwood]

For a minimum rock bottom reserve at a stressed SAC of 1.5cuft/min (double)
[snip]
Someone could check my math as it is past my nappy nap time

I'm not going to check the math, but I'll take you at your word. If you double your consumption rate for stress, you should be carrying 3CFM/ATA in your calcs since rock bottom is traditionally about the minimum gas required for two divers.

Edit: never mind, just saw the step where you double the total requirement.

 

[elan]

Sounds like you know where the limits are, and want to push them...slightly.

For dives at 120 feet EAN32 is beyond of the 1.4 partial pressure safety zone (~106ft), so Nitrox is out of the question for the first dive - but OK for the other 3.

Since you brought it up... Are the dives part of the AOW class? Typically first AOW dive is just to 100 feet, to get your feet wet and focus on just the depth, and not combined with something like a wreck dive or maximum bottom time. Keep it simple, don't try to do too many things at once, work up to it.

It sounds like an awesome series of dives, but you might focus on simply doing a deep dive first, a few more to get comfortable, and then start looking at extending bottom time on some future dives.

Even if you were AOW cert you really can't extend your bottom time beyond your DC limts without wandering into the territory of mandatory decompression stops. You can, however, use a very gradual and slow ascent to keep you closer to the bottom and work up to extend overall dive time. But that takes practice.

- Dack

Nitrox is not out of question. The 1.4 PPO for 32 gives a 111ft depth limit. depending on the vis. the OP can probably stay at 110 and be within the limit. I would me more concerned with the narcosis at that depth.

Another thing to consider is amount of gas you have to take with you. I did my calculations yesterday for a 100ft dive that we plan to do this weekend and for ~ 30 mins on the bottom on 32 I will have to take at least 130cuf of gas, given 0.7 cuf /min and rock bottom 40cuf.

Well this is pushing the realm of a single HP130 tank. If you take the pony you can reduce the rock bottom for your back tank in half but that would still be 110 cuf. We do such dives with double AL80 or LP108.

 

[DaleC]

I do have some sympathies for the OP. It is a problem when doing destination dives to want to cram as much as possible into the trip. It requires a certain degree of self discipline to say no when the opportunity presents itself like that. Looking at the link provided though, someone in the club ought to be raising at least a yellow flag at the thought of taking a non AOW diver on 4 advanced dives in two days.

 

[Dive Bug Bit Me]

I do agree that this does not look like an appropriate dive trip for the OP. However, in his defense only he will know his experience level. Has he been to 120 ft in a dry suit before?

My throw away piece of advice for the OP is that while the wrecks lie in 90ft to 120ft water, the best part is generally not in the sand at the bottom. I do not know the wrecks in question, but relief could easily be 20ft to 30ft. By staying on top of the wreck (possibly in the 60ft to 90ft range) you can see more interesting sights and have longer bottom time. Penetration is a really bad idea.

As an off topic comment, it did astonish me when I moved to the US how scuba rules are enforced here. In South Africa you could not get onto a dive boat without a cert that is required for the dive in question. In the US I have been to 125 ft and on countless night dives with OW divers. Conversely in South Africa all I needed to do was arrive at a shop to get a fill. Here I seem to need to present my C-card each time. Sadly my take away is the choice of which "rules" are important seems to be regional.

 

[ianr33]

Sounds like you know where the limits are, and want to push them...slightly.

For dives at 120 feet EAN32 is beyond of the 1.4 partial pressure safety zone (~106ft), so Nitrox is out of the question for the first dive - but OK for the other 3.

30% nitrox would work nicely though. It does come in different flavors!