Deep Dive and rule of thirds

[Charlie99]

Rather than using some arbitrary rule, one should focus on the real needs.

At any point in the dive, both you and your buddy should have enough air to safely let both you and your buddy finish the dive.

You need to carefully look at what your needs might be, such as:

1. to exit any overhead
2. time to take care of some sort of problem (1 minute is often used as a rule of thumb for how long to assume you will hang around to sort things out before starting our ascent)
3. return to an upline if necessary (figure out what depth this will have to be done. Keep in mind that you might have to stay on or near the bottom to stay in the current shadow of the wreck).
4. Ascent, and any required deco.
5. Any further exit,such as swimming out of a shipping lane (again, you need to figure out what depth this needs to be done at)

The other huge fudge factor is what SAC you assume. For some calculations, you should assume an excited 1cfm per person.

When making your calculations, you need to use some judgement as to what is a required item. For example, you may decide that you will accept the increased risk of not going back to the upline in the event of a reg or tank o-ring failure that causes one of you to lose all gas at depth.

In my case, I have made the judgement that my rock bottom will include all gas for a normal ascent, deep stops, and safety stops for two divers at normal SAC rate, but need only include gas for a 40-60fpm ascent to 3 min SS in the excited combined 2cfm state. Obviously, these are the sort of tradeoffs that you must consider carefully.

After you go through all the math, the rule of thumb of 100psi per 10' is a reasonably close approximation of the amount in an AL80 that is required to meet the basic ascent and safety stop requirement for a dive within NDLs.

Charlie Allen

 

[lamont]

IIRC, your rock bottom on an AL80 should be about 1800 psi down there and at 30 psi/min/atm (0.75 cuft/min @ surface) you'll be burning through almost 150 psi/min. that leaves you with around 8 mins minus the gas that you burn to get down there.

lets see, 8 secs * 3 atm * 2 cuft/min/atm = 48 cu ft / 2.5 = 20 * 100 psi = 2000 psi for rock bottom...

yup. AL80 is not the best tank for this dive.

 

[Michael Schlink]

Depending on the dive site conditions and surface support should we be thinking about a redundant air supply or reg.?

 

[ZenDiver]

so here is my .02. The rule of thirds works well in my experience on large wrecks where you have really no opportunity to see the whole wreck. So 1/3 for exploration and 1/3 for return and 1/3 for reserve assuming you have to come up the anchor line. For small wrecks that you can reach the anchor line from anyplace on the wreck within 2 to 3 minutes I try to ascend with a minimum of 1000 psi regardless between the depths of 80 to 130ft but I have a low sac rate as does my deep diving buddy. Your ascent gas depends on yours and your dive buddies SAC rate and what is comfortable for your safety margins. I advise conservative. What usually happens is that your first third includes your descent and your exploration so your return usually gets you back to the ascent line with about 1500 psi or so. Assuming you want to start your ascent with 1000 psi you have 500 psi to burn off which I spend close to the ascent line. Plenty of time to take pictures and relax. Always follow your dive tables or computer of course and end your dive accordingly. As someone already pointed out that sometimes your turn point is not your air but your NDL so plan that into your turn plan as well. Something like turn at 2000psi or half of your NDL minutes minus your ascent time which ever comes first. As with all "rule of 1/3's" dives the person that reaches the limits first is the one who makes the turn signal. Regardless of making it back to ascent line or not if either of you run dangerously low start your ascent immediately and safely, once on the surface deploy your safety sausage or surface marker. You also mentioned a pony bottle. Take it but do not plan your dive around it. It is for emergencies or equipment failure only IMO. I know too many people that got into trouble using it as a second cylinder to do planned ascents. If you do not know your sac rate or your buddy doesnt know his or hers then I would recommend figuring that out first before planning deep dives like the one you are planning, if you know it already then have fun and plan conservative and watch out for narcosis. Good luck and don't forget your strobe. Deep water light stinks.

 

[theskull]

So, if there is no penetration and I stay close to wreck/line (as theskull suggests) , I could theoretically use the "half tank" of 1500 as a planning number, ie, turn back point. Then the remaining 1500 should be adequate for ascent, 3 min safety stop and (if necessary) share air with buddy on ascent.

btw: I am just starting to learn how to enter a dive plan in my computer, but I started thinking about the scenario. This computer has "simulate" mode so I can "what if" the scenario and then enter as a dive plan. (Well that's my theory, I'm still learning how to use all the features.)

That "sounds" good in theory, but only with certain assumptions that may be true, but not safe to make on-line.
Such as:
1. The descent was relaxed and you both reached depth with nearly full tanks.
2. You both will be checking your air gauges often at depth, especially if you don't make dives to this depth all the time.
3. You and your buddy will stay close to each other and remain alert so that when it is time to go the signal can be made and acknowledged readily--you won't have to go looking for your buddy or swim after him to signal "up".
4. You will not be down-current of the ascent line if the current is stiff.

AND . . . a photographer typically makes a pretty lousy buddy, so the photog's buddy needs to be extra alert and attentive and assume the role of dive leader.

theskull

p.s. I also liked the previous suggestion of taking many if not all of the photos from 10 or 20 feet above the wreck. This gives you more time at depth and makes narcosis less of a factor.

 

[eod]

If I were to do this dive it would have to be on air (no trimix training and nitrox at this depth is almot air anyway) which would mean very little bottomtime. In fact after such dives I usually surface with more than half of my gassupply, as do my buddy, so why would you need the rule of thirds? This is with 12l 200bar bottles by the way, don't know how they compare to al80 at 3000psi.

 

[MXGratefulDiver]

If I were to do this dive it would have to be on air (no trimix training and nitrox at this depth is almot air anyway) which would mean very little bottomtime. In fact after such dives I usually surface with more than half of my gassupply, as do my buddy, so why would you need the rule of thirds? This is with 12l 200bar bottles by the way, don't know how they compare to al80 at 3000psi.

For a dive to 120 fsw I'd use EAN30 ... it'll give you about 8 minutes additional NDL bottom time over air. For a photo shoot, that can be significant.

... Bob (Grateful Diver)

 

[Charlie99]

This is with 12l 200bar bottles by the way, don't know how they compare to al80 at 3000psi.

A typical aluminum 80 such as the Luxfer S080 is 11 liters, 207 bar. The free gas capacity is about 2200 liters, just a bit less than 80 cubic feet.

 

[Uncle Pug]

very little bottomtime. In fact after such dives I usually surface with more than half of my gassupply

I sometimes surface with more than half my gas supply too but it isn't because I cut the dive short.

Spend only as much time as you need at the deepest part of the dive and do so at the beginning of the dive.

Spend the rest of the dive progressively shallower.

Spend as much time above 2ATA as you spent below 3ATA and make the transition from 1.5ATA to 1ATA very very slowly.

Then lay there on the world's largest waterbed for a few minutes.

 

[eod]

For a dive to 120 fsw I'd use EAN30 ... it'll give you about 8 minutes additional NDL bottom time over air. For a photo shoot, that can be significant.

... Bob (Grateful Diver)

Of course you can use nitrox and it makes a difference. When I ran the numbers in my head I thought it would be closer to 25%, my bad, not used to using feet (isn't it 3 feet = 1 meter?). But the point remains the same, even with 8 more minutes the gas supply will last much longer than the bottom time, and I still think I would surface with more than half my supply. Why even bother using rule of thirds?

Hmm, to be honest when I think about it, the rule of thirds isn't violated in this kind of dive (surfaceing with more than half the supply). I just found it confusing why someone would need such a rule for these kind of dives.