Deep Diving Gas Planning

[MarineResearch]

If I had to guess I would say MarineResearch sells air fills.

Nope, but if I did, I would sell them by the CF.

But looking at this hypothetically, and using the conservative Suunto RGBM corrected for 4,000' altitude (A1 setting):

OP dives to 100 feet using a AL80 @ 3000 psi with a SAC rate of 0.9 cf/min in cold water. OP plans a bottom time of 10 minutes. OP sticks to the plan and returns to the surface with 1220 psi. OP exceeded the 1/3 rule by a little, but had gas to spare, especially since he has that second AL80, somewhere.

OP does the same dive, but lingers an extra five minutes at the bottom. OP goes into Deco. No problem. OP returns to the surface with 380 psi. It's more than zero, so no problem. Besides that other AL80 is around, somewhere.

OP does the same dive, but has some problems. An experience level of 19 dives did not prepare the OP for dealing with a dry suit squeeze or cutting that fishing line tangled around his fin. OP manages to correct his problems, but has been down for a whole 20 minutes. OP now has a 16 minute deco obligation. Without that second AL80, OP runs out of air 15 minutes before completing deco and at about 50 feet deep. But OP has that second AL80 hanging from his float at 15'. No problem, OP just needs to do a free ascent to the second AL80, turn it on and breathe. Never mind that pesky 30 ft/min ascent rate. Gas bubbles can't count anyway.

OP should download the Suunto dive manager/planner so he can plan his dives and understand what happens if he does not stick to his plan.

Suunto Dive Manager

 

[MarineResearch]

Why not plan your gas accordingly instead of by some "rule of thumb" that doesn't apply to many OW diving situations?

Lets explore some options. For the sake of simplicity, we're going to assume buddy diving, backgas only, no ponies or stages or deco bottles etc, square profile.

Given the above, its a reasonable idea to start your ascent when you either a)hit your time limit for the dive or b)hit your minimum gas reserve pressure (typically enough for 2 divers to share gas through the ascent while making safety/min deco stops).

So dive option 1 is swim around and enjoy the scenery till one of those two parameters is met. You surface, the boat comes and picks you up, all is well.

Option 2. You'd like to get back to the anchored boat/ shore, but if you don't, its no biggie. Calm day, the boat can come get you, you can swim on the surface, etc. A reasonable plan would be to subtract your minimum gas reserve, swim out till half of the remaining gas is used (or half the time limit) and swim back. At no point in the dive do you have less than the amount it would take to surface.

Option 3. You REALLY need to get back to the upline/shore/out of a hole in the ground, etc. Surfacing immediately just isn't a viable option. This is where "thirds" comes in to play. Same as option two, but you swim out using 1/3, back with 1/3, and end up with 1/3 PLUS minimum gas reserve left over. Now, even if you need to share gas, you'll make it back to the upline with your minimum ascent gas still intact.

A blanket application of the "rule of thirds" is extremely limiting for most recreational, single tank dives. A more sensible approach is to understand proper gas management and apply that knowledge based on the situation.

I would agree with you if the OP was talking about diving in the 60-80 foot range in higher altitudes with only brief excursions to the 100-130 depths mentioned. However, the OP is talking about dives that are right at or beyond NDL for an altitude dive. The OP mentioned diving in the 100-130 foot range, which at 4,000 ft msl have NDLs ranging from 11 to 7 minutes (based on the conservative Suunto RGBM algorithm) and only slightly longer using the altitude-corrected PADI dive tables. The OP was then going to rely on a second AL80 or pony bottle to extend bottom time. Without good planning and discipline, the OP could easily transition from an OW to an overhead dive per his original statement:

I know in most cases the pony bottle should save me, but I'm wondering about those cases where I'm close to NDL and I have an emergency for whatever reason. What kind of decompression load can I get put into in that sort of situation and how much deco time should I plan for in my gas management.

I give the OP a lot of credit for asking the right questions in advance of his dives. It beats asking the question immediately after the situation is encountered. The OP clearly recognizes that what he wants to plan could easily go from an OW to an overhead dive.

If the OP were to strictly dive within the NDL and stick to a planned end pressure for a single tank dive, than the 1/3 rule would not apply. But, the original questions was about gas management for a situation that is no longer an OW dive.

 

[PfcAJ]

So once again, what does "thirds" have to do with it?

All the above guidelines I mentioned still apply, even when deco diving (which the OP shouldn't be doing at this point in time, anyway).

 

[dumpsterDiver]

I missed this one earlier. In deco diving we plan for lost deco scenarios. This means there are times when your deco bottle is not available and you must decompress another way.

You may find your pony bottle can experience failures, too, and will be unavailable and so it will not always save you. The bottom line is that gas failures happen and if you aren't prepared to do deco diving or plan for failures within NDL limits, don't depend on something that can fail because you do not want that one time to be the time it fails.

I don't know statistics on how often deco or pony bottles fail but because it can happen, it is wise to plan your dive to either not rely on it or to mitigate it in case the failure does happen. In your case, it sounds like you just need to bring a single tank large enough to facilite your gas needs/rock bottom, and execute the plan. No need to complicate it with another potential gas failure point.

So you want him to plan his dive as if he has no pony bottle,,, sounds good to me.... But you don't want him to take a pony bottle because of a potential gas failure (presumably in the pony bottle)??? I really don't see the logic in that one.

 

[ianr33]

So once again, what does "thirds" have to do with it?

Thirds is wot them cave divers use. Cave divers is good divers so thirds must be good way to dive.

 

[leabre]

So you want him to plan his dive as if he has no pony bottle,,, sounds good to me.... But you don't want him to take a pony bottle because of a potential gas failure (presumably in the pony bottle)??? I really don't see the logic in that one.

My larger point was missed on this one. He comes across as if he's putting too much faith into the pony bottle without proper planning for the original dive or what would he do of the pony experiences a failure that prevents him from using it.

Bring it, or not, my advice has no preference, but if he's expecting to use it (as a crutch or deco bail-out) and it fails, how will he deal with it? This isn't a planned deco dive so it is not necessary to have it with proper gas planning and management.

My advice to him is not whether he should bring the bottle. My advice is to do proper gas planning and if he wants to depend on the pony bottle, consider also what would he do if that bottle is unable to deliver gas for whatever reason because it is a possibility, however remote.

 

[Dr Wu]

My larger point was missed on this one. He comes across as if he's putting too much faith into the pony bottle without proper planning for the original dive or what would he do of the pony experiences a failure that prevents him from using it.

It would seem he already knows the pony will save him.

Resistance is futile.

 

[DaleC]

I would say assuming so much about people we don't know is futile. Why not stick to what we know, how we do it, and why. Then, whoever is reading the thread can decide if our experience applies to them or not.

 

[MarineResearch]

So once again, what does "thirds" have to do with it?

All the above guidelines I mentioned still apply, even when deco diving (which the OP shouldn't be doing at this point in time, anyway).

Absolutely nothing if the OP dives within his training and NDL. However, if you run his proposed dive profile and add just 5 to 10 minutes to his bottom time, he has a deco obligation that will leave him short or out of air if he doesn't start his ascent with 2/3 of his gas remaining. This was the OPs original question and concern about gas management. If the OP wants to put himself in a situation where he may enter deco, he needs to be aware of what his gas requirements are and plan accordingly. If the OP wants to dive responsibly and within his limits than your options 1 and 2 are appropriate.

 

[dumpsterDiver]

]Absolutely nothing if the OP dives within his training and NDL. However, if you run his proposed dive profile and add just 5 to 10 minutes to his bottom time, he has a deco obligation that will leave him short [/B]or out of air if he doesn't start his ascent with 2/3 of his gas remaining. This was the OPs original question and concern about gas management. If the OP wants to put himself in a situation where he may enter deco, he needs to be aware of what his gas requirements are and plan accordingly. If the OP wants to dive responsibly and within his limits than your options 1 and 2 are appropriate.

REALLY ???? You mean to tell me if a guy dives 130 ft at altitude and he stays just 10 extra minutes at depth he might end up low on air and have a problem getting to the surface safely.... Wow!