How low with spare air

[dumpsterDiver]

You are honestly going to try to make the arguement that an al 40 is heavy? or cumbersome? Im not buying it. If the added weight of a 40 is too heavy for someone, than they need to go to the gym bro.

I think the reason most divers stray away from AL 80's has less to do with overall weight and more to do with piss poor bouyancy properties.

All that being said, I have no issues with divers using 19's......i use them too, but after 100' i dont consider them sufficient. MY opinion of course. You are entitled to yours as well.

Hell yeah it is cumbersome. I dive with a 149 cu-ft tank, quite often and adding weight to it is a problem.

 

[DevonDiver]

If we adjust some of the assumptions, we can justify a much smaller bottle...

Don't assume that the diver is going to totally freak out on the bottom, you just gotta stick the reg in your mouth and go up. In other words, why assume it takes 60 seconds to leave the bottom?

Don't assume the breathing rate is going to go super high, I would be comfortable in assuming a SAC of 1,0 ft^3/min.

Don't assume that the ascent rate will be 30 feet per minutes for the start of the ascent, in reality if people are stressed they will probably go FASTER. In the very unlikely event of a total scuba failure, I am willing to revert to the "old" ascent rate of 60 fpm (at least for the first 30 or 50 feet).

Don't assume that the diver will do a safety stop (again in the extremely unlikely event of a total scuba failure).

Why not simply apply a policy of "Don't assume anything will go wrong at all"? That'd justify no pony, no spare air... no reserve in the primary cylinder.

Just always 'err on the side of incaution'. Assume that your performance will be spectacular, that your psychological strength is immense, that your physiological conditioning is superior, that your reactions will be split-second and that your problem solving will be instantaneous and flawless... Assume that all of these factors are inherently guaranteed and absolutely certain to overcome any feasible problem, without reliance of any special protocols or redundant equipment...

The reality of the matter is that scuba units rarely fail completely.

...and hence, divers rarely die. But sometimes they do. I'd suggest that many of those that perish (excluding pre-existing medical conditions) , do so in otherwise preventable accidents. Accidents preventable if training, planning and equipping had occurred in preparation for the 'worst case' scenario, not an assumption of the 'best case'.

Worst case planning has to include assumptions such as; increase in SAC due to incident stress, delays in ascent, initial difficulties with situational awareness and emergency diagnosis, etc. Otherwise is it not worst case planning. It becomes an argument of convenience, trading assurance for assumption.

 

[Foxfish]

Thanks folks for these good responses. The general consensus is that you plan your dive gas requirements ignoring the spare air. There are differences of opinion about how big the spare air bottle would need to be.

 

[redacted]

You are honestly going to try to make the arguement that an al 40 is heavy? or cumbersome? Im not buying it. If the added weight of a 40 is too heavy for someone, than they need to go to the gym bro.

I think the reason most divers stray away from AL 80's has less to do with overall weight and more to do with piss poor bouyancy properties.

All that being said, I have no issues with divers using 19's......i use them too, but after 100' i dont consider them sufficient. MY opinion of course. You are entitled to yours as well.

It is both heavy and cumbersome if you are trying to take it on a commercial flight with you. Even a 19 may just be a bit too much. A 13 is mighty attractive if you want to fly with a pony.

When you don't have to fly, then larger ponies make sense. But beware, there are some live-a-boards that limit the size of ponies and 40's may not allowed.

As far as how much gas is required. I have no problem doing my normal slow assents and long rest stops from 80 feet with a 13 in practice. A 19 is more than enough for most recreational diving. If you need to get to a mooring line, then something larger may be appropriate. When you have a pony and have practiced with it, you are not OOA. You just have to change regulators and calmly and safely terminate your dive in a normal manner. If you have to increase ascent rates and reduce SS to allow a smaller pony, then that is a planning option also.

 

[PfcAJ]

well, at least the guy giving scuba advice is the same one who ran out of air at 130' while solo.

seems legit.

 

[GrumpyOldGuy]

It is both heavy and cumbersome if you are trying to take it on a commercial flight with you. Even a 19 may just be a bit too much. A 13 is mighty attractive if you want to fly with a pony.

When you don't have to fly, then larger ponies make sense. But beware, there are some live-a-boards that limit the size of ponies and 40's may not allowed.

As far as how much gas is required. I have no problem doing my normal slow assents and long rest stops from 80 feet with a 13 in practice. A 19 is more than enough for most recreational diving. If you need to get to a mooring line, then something larger may be appropriate. When you have a pony and have practiced with it, you are not OOA. You just have to change regulators and calmly and safely terminate your dive in a normal manner. If you have to increase ascent rates and reduce SS to allow a smaller pony, then that is a planning option also.

I have a similar opinion. I have a 6,13 and 30 cu ft pony bottles. The 30 never travels, a PITA. I know I can ascend from 75' with 3 cu ft of air from practice (no stops), so the 6 gets used when I know I will not be deep, but its marginal. The 13 becomes the bottle of choice. A 30 or large pony bottle used for local dives does you no good traveling. If all your diving is local, its not really an issue but once you start traveling size becomes an issue.

 

[TSandM]

The first line of defense against needing a Spare Air is gas planning. The second is monitoring your gas.

Catastrophic failures that cause your air supply to stop instantly are rare as hen's teeth. Most failures will cause a freeflow, or a reg to breathe wet but still be breathable. (Remember, you always have another second stage, so ANY second stage malfunction except a wide-open freeflow can be solved simply by putting your other second stage in your own mouth.) Freeflows will give you time to get to a buddy, or to get to the surface.

If you are solo diving, you have given up the redundancy of having a buddy's gas available, and carrying a completely redundant gas supply is prudent. If you are buddy diving, you are better served by keeping your redundant gas supply at hand. If you don't trust your buddy, keep your dives shallower and simpler and monitor your gas.

If you do deep or complex dives with unknown buddies on a regular basis, you might have some thinking to do about things other than how big your pony bottle ought to be.

 

[DevonDiver]

There are differences of opinion about how big the spare air bottle would need to be.

There are only two sizes, when it comes to redundant/emergency gas: Sufficient and Insufficient.

We can argue hypothetically about the scope and nature of the emergency to be dealt with; limiting that scope/nature would permit a smaller bottle to be carried. Increasing the scope/nature has the opposite effect. None of those hypothetical discussions have any bearing on the risks you are exposed to - how, when, where or why Murphy might join your dive and try to kill you.

Most opinions start with an initial premise that you plan the redundant capacity based on the need for a safe, normal ascent, accounting for your SAC/RMV versus the dive depth (and/or overhead obligations).

IMHO, it is reasonable to conclude that there is little point 'prolonging' the emergency by planning for an emergency ascent, rather than a safe, normal ascent; which rules out the smallest capacity cylinders ("spare air" and tiny ponies) for most divers.

I believe it is also reasonable to account for a less than idea response by the diver. Emergencies, in most cases, do cause psychological and physical stress. That stress causes errors to be made...and causes psychological and physiological responses in the diver, for instance; the respiration rate accelerates and problem solving capacity deteriorates. I feel it is prudent to anticipate this and account for it within gas reserve/redundancy calculations. This anticipation requires us to account for more than a simple, direct ascent to the surface.

We need to account for the time it takes to recognize the emergency, to evaluate options, to choose a response...and to conduct that response. Under emergency stress, that time can be considerably longer than we might anticipate based on conducting a hypothetical scenario or training simulation. We also need to account for some time to rectify mistakes we might make; to recognize an error, re-formulate our response and put that into motion. Under stress, we can make mistakes.

We need to account for a less than optimum, timely ascent. The diver can start the emergency, or become, negatively buoyant (over-dumping)when OOA in their primary cylinder. They may have to surface using propulsion alone - raising SAC/RMV. Proper weighting therefore becomes critical; as does an ingrained skill to implement oral BCD inflation. Again, when stressed, mistakes will be made.

One factor remains constant. GAS = TIME. That is true for any emergency underwater. If the diver has TIME, they can cope with mistakes, or slow decision making, or glitches, or ascent problems, or knock-on problems arising... or a million other variables that could otherwise lead to a terminal outcome; because TIME = LIFE... and the purpose of emergency planning is to ensure we live.

 

[tomfcrist]

You are honestly going to try to make the arguement that an al 40 is heavy? or cumbersome? Im not buying it. If the added weight of a 40 is too heavy for someone, than they need to go to the gym bro.

I think the reason most divers stray away from AL 80's has less to do with overall weight and more to do with piss poor bouyancy properties.

All that being said, I have no issues with divers using 19's......i use them too, but after 100' i dont consider them sufficient. MY opinion of course. You are entitled to yours as well.

Hell yeah it is cumbersome. I dive with a 149 cu-ft tank, quite often and adding weight to it is a problem.

Maybe our idea of cumbersome is different. I dive double lp121's and an al 80 and can manage it.... I would bet that the majority of divers can handle diving an AL80 and slinging a 40. Traveling on planes with that stuff is quite another story.

 

[dumpsterDiver]

well, at least the guy giving scuba advice is the same one who ran out of air at 130' while solo.

seems legit.

Who better to give advice on the use of the gear in a real emergency? LOL. I did use a 6..

---------- Post added November 20th, 2013 at 10:06 PM ----------

Why not simply apply a policy of "Don't assume anything will go wrong at all"? That'd justify no pony, no spare air... no reserve in the primary cylinder.

Just always 'err on the side of incaution'. Assume that your performance will be spectacular, that your psychological strength is immense, that your physiological conditioning is superior, that your reactions will be split-second and that your problem solving will be instantaneous and flawless... Assume that all of these factors are inherently guaranteed and absolutely certain to overcome any feasible problem, without reliance of any special protocols or redundant equipment...



...and hence, divers rarely die. But sometimes they do. I'd suggest that many of those that perish (excluding pre-existing medical conditions) , do so in otherwise preventable accidents. Accidents preventable if training, planning and equipping had occurred in preparation for the 'worst case' scenario, not an assumption of the 'best case'.

Worst case planning has to include assumptions such as; increase in SAC due to incident stress, delays in ascent, initial difficulties with situational awareness and emergency diagnosis, etc. Otherwise is it not worst case planning. It becomes an argument of convenience, trading assurance for assumption.

Very much so. It is an argument of convenience. Of course a 40 would be "safer" than a smaller tank, but does the inconvenience justify the marginal safety? The answer is right in your face and the answer is "NO"! Look at the diving population.. are they wearing 40 cu-ft pony bottles for 65 ft dives????