Any recreational diver wishing to consider an alternative air source should be commended for foresight and contingency planning.
But those of you who debate one bottle or another are approaching the issue from the wrong direction. The size of the bottle is not the first consideration.
First, figure out what you need IN the bottle. THEN, select a bottle.
Any recreational diver should be able to calculate their Surface Air Consumption rate. This is simple arithmetic. To turn your SAC into your Respiratory Minute Volume (RMV) is more simple arithmetic. Basically this tells you how rapidly you will consume your gas supply.
When you figure yours out, chances are likely that under relaxed feel-good conditions, you'll have an RMV somewhere between .5 to .8. That means that each minute you will consume an average of approximately .6 or .7 of a cubic foot of gas.
When you respond to an emergency situation, however, your heart starts pounding with anxiety and your respiration rate soars rapidly. One general planning figure is 1.0, although some divers have suggested using a figure of 1.2 or 1.3 and they are likely closer to reality. This means for planning purposes, plan on consuming a minumum of 1 cubic foot per minute (cfm) during a real emergency, and its safer to plan using a figure of 1.2 cfm.
You carry an alternate air source so that you don't have to bolt dangerously to the surface. Particularly if you've been diving deeper than 80'-90', this includes allowing for a 3 minute safety stop. A safe ascent rate if you've been diving to depths of between 80' and 120' would be 30 feet per minute.
As an example, planning an emergency ascent from 120', with an ascent rate of 30 fpm and a 1 minute ascent the last 10', would look something like this:
time to assess emergency and transition to ABS: 1 minute
time to ascend from 120' to 10': ~ 4 minutes
safety stop at 10': 3 minutes
ascent to surface: 1 minute
Add it up for 9 minutes and multiply by an emergency breathing rate of 1.2 cubic feet of gas per minute to get a planning figure of 10.8 cubic feet, or round up to 11 cubic feet.
Then consider that emergencies at 120' may be resolveable in one minute, but then again, what if you need three minutes? What if someone needs assistance? If you give yourself four minutes at depth to resolve some unforseen problem, your total cubic feet needed increases to 14 cubic feet (using the 1.2 cfm RMV).
After you've calculated that you might require 14 cubic feet of gas to safely respond to a problem at 120' and make a safe ascent, THEN select the bottle you want to use to carry it in.
Well, you could use a 13 cu ft Luxfer (at a minimum). There is a 14 cubic foot Luxfer (LP @ 2015 psi). Or, you could step up to a 19 cubic foot Luxfer. You likely would reject a 3 cubic foot Spare Air, for obvious reasons.
[And for those who wondered about the scorn that most experienced divers have for spare air systems, it has to do with the above calculations. If you habitually stick to depths of 20 to 30 feet, the 3 cfm in a spare air may work for you. But for the majority of divers, under a majority of circumstances, your own calculations are likely to indicate that Spare Air holds an insufficient amount of gas for your circumstances. Some divers perceive the company to be selling a false sense of security to the uninformed.]
The point is that arguing over which bottle you'd prefer is the wrong way to start. Figure out how much gas you'd need, under your own circumstances. Then pick the bottle.
If you don't know how fast you'd suck gas in a real emergency, use the 1.2 cubic feet per minute figure. You certainly won't be far off. If you want to live dangerously, use a faster ascent rate. It isn't necessary, but if you want to use another figure, fine. If you're not sure, use an ascent rate of 30 feet per minute. But first calculate how much gas you'd require under your own circumstances, THEN pick the tank it will go in.
I suggest that's a better way to go about contingency planning.
My .02 worth. YMMV.
Doc
