Should I get a Spare Air?

[TX65]

The spare air is not going to give, nor was it designed to give, a diver at 100 feet enough gas to do a normal ascent. At best, it will give a couple to a few breathes as you "blow and go" to the surface - aka submarine escape 101.

Spare Air is a nice accessory when used within its operating parameters. For those seeking true bailout, a better option is a compact regulator and an appropriate size pony bottle (13-45 cube).

My CYA plan is to have enough gas to relax, ascend, safety stop and be out of the water.

 

[jonnythan]

loosebits once bubbled...
Assuming a very stressed out diver with an at-surface breathing rate of 1 cu. ft./min (always estimate on the conservative side), a 3 cu. ft. spare air will bring you to the surface from 60' using a 30'/min ascent rate.

No it won't. That ascent requires 3.8 cf at an SAC of 1 cf/min. The ascent will take 2 minutes at an average depth of 30 feet, or 1.9 atm. Your math is quite wrong.

Volume = (depth / 2 / 33 + 1) * (depth / 30)

BTW, a "very stressed out diver" will consume more than 1 cfm.

For more info and some sample calculations go to http://jonnythan.com/rockbottom.html

 

[jonnythan]

loosebits once bubbled...
Oh, 30ftsw is 1 atm.

Not even close. 30 feet is 1.9 atm. 33 feet is 2 atm.

And you don't need any calculus for this stuff. It's completely straight algebra.

 

[WileEDiver]

TX65 once bubbled...
The spare air is not going to give, nor was it designed to give, a diver at 100 feet enough gas to do a normal ascent. At best, it will give a couple to a few breathes as you "blow and go" to the surface - aka submarine escape 101.

Not that I'm an advocate, or even own a Spare Air, but I have heard some divers say that it just has to be enough to get you to your buddy (and his octo).

 

[Stone]

Effect of differing SACs on a 3 cf bottle from 100 fsw (60 fpm ascent)

 

[donacheson]

Uncle Pug once bubbled...

...at least you got the title of the thread right!

SA doesn't cut it at any depth.

Learn proper gas planning and good buddy skills and you will need neither pony nor SA.

You're right. If you solve the first of the equations I gave, which is the one yielding the total air consumption required to get from depth d to the surface, for d as a function of volume of air available (V), surface air consumption rate (b0), and ascent rate (r), then:

d = 33*(sqrt(1+4*r*V/66*b0) - 1)

For V=3 cubic feet, r=30 ft/min, b0=1 cubic ft/min, then d=50.8 feet.

For V=3 cubic feet, r=60 ft/min, b0=1 cubic ft/min, then d=80.9 feet.

For V=3 cubic feet, r=60 ft/min, b0=0.5 cubic ft/min, then d=80.9 feet.

For V=3 cubic feet, r=60 ft/min, b0=0.5 cubic ft/min, then d=124.6 feet.

Not impressive.

 

[RichLockyer]

Stone once bubbled...
Effect of differing SACs on a 3 cf bottle from 100 fsw (60 fpm ascent)

Very good.
IOTW, most of us, in a non-panic state, would barely make it.

 

[Charlie99]

RichLockyer once bubbled...
IOTW, most of us, in a non-panic state, would barely make it.

Actually, in a non-panic state, ESA from 100' at 60 fpm with 0 cubic feet isn't all that hard. What's hard is keeping that "non-panic state" when you have a major reg malfunction at depth.

 

[RichLockyer]

Charlie99 once bubbled...
Actually, in a non-panic state, ESA from 100' at 60 fpm with 0 cubic feet isn't all that hard. What's hard is keeping that "non-panic state" when you have a major reg malfunction at depth.

True. I tend to exhale too quickly and generally run out after rising less than 30ft.

 

[Charlie99]

RichLockyer once bubbled...
True. I tend to exhale too quickly and generally run out after rising less than 30ft.

Before anybody gets to freaked out, I'll explain that I mean that you should keep the airway open and just let the excess air flow out on its own rather than changing your lung volume by actively exhaling.

A 5 minute no reg ascent from 30 meters was described a couple days ago by Dr. Paul Thomas. Scroll down to 7 Jul messages http://www.scubaboard.com/showthread.php?s=&threadid=428&perpage=25&display=&pagenumber=3