Spare Air or Pony?

[Stone]

Spydertek,

Try refreshing the screen after you click on the link. If that doesn’t work, here is the text on the “delayed ascent” chart:

Each Diver starts breathing a 3 cubic foot Spare Air at 100 fsw. Both will ascend at 30 feet per min. Chart on left has Diver A ascending immediately, Chart on right has Diver B taking about 25 seconds longer to begin ascent. Read charts from right to left. As each Diver ascends, he is using air; however, Boyle's Law increases the volume of air available. All things being equal, just 25 seconds longer at depth will cause Diver B to run out of air at about 45 ft

Tank Volume (cf): 3
Breaths per minute: 12
Volume per Breath (cf): 0.057
Ascent Rate (ft/min): 30

The Y axis is Minutes of Air Remaining because, starting at 100 fsw, the divers start using the available air. The graphs are curved because Boyle's Law is non-linear in relation to the pressure in the water column. Volume decreases by half in the first 33 feet of salt water, but if you double the depth to 66 fsw the volume is decreased by one third. The volume doesn’t decrease by half again until you are at 99 fsw. The deeper you are, the “flatter” the curve is that describes the volume of air available. When you are deep, but ascending, you consume the air faster than Boyle's Law can increase the volume. As you get shallower, Boyle's Law increases the volume of air faster than you can consume it. This give the curves their "bathtub" shape.

I hope this is clearer than mud (I mean silt)

 

[fazlo]

i would have to vote for Sapre Air, which I have been using since they came out with it years ago. The idea is to use it in an emergencey, at least till you spot your buddy. S a rule, IF I go diving with a new buddy, we Have to agree on the rules down there, afterall, we are challanging mothernature and we have to understand and avoid risks. I plan my dive, and I dive my plan.

 

[trymixdiver]

I vote for a pony, but Spare air has its place. If your only doing OW diving and your shallower than 50' theres no reason it wouldnt work just fine. i just feel for the price tag of the spare air its not worth it. i would much perfer to spend that money on a larger capacity pony and another 1st and 2nd stage.

TMD

 

[SpyderTek]

Thanks Stone!

Yup, now it makes perfect sense. Appreciate the clarification.

Clearly,

SpyderTek

 

[Rooster1]

No doubt about it PONY! I once had an instructor tell me a spare air gives u just enough air to bend over look between your legs and kisss your ass good bye. Tests have been done by divers where at 120 feet under controlled conditions (no panic or real emergency) that spare air ran out before they made it to 60 feet.
NO SHUCH THING AS TOO MUCH AIR IN RESERVE! Go pony all the way, bigger better.

 

[solodiver]

OK if a 3 cf air lasts you 2 min on surface then at 33 ft that same air would last you 1 min. so at 100 ft it would only last you 30 sec. or 2 deep breaths

For you to know how long a 3 cf of a 30 cf tank well last you , you need to know your sac. surface air consumption rate.
I have a sac of about .7cf min so at 33 ft i would use 1.4cf per minute 3cf div by .7 = 4.28 on surface. now divide by atm. so for 100 ft 4 atm i would have 67 sec of air

hope this helps
Barry

 

[Stone]

Solodiver,

Your logic is correct; however, if your SAC is .7, then a 3 cf tank will last 4.3 min at the surface.

Coincidentally, the graphs I created use a diver with a SAC of .7
(12 breath/min * .057 cf/breath = .684 cu/min)

 

[solodiver]

Hi Stone.
seems our calculators don't agree
mine says that 3 cf / .7 = 4.28571428571428571428571428571429
Hence the stated 4.28 I always round down.. never up.. or use exact figure. but we are only talking a differance of 2 sec.

and yes you are right thats not 67 sec of air but rather breathing time.

My conclution. Pony is the way to go . that or twinned..

Barry

 

[Stone]

Solodiver,

Here's what confused me. Your first paragraph says:

OK if a 3 cf air lasts you 2 min on surface then at 33 ft that same air would last you 1 min. so at 100 ft it would only last you 30 sec. or 2 deep breaths

I see, now, that this paragraph is just an example and not related to your actual SAC of .7, and the second paragraph is the only one related to your SAC.

I thought you had come up with 2 minutes by accidently multiplying 3 cf by .7 instead of dividing by .7. So as you can see, I thought I was correcting a 2 minute error, not a 2 second rounding error.

My mistake!

 

[solodiver]

of course all this relates to a relaxed calm diver. Once your stressed or Paniced your breathing rate well change. and find you could probibly cut your figures by a third or half.

I know.. i'm a pain.. LOL

Sorry if my earlier post confussed you.. about my sac.

Barry