19 or 30 cubic feet?

[stimpy4242]

19 cubic feet equals 532 litres. Average breath is 500ml. So in one minute an average person breathes about 8 litres/minute.
For Argument I went ahead and assumed a respiratory rate of 40 and the same tidal volume. Truth be told, tidal volume would probably decrease as rate increases, that is how it works...but just to continue to prove my point...Look for the new information in RED.

So at 60 fpm it takes 2.5 minutes to ascend plus a 8 minute safety stop? Lets just say 10 for the heck of it. So that is 12.5 minutes or 100 litres.

Ok great, well thats all if you were breathing on the surface, so lets take it in chunks, but I will show you that for right now, there is no need to even take it in chunks, because if you were doing 12.5 minutes at a depth of 132, then you can just multiply that 100 by 5 since you are breathing 5 times more than at the surface. So that would be 500 litres. Which still give you a remaining 32 litres. We of course know, that we are ascending this whole time and doing a conservative safety stop, so a 19 cf bottle is by far enough for a recreational dive.

Lets break it into chunks for the heck of it assuming that we are breathing the greatest amounts until we hit the next atmosphere:
132 ft 5 atmospheres - takes us 33 seconds to get 33 feet to 4 atmospheres. So that is 8 litres/60 seconds x 33seconds x 5 (atm)= 22 litres. or 55
99 ft 4 atmospheres - take us 33 seconds to get 33 more feet to 3 atmospheres. So that is 8 litres / 60 x 33 x 4 = 17.6 litres. or 44
66 ft 3 atmospheres - takes us 33 seconds to get 33 more feet to 2 atmospheres. So that is 8 / 60 x33 x 3 = 13.2 litres. or 33
33 ft 2 atmospheres - takes us 18 seconds to get 18 more feet to 1.5 atmospheres (15 feet). So that is 8 / 60 x 18 x 2 = 4.8 litres. 12
15 ft 1.5 atmospheres - We spend 10 minutes doing a safety stop here. So that is 8 litres/min x 10 minutes x 1.5 = 120 litres. or 300
15 ft 1 atmospheres - takes us 15 seconds to get 15 remaining feet to the surface. So that is 8 / 60 x 15 x 1 = 2 litres. or 5

So what does that give us for a more realistic grand total? Well - 22+17.6+13.2+4.8+120+2 = 179.6 litres total. Given a 19 cf bottle which holds 532 litres that gives us a safety factor of 2.96. So nearly 3 times safety factor.

New grand total = 55+44+33+12+300+5=449 litres. Which is still a safety factor of 1.2. And it ASSUMED An incredibly generous Minute Volume of 20 litres/min which is attributed to a human doing strenuous excercise, which I argue most of the dive isn't! Most of the dive is leisure swimming. You are supposed to be neutrally bouyant right???

IMHO that is more than enough and the bulkiness of a 30 is not worth it for a recreational diver. Find out how many people that answered you are technical divers and either use their bottle for a stage bottle as well. I mean most the people mentioned using it as such....

Please read above and notice all the things in red, this assumes a minute volume of 20 litres/min. This is medically considered what a human being breathes at during strenuous exercise, which arguably diving is not that strenuous all the time especially when practicig good bouyancy. 20 litres/min if you aren't medical is a Sac of .71 which is pretty realistic. They say an athelete at PEAK PERFORMANCE breathes at 65 times per minute with tidal volume of 500ml is a SAC of 1.07 and I wouldn't come close to comparing any of our dives to a peak performanc athlete!

 

[stimpy4242]

(2) if I decide to use it myself, I want enough gas left in it to be useful on at least one or two more dives.
Rick

Just curious when would you not refill your pony after just having an OOA experience and then choose to go back down knowing the pony you just used, now has a lot less air in it??? If you had tanks, it would almost seem worth it to get a cross fill valve assuming tanks of the same pressure rating.

 

[SparticleBrane]

Please read above and notice all the things in red, this assumes a minute volume of 20 litres/min. This is medically considered what a human being breathes at during strenuous exercise, which arguably diving is not that strenuous all the time especially when practicig good bouyancy. 20 litres/min if you aren't medical is a Sac of .71 which is pretty realistic. They say an athelete at PEAK PERFORMANCE breathes at 65 times per minute with tidal volume of 500ml is a SAC of 1.07 and I wouldn't come close to comparing any of our dives to a peak performanc athlete!

You are confusing an overweight once a year couch-potato of a "scuba diver" with bad buoyancy, bad trim, waving arms...with an athlete. I can certainly see how you would have mistaken the two. Diving IS strenuous exercise to people like that -- they breathe a LOT due to CO2 buildup from the aforementioned actions.

Someone who only moves when they need to, practices good trim, buoyancy, and technique, will have a breathing rate much lower than 0.71cf/min.

 

[doczerothree]

Depends on the type of diving your doing. a 30 ft3 or 40 ft3 would more than likely hang D-rings off your BC (that is where I put mind although I've seen the rigged off tanks) A 19 you could attach easily to your plate, BC or rig to your tanks. it depends on where you are comfortable haviving it.Equipment Configuration | Global Underwater Explorers Check out this site.

 

[stimpy4242]

You are confusing an overweight once a year couch-potato of a "scuba diver" with bad buoyancy, bad trim, waving arms...with an athlete. I can certainly see how you would have mistaken the two. Diving IS strenuous exercise to people like that -- they breathe a LOT due to CO2 buildup from the aforementioned actions.

Someone who only moves when they need to, practices good trim, buoyancy, and technique, will have a breathing rate much lower than 0.71cf/min.

Come on, lets be honest that person you just described doesn't even know what a pony bottle is. They are not who we are talking to. Someone who is interested in a pony bottle probably dives a bit more than what you described. Lets keep everything realistic and not shoot for the outliars? With that being said, I would still disagree that the person you described would still not be huffing the entire time like that.

 

[dannobee]

The 1.0 cu ft/min SAC rate that Sparticle claims is common. On a sedentary dive in warm water, mine hovers around 0.35. In cold water with a drysuit and not so sedentary, it's around 0.5-0.6, and that's considered relatively good.

Go do your diving with a tiny pony. Pick a 6 cu ft tank for all I care. But don't recommend that to someone without giving them all of the correct facts.

 

[jeraldjcook]

Come on, lets be honest that person you just described doesn't even know what a pony bottle is.

You're right. The sad part it "that" person is going to go to 130' with a 3cf Spare Air.

 

[SparticleBrane]

Come on, lets be honest that person you just described doesn't even know what a pony bottle is. They are not who we are talking to. Someone who is interested in a pony bottle probably dives a bit more than what you described. Lets keep everything realistic and not shoot for the outliars? With that being said, I would still disagree that the person you described would still not be huffing the entire time like that.

Most divers lack good trim, buoyancy, non-silting propulsion techniques, and other basic skills...

 

[stimpy4242]

But truth be told a spare air is a rescue device, but for an ascent straight to the surface, dealing with any complications that arise later...hard to deal with complications when you are dead at 130 feet after drowning.

 

[jeraldjcook]

Go do your diving with a tiny pony. Pick a 6 cu ft tank for all I care. But don't recommend that to someone without giving them all of the correct facts.

We've gone well past the "correct facts" of the OP's question. What if you're at 130ft? What if your SAC is 1.5? What if a sea monster punches you in the face, grabs your leg, and slows you ascent? That puny 19cf isn't going to be enough.

I'm planning to use this as a back-up air source for diving in high mountain lakes (8,000') at depths no greater than 60' (usually much shallower) and always within no decompression limits.

If you read the question the OP wants answered, we're talking about depths less than 60ft (add 10ft for altitude if you want). 19cf should be more than adequate for even the heaviest breathers.