vshearer
Contributor
The more I read this thread the more I think buy both. Then just see what you feel like the morning you go diving. 
19 or 30 cubic feet?
- Thread starter rye_a
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19 cubic feet equals 532 litres. Average breath is 500ml. So in one minute an average person breathes about 8 litres/minute.
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.
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.
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.
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.
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.
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.
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.
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....
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.
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.
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.
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.
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.
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.
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.
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....
Are you sure? I think the average person breaths about 15-20 liters per minute and alot more in an out of air situation. From 30m depth out of air you need about 1 min of getting your act together, keeping ascent rate at 9m/min, 1 minute stop at half depth and 3 minutes of safety stop within the first ATA of water an AL19 just does not cut it.
jeraldjcook
Contributor
Respiratory minute volume - Wikipedia, the free encyclopedia
WIKIPEDIA:
Still, even at 4x the normal rate/volume of breathing you'd still make it okay. The calculation below accounts for a 10min safety stop. Within rec limits I do a 5 minute stop and you shouldn't technically need to to a stop at all, it's a safety stop not a deco stop. Cut that 10min down to 5 min and you're only at 120 liters. 120 x 4 = 480 < 540 (amount in 19cf). Plus, chances are once you hit your safety stop you would have calmed yourself down a little and your breathing rate would have dropped back down.
I am sorry, look at jerladjcook trying to take all my glamour...didn't I just post a nice elaborate response with numbers and all? Just messing with you.
Again, I don't need wikipedia to tell me what the rate is above, but I do thank you for posting it to support my calculations. I know from being in the medical field. Truth is that a tidal volume of 500ml is pretty high. Thats why wiki came up with 5-8, with 8 litres per minute, being on the high end so it was the calculation I used which still shows the 3x safety factor.
A human breathes 12-20 times per minute, using a 500ml tidal volume and 16 as the mid range of the rate, we get 8 litres per minute, or an 8 litre minute volume.
Also shellbackdiver1. I don't recall a "safety stop" at half depth in my no decompression diving table rules. I do know of a 8 minute safety stop at 15 feet / 5m. That 8 minutes is for any dive deeper than 100 feet, a safety stop of 5 minutes at 15 feet/5m is recommended all the time.
Again, I don't need wikipedia to tell me what the rate is above, but I do thank you for posting it to support my calculations. I know from being in the medical field. Truth is that a tidal volume of 500ml is pretty high. Thats why wiki came up with 5-8, with 8 litres per minute, being on the high end so it was the calculation I used which still shows the 3x safety factor.
A human breathes 12-20 times per minute, using a 500ml tidal volume and 16 as the mid range of the rate, we get 8 litres per minute, or an 8 litre minute volume.
Also shellbackdiver1. I don't recall a "safety stop" at half depth in my no decompression diving table rules. I do know of a 8 minute safety stop at 15 feet / 5m. That 8 minutes is for any dive deeper than 100 feet, a safety stop of 5 minutes at 15 feet/5m is recommended all the time.
jeraldjcook
Contributor
SparticleBrane
Contributor
I would disagree on how much people breathe, based on acedotal evidence.
Most people diving have a SAC rate between 0.5 and 1 cubic feet per minute. This is between 14 and 28 L/minute.
8L/min is a SAC rate of 0.28 cubic feet per minute, which is almost unheard of.
You can quote Wikipedia all you want to, but people breathe more underwater, especially newer divers. The 'stress' of a new environment, CO2 buildup from waving their hands around, comfort levels, cold water, and the density of water (800x more than air) makes it a bit more difficult to move through -- especially if the divers don't have good buoyancy and trim.
Using an OOA calculation of at LEAST 1cf/minute is necessary. Anything less is a bad idea...
Most people diving have a SAC rate between 0.5 and 1 cubic feet per minute. This is between 14 and 28 L/minute.
8L/min is a SAC rate of 0.28 cubic feet per minute, which is almost unheard of.
You can quote Wikipedia all you want to, but people breathe more underwater, especially newer divers. The 'stress' of a new environment, CO2 buildup from waving their hands around, comfort levels, cold water, and the density of water (800x more than air) makes it a bit more difficult to move through -- especially if the divers don't have good buoyancy and trim.
Using an OOA calculation of at LEAST 1cf/minute is necessary. Anything less is a bad idea...
Sparticle's right. Especially if stressed, the SAC rate could go well over 1.0 cu.ft./min (28lpm). Some people have a "normal" SAC rate in the 1.0 range-without the added stress of being out of air. Diving is not a sedentary sport and RMV's can't be compared to sitting in a chair or laying in a bed.
If you err on the small side for a pony, you could be out of air AGAIN.
Stick with the 30 cu ft cylinder. Even if you don't go tech, if you ever go to sell it, the 30 cu ft has a wider market than a 19. The 30 is fairly common for an oxygen bottle, but the 19 is too small for oxygen and too big for an argon bottle.
Also, as stated, there are two physical sizes for 30's. The Catalina is shorter and fatter, the Luxfer is taller but skinnier. Either one slings nice and tucks out of the way, relatively unnoticed.
If you err on the small side for a pony, you could be out of air AGAIN.
Stick with the 30 cu ft cylinder. Even if you don't go tech, if you ever go to sell it, the 30 cu ft has a wider market than a 19. The 30 is fairly common for an oxygen bottle, but the 19 is too small for oxygen and too big for an argon bottle.
Also, as stated, there are two physical sizes for 30's. The Catalina is shorter and fatter, the Luxfer is taller but skinnier. Either one slings nice and tucks out of the way, relatively unnoticed.
Here's an old post that deconstructs an incident and arrives at a requirement. Adjust the values to cover your intentions and go from there.
http://www.scubaboard.com/forums/1590880-post4.html
Pete
http://www.scubaboard.com/forums/1590880-post4.html
Pete
I rarely carry a pony with backgas in it, but when I do, my personal minimum is 30CF, for a couple of reasons:
(1) if I have to hand it off, I want it to be plenty for the extra-stressed OOA diver.
(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
(1) if I have to hand it off, I want it to be plenty for the extra-stressed OOA diver.
(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
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