NAUI Master Diver Course: understanding theory regarding amount of air needed in lift bag
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The volume is not compressible, what's in it may be.
You need to displace 34 litres of water to lift that outboard. At any depth.
You can do it by filling a 34-litre lift bag with 34 litres of lighter-than-water uncompressible liquid, like ether, at any depth. If you're filling it with compressible gas, like air, you'll need 34 litres of it at sea level and 1.5x34 litres at 15 metres.
You need to displace 34 litres of water to lift that outboard. At any depth.
You can do it by filling a 34-litre lift bag with 34 litres of lighter-than-water uncompressible liquid, like ether, at any depth. If you're filling it with compressible gas, like air, you'll need 34 litres of it at sea level and 1.5x34 litres at 15 metres.
With ether, the lift you'll get will be the 34 litres displaced minus the weight of the ether, so far from the needed lift. When reasoning with air, we just simplify the maths by not taking into account the weight of the air in the lift bag (btw, I suspect a typo when you say 1.5x34 at 15m, instead of 2.5x34). With ether, we have a density of 0.713 (0.7 for the ease of claculation), so, when using 34L of ether, you get 34-0.7x34=10.2L of lift.
I know...but even though his post was superfluous because the info in those first 5 posts was uber helpful, his explanation was written in a bit of a confusing fashion and I could not think of a tactful way of stating either of those things, so I thought I would entertain the issue by asking him to clarify what he wrote.
Tough crowd here on Scubaboard.com
-Zef
That's why I intervened: it doesn't illustrate the point because simply displacing 34L with something lighter than water isn't enough, the lift needs to be sufficient so the weight of the "displacing" material may need to be taken into account.
But since we almost exclusively use air, we can forget about this step. I just wanted to be sure that no one got any wrong idea about lifting an object. Even though we're un the advanced scuba discussions part of the forum, many readers (me included) aren't "advanced" divers.
Wow. Looks like the OP is looking at this as a trick question. There is no factors of air volumn expansion in the problem. if you have 50 #'s of weight to lift you need about 50+/- pints or 22+/- liters of air at the lifting atmosphere to make it neutral. Yes when it moves to the surface the air volumn will expand and you will have to control the change in lift. But the question appears to ask to make it neutral. Yes the cuft referenced to the surface will be different at 50 ft compared to 100 ft but the lift bag at depth will have the same amount of gas in it. Likewise when the object moves to the surface the 100 ft bag will vent off twice as much s the bag from 50 ft.
For the OP look at it this way if you take a breath on the surface and you inhale 2 liters of air how much do you inhale at 100 ft. answer 2 liters. On the surface its 2 liters at 1 atm compression at 100 ft its is (2 liters) air compressed to 4 atmosphers.
For the OP look at it this way if you take a breath on the surface and you inhale 2 liters of air how much do you inhale at 100 ft. answer 2 liters. On the surface its 2 liters at 1 atm compression at 100 ft its is (2 liters) air compressed to 4 atmosphers.
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