Depth Rating BC's?

[paddler3d]

A friend of mine has this idea that BC and Wings have a depth limit. He contends the deeper you go, the more lift you will need because you have that much more pressure on top of you.

He contends that a wing that provides 60#'s of lift at 33 FSW, 2 ATA isn't going to give you that same 60#'s of lift at 132 FSW, 5 ATA.

I cannot find where Archimedies Pinciple, which states any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object, is a function of depth.

I contend that 60#'s of lift at 2 ATA is still the same 60#'s of lift at 5 ATA, but it is going to take a lot more gas to provide 60#'s of lift at 5 ATA vs. 2 ATA.


I've got beer riding on this...any help would be appreciated!

Chris

 

[JeffG]

He's wrong, you're right.

 

[cool_hardware52]

A friend of mine has this idea that BC and Wings have a depth limit. He contends the deeper you go, the more lift you will need because you have that much more pressure on top of you.

As you describe it, Jeff's spot on, your correct and you friend is wrong.

A diver could need more BC (wing) capacity at greater depth if they are diving a compressible exposure suit, i.e. neoprene wetsuit.

Neoprene is a closed cell foam, and will compress and loose volume as the ambient pressure increases, up to some maximum. At about -165 fsw neoprene reaches maximum density. Between the surface and 165 fsw a wetsuit wearing diver will need to add gas to his bc as his depth increases to offset the compression of the suit.

If a diver was going to dive over a hard bottom of say 20 ft. he could do so with a smaller wing than if he was to dive over a bottom 165 ft deep or deeper.

In practice this is not done, and wings should be sized to allow for the maximum possible change in buoyancy of the divers exposure suit.

Tobin

 

[PerroneFord]

Sounds like your buddy needs to drink the beer AFTER the contest, not before!

 

[ianr33]

The wings will have SLIGHTLY less bouyancy at depth as the density of the air in them will increase with depth. Insignificant for any practical purpose though.

Anybody feel like putting some numbers on it?

 

[GDI]

Yeah that Archimedies dude what a kidder eh, All that stuff about mass, density, buoyancy and displacement.

You may want to point out to him that other guy named Robert Boyle I believe he had something to do with that volume and pressure thing.

Your friend maybe thinking that the deeper you go the more negative you will become respectfully. And this is true, thus requiring more LIFT, but 60# at the surface will still be 60# at depth and you will need more gas to have it reach that amount at depth. The higher amount of gas filling it up will cause it to displace more water and thus give more lift.

MIT Physics Lecture: Classical Mechanics - 28 - Archimedes' Principle

 

[lamont]

water has a density of about 1000 kg/m^3, air has a density of about 1.2 kg/m^3, so if you increase the density of the air by a factor of 833 then air will have the same density as water and BCs will cease to function. treating it as an ideal gas (which it won't be, but i don't have the copious free time to look up a real equation of state for air right now), this happens when the pressure is 833 ata, or 27,489 ft below the surface. the depth of the mariana trench is actually deeper than this at about 6.8 miles or 36,000 ft.

at 800 ft, you will have only lost about 3% of the lift of your BC due to the compression of the gas in the wing due to pressure (25 ata / 833 ata = 0.03)

 

[bleeb]

The wings will have SLIGHTLY less bouyancy at depth as the density of the air in them will increase with depth. Insignificant for any practical purpose though.

Anybody feel like putting some numbers on it?

Sea water: 64 lb/cu ft
Air: 0.08 lb/cu ft/ata (0.125%)

So, a rigid container with 60 lb lift at the surface has a volume of 0.94 cu ft.
At 132 ft, filling the same container to ambient pressure requires an extra 0.3 lb of air 'cause there are 5x as many molecules in it.

But, BCs and wings aren't rigid. Nor are they regularly shaped, so I don't imagine they fill exactly the same way every time, especially with a few bungees wrapped around the thing. The extra 0.5% volume is very probably lost in the noise. If nothing else, if you need the extra 0.3 lb lift, the material will stretch that far.

So, while your friend may be correct about the details of the theory, when you assemble all the bits of reasoning, I vote that you get the :beer: for being more right with the real-world application.

 

[GDI]

You could always just start a argument of the facts or just argue on principle, If neither one of those methods works just start calling him names and drink the beer!

 

[Hickdive]

I suggest giving serious consideration to what kind of beer he's going to buy you.

You don't want him to cheat you with some ghastly, palid fizz. You need to be looking at a good imported Belgian or something from a micro-brewery on the far side of the country.

When it comes to beer never accept the sub-optimal