Lift capacity and depth?

[Solly]

I have been reading recently about the BC lift capacity. I understand that it should be the maximum of 2, the first is to be able to keep one comfortable on the surface with all the gear and weights on, the other one is while underwater... I don't really understand the relation between lift capacity and the depth, I mean that for a certain BC with X lift capacity with your weight and compressed volume at a certain depth, you can't exceed this depth orelse you will start sinking and the BC will not be able to help you then...
Some physics will help here, at the surface you are floating which means your body and gear weight is balancing the buoyant force, then you add some extra weight to compensate for the breathed gases and you start to go down. At certain depth, your volume (body and suit) will decrease hence the buoyant force decreases (in other words you are heavier than what you would expect) then you have to inflate the BC or wing to hover... at this point the forces acting over your body is the extra weight and the volume decrease and the weight of the water column above you all 3 pulling you down while the buoyant force of the BC or wing pushing you up hence you hover... using this equation I tried to find a relation between the maximum depth that one can reach if the bladder volume is 20 litres (or giving lift capacity of 20kgms) for example but it failed, always giving me depth in few centimeters... help is really appreciated ... thanks

 

[captndale]

Your initial assumption that the lift capacity of the BC is depth dependent is incorrect. The lift of the BC is equal to the weight of the water it displaces when fully inflated and this does not change with depth.

As for what lift capacity you need, the answer is a lot less than you probably think. If you are properly weighted, you will have only enough ballast to make yourself neutrally buoyant with an empty tank, near the surface. Your BC only needs to have enough lift to support the weight of the gas in your full tanks plus enough extra to make it easy to comfortably swim on the surface. If you are diving a single 80 that is 6 lbs for air in your tank plus about 10 lbs to support yourself on the surface. Sixteen pounds of lift will do it! Of coure, that does assume correct weighting.

 

[Diver0001]

I have been reading recently about the BC lift capacity. I understand that it should be the maximum of 2, the first is to be able to keep one comfortable on the surface with all the gear and weights on, the other one is while underwater... I don't really understand the relation between lift capacity and the depth, I mean that for a certain BC with X lift capacity with your weight and compressed volume at a certain depth, you can't exceed this depth orelse you will start sinking and the BC will not be able to help you then...
Some physics will help here, at the surface you are floating which means your body and gear weight is balancing the buoyant force, then you add some extra weight to compensate for the breathed gases and you start to go down. At certain depth, your volume (body and suit) will decrease hence the buoyant force decreases (in other words you are heavier than what you would expect) then you have to inflate the BC or wing to hover... at this point the forces acting over your body is the extra weight and the volume decrease and the weight of the water column above you all 3 pulling you down while the buoyant force of the BC or wing pushing you up hence you hover... using this equation I tried to find a relation between the maximum depth that one can reach if the bladder volume is 20 litres (or giving lift capacity of 20kgms) for example but it failed, always giving me depth in few centimeters... help is really appreciated ... thanks

The clue you're missing is that it's not your body that gets compressed, it's the suit you're wearing. Your body dosn't compress because it's composed almost entirely of water.

There are limits to how much a suit can/will compress within normal diving parameters.

[edit] Oh.... and just getting to your question, the lift need is to compensate for suit compression so the amount of lift you need at depth won't be more than the buoyancy of the suit on the surface plus the weight of the air in the tanks assuming you're carrying the perfect amount of lead. [/edit]

R..

 

[Charlie99]

I have been reading recently about the BC lift capacity. I understand that it should be the maximum of 2, the first is to be able to keep one comfortable on the surface with all the gear and weights on, the other one is while underwater...

Others have answered your main question. I'd just like to point out that in addition to the 2 requirements you mention above, there is a 3rd, which is particularly important in regards to BCDs with integrated weights. That third requirement is for the BCD to have enough lift to keep afloat the BCD, a full tank, and any lead that you carry in the BCD pockets.

A typical situation where this 3rd requirement for lift is needed is when you need to partially degear in order to reboard a small boat.

 

[Solly]

thanks guys, this was really helpful.. so in other words, if you are perfectly weighted and have a bcd with the proper lift capacity (maximum for any of the 3 criteria mentioned above) then you can dive to 10 meters depth or 50 meters depth.. the depth is not of any effect here except that the suit is more compressed at lower depth but if covered by bcd lift capacity then it all should be ok...
very helpfull.. thanks a million

 

[Diver0001]

thanks guys, this was really helpful.. so in other words, if you are perfectly weighted and have a bcd with the proper lift capacity (maximum for any of the 3 criteria mentioned above) then you can dive to 10 meters depth or 50 meters depth.. the depth is not of any effect here except that the suit is more compressed at lower depth but if covered by bcd lift capacity then it all should be ok...
very helpfull.. thanks a million

Yep, you got it.

R..

 

[rstofer]

Why not play with the spreadsheet that is 'sticky' at the start of this forum? "The "Ultimate" wing lift calculator"
It's a very interesting tool

Richard

 

[Specmac]

The lift relationship to depth has to do with the effects of pressure on the air inside the BC bladder. This is very noticeable as you change depths, but most noticeable during ascent. In other words as you descend you will need to add air to the BC to maintain neutral buoyancy and of course release air as you ascend to avoid becoming positive.

This is partially do to the compression/expansion of your suit, but also attributed to the compression/expansion of the air in your BC bladder.

Now for the part someone more knowledge than I can explain. I have read discussions claiming that if you go deep enough you could exceed the volume of your bladder. In other words need to add more are for depth, but the bladders is at it's max and the relief value lets air out every time you try to add. The theory is at this point you become negative because you can't add any more air. This seems counter intuitive to me because as depth increases the air in the bladder compresses and more volume becomes available again. In other words I don't think the scenario or theory is accurate. For example I have an old BC (first one I bought) that only has 28lbs of lift. In my dry suit the bladder is full on the surface to maintain positive buoyancy. It's about 3/4 full to be neutral at 15 feet. But at 95 feet it is not even half full to maintain neutral. Yes I have to add air as I descend, but because the air is being compressed the total volume of air is less.

Some of you engineer types might want to jump in and correct me if I'm wrong or do a better job of explaining it.

 

[Diver0001]

I have read discussions claiming that if you go deep enough you could exceed the volume of your bladder.

No that's not it.

What happens to some suits is that they sort of "collapse" at certain depths. Take the Bare CD4 drysuit, for example. at a depth of between 70 and 75 metres it quite suddenly collapses and loses a lot of buoyancy which can make you suddenly much more negative than you were a few moments before.

Another effect that can happen is that a diver descends to very deep depths and the the speed of the descent accelerates to the point that the increase in compression from the descent exceeds the inflator's FLOW VOLUME to compensate for it. In other words, you're sinking faster than you can put air in your BCD.

The first effect is easily countered by using a trilam drysuit, which most technical divers do anyway.

The second effect is easily countered by (a) using the right amount of ballast and (b) controlling your descent rate and not descending like a free-diver on a mission.

R..

 

[cool_hardware52]

Now for the part someone more knowledge than I can explain. I have read discussions claiming that if you go deep enough you could exceed the volume of your bladder.

Not if you sized your BC correctly, and at "Man survivable" depths.

Consider the diver wearing a medium to thick wetsuit, who is weighted so they are neutral at the surface, no gas in their BC and a full single tank.

Will they ever be "heavier" as the dive proceeds? No, they will actually loose mass due to air consumption.

Can they become more negative, i.e. denser than the surrounding water? Sure, as they descend their wetsuit will compress. That reduces their volume without reducing their mass, increasing their density relative to the surrounding water. Simple physics.

By inflating their BC they will increase their effective volume, and adjust their density relative to the surrounding water.

There is a limit to how much volume a wetsuit can loose. Once it is compressed to the point where it is effectively a "solid" it will not loose any more volume. This occurs at about 165 fsw.

The total change in buoyancy is limited to the initial buoyancy of the wetsuit at the surface.

If the diver is using a BC with a lift capacity equal to or greater than the initial buoyancy of their wetsuit, and they are not over weighted they will have sufficient lift capacity for all reasonable "Man Survivable" Depths.

Having said that at great depths an "air" BC could become ineffective.

At 1 ata air is about .078lbs / cuft. and water is about 63 lbs / cuft.

At 33 ft the air in a BC is at 2 ata and is ~.156 lbs / cuft Water remains at 63 lbs / cu ft.

63 / .078 = 807. At 807 ATA, in theory the air in a inflatable would be at the same density as the surrounding water. Of course this require extreme depths, over 26000 ft. These numbers are just for illustration, gases at high pressures, ~12k psi won't follow ideal gas laws. Ever wonder why the bathyscaphe used incompressible gasoline for buoyancy? (Bathyscaphe - Wikipedia, the free encyclopedia)

What about lessor depths? At the surface a cuft of air offers 63 -.078 = 62.92 lbs of "lift".

At 10 ata (297 fsw) a cuft of air offers 63 - .78 = 62.22 lbs of lift. A 50 lbs wing will "loose" about 1/2 lbs. of lift from the surface to ~300 ft. Not even a noticeable reduction.

Tobin