BC Lift Question

[turnerjd]

I've never completely flooded a drysuit, but where does this extra 'weight' come from?

<snip>

As there is no extra 'weight' added by a flooded drysuit, then there is no need to increase the wing capacity beyond what was already calculated.

Agree. There is no extra weight. There is just no buoyancy whatsoever from the suit.

If you start the dive negative (to allow for the change in buoyancy in the tank, aiming to finish the dive neutral) and you lose suit buoyancy, you can easily find yourself needing more lift than a small BCD can provide to become neutral. Most BCDs don't provide more than 8-10 kgs of lift, often less.

I unfortunately learned this the hard way completely flooding my suit once. Luckily it was a shore dive that I was able to "walk" out from, and keep my weightbelt for future dives.

Jon

Edited to say that I just looked at some of the BCDs available now, and quite a few of them have quite good lift values from 10 to 15 Kgs. So, with one it might be possible to get back to neutral buoyancy.

 

[k ellis]

The extra weight comes from the loss of buoyancy. Most divers I know calculate to some extent a small amount of extra weight (Though trivial) to account for the fact that we really dont want to dive with a dry suit squeeze so we plan to have some air though not much in our suits. When a dry suit floods you now have no air in your bcd (Complete flood which by the way is extremely rare) and thus the extra weight you added to offset that small amount of air would be working against you.

 

[turnerjd]

Don't forget, that you aim to finish the dive neutral, but with air in your suit, otherwise you get some "interesting" suit squeeze.

Lose this air, and you are not neutral at the end, but (significantly) negatively buoyant.

Jon

 

[turnerjd]

<snip> (Complete flood which by the way is extremely rare) <snip>

ripping a latex neck seal will do it every time :-(

 

[Bubbletrubble]

Bear in mind that lift requirements for the BC also depend on whether ballast is attached to the diver's body or the BCD/wing. More specifically, attaching all of the ballast to the BCD or BP/W will likely necessitate more lift.

Can you explain the reasoning here? It would seem to me that weight is weight regardless of where it it.

It's really about starting to view your configuration from a 'Balanced Rig' perspective. Not only dealing with what weight you need, but also calculating your distribution of ditchable and non-ditchable weight, on the basis that you may need to swim the rig up in an emergency (i.e. wing failure/burst bladder).

Whilst this isn't directly connected to the wing lift necessary, it is all part of a cohesive and intergrated approach to configuring an optimum set-up. Calculating required wing lift, must be based on precise and effective weighting...and must also consider the contingency of wing failure.

@Hatul: DevonDiver is discussing the concept of a "balanced rig." Although that's good information, it's not what I was referring to in the post you quoted.

Let me see if I can explain my statement a little more clearly...

We know that a BCD/wing needs to have enough lift to float the rig by itself (without the diver) at the surface.

For several of the drysuit divers I know, their shell drysuit + heavy undergarment configuration has approx. 26 lbs. of positive buoyancy. With a HP100 tank (-2 lbs. empty), SS BP (-6 lbs.), and reg (-2 lbs.), this means that the diver is probably carrying about 16 lbs. of lead. If the diver attached 100% of that lead to his rig (using trim weight pockets or those snazzy Halcyon/Apeks/GolemGear waist strap-mounted weight pockets), his wing would need to provide at the beginning of the dive with a full tank: 16 lbs. (lead) + 6 lbs. (SS BP) + 2 lbs. (reg) + 10 lbs. (full HP100 tank) = 34 lbs. of lift.

What happens if the diver in this example decides to wear 8 lbs. of lead on a weightbelt or weight harness (e.g., DUI Weight & Trim)?
Then the wing would only need to provide 26 lbs. of lift.

If the diver is using a wing with 30 lbs. of lift (which is a very common wing size for a cold water single tank configuration), where the weight is attached (diver vs. rig) is pretty important.

On a funny note, my girlfriend was diving off of a houseboat with some friends several years ago. The houseboat didn't have a swimstep or a convenient place to do a giant stride entry, so divers had to inflate their rigs, toss them overboard, and then don the gear in the water. Not the best boat to dive off of...but, hey, it was free. Before the sloooow houseboat had come to a complete stop (anchored, etc.), one of the show-offs on the boat who wanted to get a head-start on his dive tossed his maximally inflated BC overboard and planned to jump in after it. As he contemplated the style of cannonball entry that would be most entertaining to the crowd on-deck, he received a little surprise of his own. To his dismay, his weight-integrated BCD (loaded with lead in the pockets) had already sunk out of sight -- turns out that with a full tank (HP130) and all of his lead attached to it, the BCD didn't have enough lift. He jumped in quickly as the boat continued to drift away from where his gear was last spotted. There was a little current at the site, so the guy had to hang onto some kelp where he thought the BC disappeared. He ended up bribing someone with a 6-pack of beer to do a bounce-dive down to about 80 fsw to retrieve his rig. Ah, good times!

 

[turnerjd]

Using BubbleTroubles dry suit example above with 26Lbs ~13Kgs of positive buoyancy, in a flooding context this plus the difference in buoyancy between a full and empty tank would make you suddenly go from -7kgs to somewhere in the order of 19kgs negative at the start of the dive, or from neutral to 13kgs negatively buoyant at the end of a dive.

Most BCDs will deal with the 13kgs of negative buoyancy, but few will deal with the 19Kgs.

Jon

Thanks bubble trouble for the numbers, as I couldn't remember off the top of my head what my weight for the suit alone was.

 

[DevonDiver]

ripping a latex neck seal will do it every time :-(

Invert... to a slightly head-down position. You can prevent a 'complete' flood... but you'll still be wet and cold.

This discussion makes me glad I moved to the tropics

 

[turnerjd]

This discussion makes me glad I moved to the tropics

Well I moved to France, and used to dive often in the Med (and am hoping to once again be regularly diving there). But even mediteranean wrecks like the Donator at -50m are far more comfortable in a dry suit than a semi-dry. Especially hanging on the deco stop.

 

[danvolker]

The other side of this discussion is why we don't use much larger bouyancy than needed---in addition to causing more drag ( meaning you will have to work harder swimming on your dive-maybe even having to carry a larger tank because of this), a wing or bc that has a lot more lift potential than needed can be dangerous--particularly to a diver without instant reflexes....if the inflator hose blows, and this does happen, suddenly the wing or bc fully inflates....a fast thinking and fast acting diver can disconnect the inflator hose in time to prevent an "express ride" to the surface....But if not, this is a ride that could pull "normal/slowly reacting divers" to the surface at speeds that could cause a lung over expansion injury, and could certainly get them bubbling in an unhealthy way

 

[vixtor]

I've never completely flooded a drysuit, but where does this extra 'weight' come from?

I understand that it would be pretty difficult to climb back up the boat ladder in a flooded suit, or even to maintain postive buoyancy at the surface, but whilst submerged a flooded drysuit won't add 'weight' to the diver. It would just remove buoyancy. But that wouldn't be a problem, as they have their BCD anyway for that purpose...so there is no question of having a need to ditch weights.

As there is no extra 'weight' added by a flooded drysuit, then there is no need to increase the wing capacity beyond what was already calculated.

When water enters a drysuit, you are losing all its buoyancy. But you were wearing a big weight belt (probabil something in the range of 8-12kg) to compensate the huge buoyancy of a working drysuit. Now, all that weight is not compensated anymore by the drysuit, so you need to have means to "float" it. So you need to be able to put enough air in the BCD in order to float your weight belt (added over whatever you had to float before the incident).