BCD Lift

[jonnythan]

Your BC should be able to handle the most negative you will be during a dive.

The difference between empty and full should be greater than the sum of air in your tanks and amount of lift you lose as your wetsuit compresses (or the amount of lift your drysuit loses if it floods).

Everything else is essentially static. The two things besides BC that change buoyancy during your dive are your tank and your exposure protection. If you have an Al 80 that's about 6 lbs of air. A 2-piece 7mm full suit will compress something like 10-15 lbs. That means that if you're perfectly weighted, you will be maximum -21 lbs at depth at the beginning of the dive and neutral in the shallows with an empty tank. Of course, you're never perfectly weighted (not that you ever want to be) so you're going to end up 2-7 lbs more negative than you want to be, so the BC has to account for that as well. That means that for diving an Al 80 in a 2-piece 7mm wetsuit you really only need around 25 lbs of lift.

If you dive something that carries more gas, like an E8-130, that's almost 10 lbs of gas. That means you need more like 30 lbs of lift.

That's about as bad as it gets for single tank diving. A little extra lift capacity certainly doesn't hurt, to give an additional boost at the surface or to give your buddy a couple extra in an emergency. That's why you see the single tank wings from Halcyon and Hog topping out at 35-40 lbs maximum.

 

[NAM001]

first i think your numbers are a bit off. if you look at the charts for tanks the bouyancy i think yyou are using is without valve. my lp85'si think are -6 or better when full and my 95's are right at -9. air is 8# / 100 cu ft

Any way....figure the worst condition you can be in full tanks bouyancy... if dry suit its bouyancy (which will be negative if it floods) or your heaviest wet suit ( suit compression ) 10# for your head.

a couple of examples.

1. 3-5mm wet suit +19 , full doubles 85's, -13 , 5# of crap lights reels camera and stuff. so at the surface you are +19 -13 -5 youare even at depth your wet suit is now +4 so you are now about -15.

your wing has to carry your gear weight which is about -15 or so and it has to hold you in your gear with your head out of the water ... so that would be a 10# wing needed on the surface and a min 15# wing at depth.

now lets look at a dry suit. dry suit is 30 light so you add 30# of lead to your rig ,,, your tank -13 ,, and crap still -5 and your head is still 10#. ok all is well and you flood your dry suit. you have non ditchable wight on. so you are now 30# heavier than before flooding. so you are -30 -13 -5 -10 so you need 58# wing to break even when floating yourself in your gear (flooded suit) and keeping your head out of the water.

I am looking at the new Nomad LT on the DiveRite website and they say the largest cylinder you can use with that rig are Faber 85s due to it only having 25lbs of lift. Faber 85s are -3.80 lbs when full. I prefer Faber 95s which are -5.37 lbs full. I get the concept of putting on all my gear with full tanks and making sure I can float on the surface but what I am wondering is how to really determin how much lift I need. The 95s are only another 2lbs negative. Can the rig really not support the extra weight or is DiveRite just trying to play it safe? What if I dove AL80s which are -1.6lbs ful (catalina)? Does that mean that the rig could support up to four AL80s or more?

 

[decompression]

Just a small point, a flooded drysuit does not become negative, just cold. You do lose lift as the under garment absorbs water (dependant on type) and the suit may not hold air but it will not become negative.

 

[PfcAJ]

Keep in mind that your BC needs to be able to lift you AND your partner (at full tanks, and with all your weight) in the event of an emergency. For instance, if you ever have a tox event, or go unconscious for whatever reason, your buddy needs to be able to use YOUR wing to lift both of you out of the water. If he/she used their own BC to do this, it would be incredibly full, and in the event that they dropped you, they would now have an uncontrolled ascent. Now two people are in danger. The dive on top (your buddy) stays slightly negative while using your wing for buoyancy on the way up.


Refer to this:




I have not taken Tech1 yet, so I do not have formal training on this. I am explaining this to the best of my understanding. If I got anything wrong, someone please correct me, because I would hate to be doling out false information.

The point is that you should have a balanced rig though

The move is to have enough lift to float your main gear (backgas tanks, lights, heavy stuff) without you in it. Divers themselves are almost always positive (drysuits, wetsuits, and usually with no exposure suit, people float), so if it can float your (or your buddy's) heavy gear without you (or your buddy) in it, it'll be enough to do the unconscious diver thing. Stages/deco bottles (as shown in the video) are pretty darn close to neutral, and if you have to dump them, you can. Your left hand (the one that controls the inflator and butt dump) is in position to unclip a heavy tank, if need be. In practice, its not really needed. In training, I've successfully (although not gracefully) brought a diver to the surface who was wearing an RB80 with lp120s, stage bottle, two big-ass can lights. That's a lot of mass.