How much BC lift do I require?

[nobends]

Herman, if it weighs 10lbs on the surface then its weight in water will be:

10lbs - (weight of water displaced by object) = apparent weight.

Lead weights don't displace much water relative to their mass, so their apparent weight is close to their surface weight. Steel tanks weigh around 50lbs on the surface, but only a couple of pounds underwater because the weight of water they have displaced. Make sense??

 

[Aegir]

Ok lets say a 15 litre steel tank which would probably be around 8lbs negative when empty and 12kg of lead. Not sure what the stainless steel backplate would weigh but it would be an exact copy of the Halcyon backplate if not the real thing. Does anyone know roughly how much that sort of backplate would weigh? Also a single tank adapter also.

Hmm would a wing of 36lbs lift be enough?

 

[herman]

Ah yes, appearant weight, knew there was something I was missing. Guess solid, heavier than water objects like lead or a backplate could be taken at face value but everything that has some bouyancy of it's own will have to be tested to see what it's appearant weight is. Aegir, did you mean to mix lbs and kg's? Looks to me like 36 would be enough, you have about 14 lbs accounted for (assuming that was 12 kg) and I can't imagine the rest of your equipment having an appearant weight of 22lbs. I am guessing but I suspect that a backplate does not weigh much over 10 lb which still leaves 12 lbs in reserve. On the other hand, if you are buying new and the cost/size of the next larger size is not significant, why not go up some?

 

[scubanole]

I am looking at getting a back plate and wing, and I need to figure out what wing size I should have. I know someone that is selling a Halcyon back plate and 18 lb wing. I will only be diving singles and no tech diving. I think the 18 lb wings will be ok but if anyone can tell me how to calculate how much lift you need I would appreciate it. Thanks

 

[Frog]

Hi there Scubanole. I dive a 12L steel 232 BAR single on a Halcyon 37 lb wing. I had to go for the 37lb wing rather than the 27lb because i dive dry and if my dry suit floods then i need enough lift to get me back to the surface. But to answer your question we really need to know a little more, such as:

Are you diving wet or dry
What size tank are you going to use
Will the tank be 232 or 300 BAR
SS backplate or aluminium (usually alu if your diving wet)
What other (if any) items will you be taking under with you

 

[Rick Murchison]

The way I approach this question for myself is to consider my wetsuit to be fully compressed at depth and of no buoyant value - same with my neoprene drysuit. With that in mind, I take all my other gear (tanks, backplate or BC, lights, ditchable and non-ditchable weights, knife, scissors, any other nits & nats) to the pool, and see what it takes to float the whole mess. That's the minimum lift you need.
Now, a word on safety. If you're diving wet, your whole kit should be either buoyant or close to it without any air in your wing or BC once you've dumped your ditchable weight (you can include your main light and other ditchable equipment too), without depending on your wetsuit for any added buoyancy. That way if you do have a wing failure you won't be glued to the bottom, or unable to arrest a descent into the abyss. (If you're diving dry you can consider your drysuit a backup to your wing in an emergency)
If you're diving in warm water with an AL 80, your 18 pound wing is probably ok, but for anything else, you'll more likely find you'll need more lift. For example, with my standard cave rig, I use a 55# lift wing.
Rick

 

[scubanole]

more info. I will use a single AL80. Most diving will be in a 2 mil shorty, skin, or no protection. In some instances I will be using a 2 piece farmer john. I take very few other items under with me. Knife, signal device and that is about it. I plan to get a camera in the future, but I don't think the reefmaster weighs a lot. Thanks for your replies, and any future replies.

 

[100days-a-year]

I use the 18 for most rec diving,spearfishing,lobstering,etc......Needs no weight with a single tank adaptor and a polartec or skin.It is so stearlined.Like RickM I use bigger wings(up to 65-85) when doing more involved stuff.If you plan on doing cooler water 68- then perhaps a bigger wing is called for due to the bouyancy loss at depth from a wetsuit.

 

[SunshineFish]

Hi all - I've got a question about selecting a BC....

I've noticed that most BCs my size (XS) like the Genesis Athena or Seaquest Diva only offer about 18-25 lbs of lift. This would be okay if I were diving in tropical waters on vacation. With a 3 mil shorty, I've been using 6 lbs of weight. But what if I wanted to go diving around my hometown of NY? With a 7 mil fullsuit and jacket, hood, gloves, and extra weight - Is 18-25 lbs enough?

What is the formula (if there is one) to determine sufficient BC lift?

Sandy

 

[Campana]

You need enough lift to assist you to the surface with a full tank and all your gear. If you use an Aluminum 80, you need about 6# of lift to pick it up. With the 7 mm wetsuit, you'll need more weight to offset the wetsuit. I would guess 8-12# more weight for a small size wetsuit. Now say you dive to 130', and your wetsuit compresses and only has 1# of bouancy left, and your tank is full. That would mean you needed 6# of lift for the tank, and 7-11# of lift for the wetsuit compression, until you started getting nearer the surface. Total 13-17#. You should be fine.

Now if you are wearing a huge steel tank, that's different.

I use a 27# lift wing with single tanks, and it has always been fine with up to 95 cubic foot steel tanks. I wear an XXXL wetsuit. My 7 mm wetsuit required 32# of weight to get down, but it's got about an acre of neoprene as opposed to your Small wetsuit, which would need quite a bit less.

Remember, people used to dive without BC's. When they needed to surface, they swam up. Even if you are a bit negative when you are very deep and your wetsuit is compressed, you can always swim, but my opinion is that you should be fine with the Athena.

There can't be a formula, there are too many variables.