How much BC lift do I require?

[DomRep Diving]

I guess This is the one Hallmac said....

 

[Hallmac]

Sorry O-Ring

How much lift

Hallmac

 

[cstreu1026]

I guess I should have mentioned I don't dive doubles and have no plans to any time soon. It is been nothing but Al 80's so far though when i by my own tanks soon I might go larger.

 

[Hallmac]

The lift required would follow the same kind of thought. To make it simple I think most on the board would recommend the 27lbs to 36 lbs range. With the amount of weight you are carrying the 36lbs would be a good choice. Top choice would be the Halcyon due to their quality.

Hope that helps!

Hallmac

 

[kalvyn]

cstreu1026 once bubbled...
I am going back and wing. The only problem is that I don't know how much lift i need. Around home I will be doing mostly cool-cold freshwater dives int eh quarries. Typically I will be in a 7mm wetsuit with i beleive in the neighborhood of 27lbs of weight but I don't have my log book so that is just from memory. I have been looking at the Halcyon 27lb Pioneer but I don't think that is enough for cold water diving. I hae also been looking at a 40lb setup from Deep Outdoors. Anyone have a recomendation on wing size?

I bought the Halcyon Pioneer 36 Lb wing setup, as recomended by the guy at 5th Dimension when I asked. I dive dry (46 F water pretty much year around here), and currently wear 22 Lbs, plus the 6 Lbs of the BP itself. I'm willing to bet your best choice would also be the 36 Lb wing.

Jimmie

 

[PnL]

Email Halcyon and get a recommendation. That's what I did and the 36# was recommended and works great for both my wife and I. We dive wet 7mm full w/ Al80's. I think the 27# might be on the short side personally.

 

[MikeS]

cstreu1026 once bubbled...
I am going back and wing. The only problem is that I don't know how much lift i need.

An interesting question that I’ve never heard answered well. Enquiring minds want to know how much lift we need but also understand the principles involved and hopefully learn to calculate the lift requirements for any given situation. The following is the best explanation that I’ve been able to come up with. Anyone that can add anything please do so.

The reason that we need lift is to overcome any negative buoyancy to become neutrally buoyant. Why would we be negatively buoyant without air in the bladder? For the purpose of this discussion, let’s assume we are properly weighted. There are two reasons I can think of, equipment failure (i.e. drysuit puncture) and the negative buoyancy required to offset dynamic changes in buoyancy.

What are the dynamic buoyancy factors during a dive? The swing weight of our tank(s) as gas is consumed, the loss of lift resulting from compression of our exposure protection (i.e. wetsuit) and the buoyancy of anything we might detach (i.e. stage bottle).

When properly weighted we are neutrally buoyant with an empty tank just below the surface with anything detachable detached and no air in the bladder. At this point, we are as positive buoyant as we will ever be without adding air to the bladder and we’ve added enough weight to overcome the dynamic buoyancy factors other than air in the bladder and their most buoyant point.

We now need enough lift to overcome the negative buoyancy resulting when all of the dynamic factors are the most negative or worst case. This would be at the beginning of the dive (full tank), at maximum depth (maximum compression) with any detachable items with negative buoyancy still attached.

So to calculated the lift requirements we would add:
The swing weight of our tank(s)
Plus the swing in buoyancy of our exposure protection
Plus the negative buoyancy of any detachable objects

When purchasing a wing I calculated it as follows:

PS HP 120 swing weight 11 pounds
Swing in buoyancy of Xcel 7-6-8 wetsuit 7 pounds
Negative buoyancy of stage slung 30 cf pony 5 pounds
Total required 23 pounds

So to have a safety margin I went with the 36-pound wing.

Hope this helps,
Mike

 

[NuttyGambler]

My choice was not as complicated. In freshwater, I will have a drysuit (built in extra air if need be). In salt, more bouyant.

I went with the 27lb, mostly because it was cheaper.

It works well, UNLESS you are overweighted. :whoa: :drown:

 

[MikeS]

NuttyGambler once bubbled...
My choice was not as complicated. In freshwater, I will have a drysuit (built in extra air if need be). In salt, more bouyant.

I went with the 27lb, mostly because it was cheaper.

It works well, UNLESS you are overweighted. :whoa: :drown:

Nutty,

I don’t think the physics behind your choice are any different, whether you chose to think about them or not.

There is still an issue with the drysuit. You need enough lift to compensate for the loss of buoyancy resulting from a flooded drysuit. So actually, I think that a drysuit makes things more complicated because it’s more difficult to measure the swing in buoyancy of the drysuit from having enough air to remove squeeze to flooded.

While freshwater versus saltwater effects weighting it does not effect lift requirements. It does effect the amount of lift that your bladder has though.

 

[leadweight]

One rule of thumb for wing size is the amount of weight you need (including weight of the BP) plus 10 lbs.

The method used by Mike S is also a good one, but allow something for margin of safety. As a general rule a 7 mil suit is 15# bouyant and will lose 75% of that bouyancy at 100 feet depth.

Especially if you are using integrated weights you would like for your rig to float without ditching anything.

Personally, I think the Pioneer 27# is too small for your application, get the 36# in a Pioneer. There is also a Diverite Trek wing with 40# of lift.