BCD Lift, what is Required

[beaverdivers]

I think you have to ask three questions:

How positively buoyant are you in your 7mm suit?

How comfortable are you in the water?

How extreme are the surface conditions going to be?

35lbs. of lift is a fair amount.

However, you may be better off with more lift.

 

[tboulware]

Folks, this is all great advice and gives me something to think about.

One final question.

How much margin do you think you should have? For example, a BC is rated at 35 LBs and the weight guidelines suggest that 28 pounds is required for a full 7MM suit with hood and gloves. Is that 7 pounds sufficient margin?

Thanks again.

 

[beaverdivers]

Please reference questions #2 & #3 above. If you are not very comfortable in the water or if you are going to be in extreme conditions on the surface, then no it might not be enough " extra " lift.

Personally, I really like the Scubapro classic due to the fact it has 60+lbs of lift and it floats you face up w/o any effort.

This feature is nice when you are in extremely rough surface condition, when you have a panic diver on top of your head or when you are teaching.

 

[fnfalman]

Folks, this is all great advice and gives me something to think about.

One final question.

How much margin do you think you should have? For example, a BC is rated at 35 LBs and the weight guidelines suggest that 28 pounds is required for a full 7MM suit with hood and gloves. Is that 7 pounds sufficient margin?

Thanks again.

That's 25% err on the side of caution. Plenty good.

A rig either floats you or it doesn't.

You will find that depending on how you rig your equipment, that 35-lbs lift will have a lot more than 25% margin of error.

Take me for example, my current wing is 30-lbs in the lift capacity. I wear a 1-pc 7mm wetsuit, hood, etc.

With the suit on and a weight belt, I need about 18-lbs in order to do a proper weight check (brand new suit that is, later on the weight requirement will be less because the suit is well worn). So everything on my rig should have about -18lbs buoyancy characteristics.

-6lbs (BPW with stainless steel plate) + -6.5lbs (tank's negative buoyancy when empty) + -2lbs (Single Tank Adapter) + 2lbs (tank valve) + -2lbs (regulator) = 18.5lbs. This means that by the end of the dive when the tank runs low (500-psi or less), I can dump all the air out of my BC and do a 15-ft safety stop with nothing but my breathing regulating my buoyancy control.

That means that my 30-lbs BC has a margin of error of 11.5-lbs or 38%.

Let's look at the beginning of the dive where my 100-cuft HP steel tank is filled to the brim:

-6lbs (BPW with stainless steel plate) + -14lbs (tank's negative buoyancy when full) + -2lbs (Single Tank Adapter) + 2lbs (tank valve) + -2lbs (regulator) = 26lbs. My 30lbs wing has a margin of 4lbs (4lbs/30lbs = 13%).

The bigger the BC's air bladder, the more difficult it is to vent. And in the case of back inflate BC, the bigger the air bladder, the more it wraps around the tank and makes it even harder to vent.

I can tell you that venting from my Dive Rite 45-lbs Venture wing is a lot more difficult than venting from the same wing but at 30-lbs lift.

Some people who harvest scallops or other shellfish would use an oversized BC air bladder to act as a lift bag. I personally would recommend using a lift bag as a lift bag and let your BC functions as a BC.

 

[halocline]

Personally, I really like the Scubapro classic due to the fact it has 60+lbs of lift and it floats you face up w/o any effort.

Personally, I think that 60lbs lift for single tank recreational diving is insane. And I suspect that you "really like" everything you sell.

And while it's nice to have an ersatz life preserver on the surface, consider buying dive gear based on how it performs underwater, while actually diving. And a monster-size BC with 60 lbs bladder flopping around is not going to make for enjoyable diving, IMO.

---------- Post added March 1st, 2013 at 11:25 AM ----------

One final question.

How much margin do you think you should have? For example, a BC is rated at 35 LBs and the weight guidelines suggest that 28 pounds is required for a full 7MM suit with hood and gloves. Is that 7 pounds sufficient margin?

It's plenty of margin. At the surface you'll have the added buoyancy of your suit, and if you get in trouble on the surface and need to be even more positively buoyant, you drop weights. And while your suit theoretically 'could' lose all its buoyancy at extreme depth, in truth it probably loses, I don't know, maybe half, at recreational depths.

35 lbs is a lot of lift, and most recreational BCs have WAY more than you actually 'need' if you're configured correctly. Technical diving with double tanks and lots of additional gear, then you need more lift and redundant lift. But not for 'normal' single tank diving.