BCD lift requirements

[somegum]

I am looking at purchasing a BCD. The manufacturer recommends (based on my weight and height) that my BCD size is ML. For the model I am looking at (Aqualung Sea Quest Pro QD), the lift capacity is only 34 lbs, which is LESS than the amount of lead that I wear. The folks at the dive shop say that the BCD will provide enough lift. They tried to explain why, but I couldn’t understand. Can someone please offer an explanation why I shouldn’t be concerned? Does it have to do with the lift provided by my dry suit?

I have a Sea Tux dry suit (I think it’s 6.5 mm high density neoprene) which is quite buoyant. I use a single aluminium tank, which sinks (-3.6 lbs) when full, but floats (+2.3 lbs) when empty. I wear 38 lbs of integrated weight (24 lbs of which can be dropped in emergencies), which makes me negative at the start of a dive, but I gradually become positive as the dive progresses. I have difficulty staying down at the end of the dive, and in attempts to stay down, I rid the dry suit of air, and sometimes get bruises from the ‘dry-suit squeeze’. Sometimes, I strap a few pounds to my tank to account for this buoyancy at the end of the dives.

I have been renting a BCD (Aqualung Sea Quest Pro QD), which has a lift capacity is 46 lbs -- the dive shop only rents size L. Thus I have had no issues staying at the surface at the start and end of dives. But I do not have the option of renting an ML size to see how it goes.

Thanks in advance,

somegum

 

[RJP]

Please allow me to be the first to suggest a BP/W...

:d

But seriously, you're toting a LOT of lead, and a steel plate will let you lose at least 6lbs of it. Add a weighted single tank adapter and drop another 6lbs. Factor in the inherent buoyancy of the BCD you're sporting now and you could probably drop another 4-5 lbs.

Then switch to a steel tank and drop another few pounds.

Then you'll be down to a MUCH more manageable amount of lead.

PS - fill in your profile with appropriate info, will help folks provide as specific info as possible.

 

[keithdiver]

Go bp/w you will not look back. believe me after 3bc, i finally did and have never been happier!!!!

 

[TSandM]

What do you need the lift in your BC FOR? You need it to float your rig on the surface (in case you're getting back on a boat where you have to doff gear first), and you need it to compensate for the gas you are going to use during the dive, and you need it to compensate, in an emergency, for the greatest amount of lift you can lose from your exposure protection.

About the most negative single tank I know is -14 full. So, with a 34 lb bladder, you could add up to 20 lbs of integrated weights, and the BC should still float. (Note that lift ratings aren't always very accurate, so it's best not to push things to the limit.) If your tank is less negative, you have more room to work with the integrated weight.

Now, the exposure protection issue can be thornier. If you are in a dry suit, you probably have quite a bit of lift from that and the undergarments you wear. If the suit is neoprene, you will lose some of that lift at depth. How much depends on the type of neoprene and its age. Undergarments will lose some of their lift if the suit floods (something you ALWAYS have to be prepared for with dry suits!) How much, again depends on the material of the undergarments, but it can be substantial.

It takes 26 lbs to sink me in my drysuit and undergarments, with no gear on, so I figure that's the maximum amount of lift I can lose. If the tank is -7, which is about what mine are full, I'm right on the edge of what my wing can lift. So I configure some of my weight to be ditchable, so that I can be sure of staying on the surface once I get there. Most people can swim up about ten pounds, so as long as I'm no more negative than that, I know I can GET to the surface.

So hopefully that gives you an explanation of how a wing can not need as much lift as your ballast. Some of your ballast is compensating for things like an empty buoyant tank, which isn't something that will change. And as long as the ballast isn't all attached to the rig, the bladder doesn't have to do all the lifting.

 

[somegum]

Hi again,

The advice to reduce my lead load is helpful. Thanks for that. Regardless, I will still need to lift the same amount of weight, whether it is in lead-form or 'steel tank/tank adapter/etc' form. So, shouldn't I be concerned about a BCD with a lift capacity less than the total weight of my gear?

If I did so BP/W, how do I select the amount of lift that I need. Is it not true that the lift needs to exceed the weight of the gear?

Thanks again,

somegum

 

[TSandM]

By going to a backplate, you can usually drop two to three pounds of weight that you were using to sink the padding on your BC. That's the only amount you drop from your total ballast. But you do get to integrate 5 pounds (or more) of your ballast into your rig, thus reducing the amount on a belt or in pockets.

Again -- you add up the total negative stuff in your rig, and for me, that's:

Full tank -7
Backplate -5
Regulator -2
Hardware -2
Camband weights -6
Canister light -1

That's 23 pounds negative on the rig. I have a 30 lb wing. It's enough.

Exposure protection:

Suit and undergarments +32 (This is the maximum lift I can lose with a complete suit flood, and having had some major leaks, it takes quite a bit of time to lose all of it)

Weight belt -20 (This I can jettison if need be)

So, if I have a total flood and lose ALL the lift from the suit, I'll probably have to drop some weight to remain buoyant at the surface.

Does this make sense? It's not the total weight, it's the balance between the weight and the things that float, and where all those things are distributed.

 

[somegum]

Thank you very much. Your help is greatly appreciated.

somegum

 

[rstofer]

A BC compensates for the change in a diver's buoyancy. I use a 7mm wetsuit and I guess it can lose 20# of buoyancy. I also use an HP100 that starts out, at most, 8# more negative than it winds up - the weight of the air in the tank.

At MOST, I need to compensate for 20 + 8 = 28# CHANGE in buoyancy. Assuming I am properly weighted, a 30# lift wing is adequate.

But there are 4 distinct conditions under which buoyancy must be reviewed:
1) start of dive at the surface - full tank with buoyant wetsuit
2) start of dive at great depth - full tank but wetsuit has lost buoyancy
3) end of dive at great depth - empty tank but wetsuit has still lost buoyancy
4) end of dive at the surface - empty tank but wetsuit has regained SOME buoyancy

There is a nice little 'sticky' spreadsheet at the top of this forum that allows you to experiment with buoyancy and flotation.

I don't know anything about drysuits but it is my understanding that they don't lose buoyancy. If that is the case, regardless of how much weight you need to carry to sink the drysuit, your change in buoyancy, for which the BC must compensate, is only the change in the weight of air in the tank. Perhaps the drysuit does have some change in buoyancy from surface to great depth - I wouldn't know. But if it does, that too must be compensated for.

One thing the spreadsheet uses is 10# of flotation to get the diver's head above water. But that's pretty easy to get (for a wetsuit diver) because one approach to adjusting weight is to get eyeball level with an empty BC and a full tank. Inflating the BC will result in lifting the diver higher in the water as long as part of the bladder is underwater. This might lead you to think that a crotch strap is helpful. You would be right.

This approach works for a wetsuit because the suit will not regain all of its buoyancy during the ascent. As a result, it is still possible to make a 15' stop with a nearly empty tank. I don't have a suggestion for a drysuit diver. If your suit doesn't change buoyancy then the weighting will need to be somewhat different. Maybe the classic empty BC and near empty tank at 15' approach.

Richard

 

[TSandM]

rstofer, a properly working drysuit doesn't change buoyancy through the dive (or shouldn't anyway, if the diver is managing the suit properly). But you do, as I wrote above, have to plan for a flooded suit and a loss of most if not all of the buoyancy from the suit and undergarment.

I've had a bad flood, a really bad one -- swimming underwater wasn't terribly difficult, but maintaining positive buoyancy on the surface was more of a challenge. I know I'm right on the edge of acceptable lift from my wing, but then again, if I really got into trouble, I have 20 lbs I can ditch.

 

[rstofer]

rstofer, a properly working drysuit doesn't change buoyancy through the dive (or shouldn't anyway, if the diver is managing the suit properly). But you do, as I wrote above, have to plan for a flooded suit and a loss of most if not all of the buoyancy from the suit and undergarment.

When I went back and read your prior post, I realized that I wasn't accounting for a flooded drysuit. But you pointed out that dropping weight is an alternative so I let my comments slide. It's pretty clear I really don't know anything about drysuits.

One of these days I just have to get with the program and buy one.

Richard