Lift Capacity...

[H20Bubbles]

When does a BCD's lift capacity really come into consideration? For example, in recreational diving wouldn't 18lbs lift capacity be sufficient?

The only scenarios that I can think of are where you are carrying so much gear that you become inherently negatively bouyant.

 

[JimC]

It needs to be able to float your gear without you in it.
It needs to be able to get your head out of the water at the surface.
It needs to provide enough lift, at depth, to get you neutral with all the gear and gas you are going to carry.

18# might be sufficient with an AL80 and a 3mm or less.
I find 27-35# to be ideal for most single tank diving.

Assumeing your a "normalish" size.

 

[H20Bubbles]

It needs to be able to float your gear without you in it.
It needs to be able to get your head out of the water at the surface.
It needs to provide enough lift, at depth, to get you neutral with all the gear and gas you are going to carry.

18# might be sufficient with an AL80 and a 3mm or less.
I find 27-35# to be ideal for most single tank diving.

Assumeing your a "normalish" size.

Given the fact that you are generally having to add weight to achieve neutral/negative bouyancy why would 27-35 lbs be an ideal? When you look at the numbers....

Tank 5lbs
Regs - 1 to 2 lbs
Small amount of misc gear 3 lbs

Once you're properly weighted, the body is perhaps 1-2 lbs negatively bouyant.

So a total of 12 lbs negative bouyancy... what am I not getting?

Again, if you are carrying cameras, lights, tech gear, etc... then I could see the numbers adding up.

How does the wetsuit come into play? Even with a 7mm suit.. with proper weighting aren't you going to achieve just slightly negative bouyancy to get under the water?

 

[spectrum]

Given the fact that you are generally having to add weight to achieve neutral/negative bouyancy why would 27-35 lbs be an ideal? When you look at the numbers....

Tank 5lbs
Regs - 1 to 2 lbs
Small amount of misc gear 3 lbs

Once you're properly weighted, the body is perhaps 1-2 lbs negatively bouyant.

So a total of 12 lbs negative bouyancy... what am I not getting?

Again, if you are carrying cameras, lights, tech gear, etc... then I could see the numbers adding up.

How does the wetsuit come into play? Even with a 7mm suit.. with proper weighting aren't you going to achieve just slightly negative bouyancy to get under the water?

Assuming the diver starts out properly weighted to end the dive neutral they already need 5 pounds or so of lift to stay on the surface at the start of the dive, probably more than 5 pounds of lift to be comfortable. In any event you are already 5 pounds negative at the onset.

Now get in to a full 7mm wetsuit, probably 2X on the core and make a fairly deep dive. If you have 18 pounds of lift and and already need 5 to stay neutral there will be a point where your wetsuit losses 12 pounds of lift and if you go too far you will plumet into the depths as you become negative fit a full BC. This improves a little as you breathe the tank down but for a wetsuited diver going deep it's on the hairy edge at best.

You also need to be able to float the rig off youeself if needed. Cylinders wil start the dive negative to some extent and 18 pouinds of lift will not afford much extra lift for trim or integrated weights.

For a warm water diver 18 pounds may be all fine and good.

Pete

 

[spectrum]

When does a BCD's lift capacity really come into consideration? For example, in recreational diving wouldn't 18lbs lift capacity be sufficient?

The only scenarios that I can think of are where you are carrying so much gear that you become inherently negatively bouyant.

It's all in the name, a Buoyancy Compensator.

Look at your configuration and decide what will require compensation. The big culprit is usually the combination of wetsuit buoyancy and the loss of that buoyancy at depth.

As an example.... If I go skin-diving in my full 7mm I wear 26 pounds of weight. This puts me comfortably on the ocean surface but I can dive down at will. Somewhere between 12-15 feet I go neutral and just hang there until I kick upwards and my suit begins to expand creating more displacement and increasing my buoyancy. If I were to go to something like 20 feet I will find a point where I continue sinking and need to swim up past my neutral depth.

In a scuba dive you have varying depth and and declining weight of air in your cylinder. You are also needing to be neutral in the full range of depths that you traverse. Ironicaly your dive weights are not a big deal at the surface since your wetsuit is at maximum buoyancy. Remember that almost all of that weight is being worn to counteract the lift of your wetsuit.

As you netions any other gear that you keep with you needs to be acounted for. An 8 c-cell dive light makes a big difference in a warm water skin dive when wearing just 3mm.

Obviously in the case of a drysuit diver suit buoyancy is configurabe and this results in a whole different discussion.

Pete

 

[H20Bubbles]

Thanks Pete... nice explanations. At what depth do you start to see the negative effects of compression on the wetsuit - thereby causing a loss in bouyancy?

 

[wabbitman]

I logged on to ask questions along just these same lines... I am replacing my BC and have stumbled into having to understand just these issues.

I dive in the Caribbean a couple of weeks each year and do several dives in the cold water of Ontario. When south I use a 3mm full wetsuit, (with 18lbs o' lead), and when diving in Ontario I use a full 7mm double-core wetsuit, (with 28lbls o' lead) - this all seems to have worked out pretty well so far but the BC is not comfortable and is not really a good fit. I am 6' and about 220lbs - relatively solid. I bought a cheap Dacor Enduro BC when I first started diving and its time to replace with a back-inflate.

I was considering a Zeagle Stilleto but my LDS is suggesting that the 34lbs of lift is inadequate for the 7mm cold water diving. I don't want to over equip here as I also want to keep the BC as low profile and light/packable as possible. It seems the next step up in lift is somewhere around the 44lb lift bladder of the conceptII/Ranger series.

I wonder if I really need all this lift - by my calcs the 34lbs seems adequate...?

 

[spectrum]

Thanks Pete... nice explanations. At what depth do you start to see the negative effects of compression on the wetsuit - thereby causing a loss in bouyancy?

Almost immediately. This applies to gas in any soft part of the system at the start of the dive. Gas bubbles in the neoprene, any remaining air in your bcd and small pockets of air still in your suit, bcd etc. They all start compressing and loose displacement as soon as you start down.

On a shore dive we may snorkel out to where we have 8-10 feet of water before going down. Even if I make a very slow patient start without dumping any more air than needed I will be adding in a number of puffs to the BCD on the way down to allow for slow equalizing to avoid touching bottom. The changes are most dramatic in the shallows and at the start of the dive. And yes my weights are dialed in. This in full 7mm.

As for specifics there is no good data. Exact neoprene formula, square inches of material and age all come into play. As neoprene ages the gas gets squezzed out through the porous rubber and it becomes a more neutral, less insulating material.

Pete

 

[divesmith]

I found that a 35 lb lift capacity worked well. However, in salt water with my 5 ml wetsuit I needed more weight than it could lift WHEN NOT ATTACHED TO ME. I opted to change my BC and increase the lift so I would have the capability of floating my gear if I needed to remove it while still in the water without having to remove some of the weight first.

 

[JimC]

Thanks Pete... nice explanations. At what depth do you start to see the negative effects of compression on the wetsuit - thereby causing a loss in bouyancy?

Immediately. Thank boyles law. At 100 feet your wetsuit will provide about 1/4 the buoyancy it did at the surface. So if you need 20# of lead for the wetsuit (normalish for a 7mm), you need 15# of lift just to offset that. Add in 6# of air and your into the 21#s of lift to get neutral at depth. In a drysuit, you need to prepared to augment the full lift of the drysuit in case you flood the thing. Which is again in the 15-20# range for most people.

And thats just underwater. At the surface your BC needs to float your gear without you. Which means tank, stuff and any integrated weight. Which could be anywhere from 6# to 30# depending on your configuration.

It also need to be able to get your head out of the water in a good swell. Your head weights somewhere in the 15# range. And again, since your tank has 6# of air in it.. you need that too for 21# of lift.



That assumes your properly weighted in the first place. So, for a properly weighted single tank diver your looking at around 30# of lift for most people when properly weighted.