Enough lift?

[DevonDiver]

Good point.. Which means I may have to rent huh?

You need enough weight to compensate for wetsuit compression (buoyancy loss) at depth, plus air consumption (buoyancy loss) over the duration of your dive.

You have least buoyancy at the start of the bottom phase of your dive.... when you are deepest (most wetsuit compression) and you have not yet consumed significant gas (weighting in the assumption of being below minimum reserve in calculations to cover all contingencies). At this stage, your BCD buoyancy is needed - providing the capability to achieve neutral or positive buoyancy.

As the dive progresses, the negative buoyancy from gas supply decreases and, if multi-level profiling/on ascent, your wetsuit regains some buoyancy. A point occurs where your weighting is needed - providing the capability to achieve neutral or negative buoyancy.

Your BCD buoyancy capacity should also be sufficient to support that weight in toto (positive buoyancy). For instance, if you should remove the BCD at the surface - it needs to be able to float.

18lbs wing buoyancy is unlikely to be sufficient for any diver using a 7mm wetsuit or thicker (for some, a 5mm and thicker) and/or using steel or high capacity (more than 80cuft) cylinders.

 

[ams511]

You need enough weight to compensate for wetsuit compression (buoyancy loss) at depth, plus air consumption (buoyancy loss) over the duration of your dive.

You have least buoyancy at the end of the bottom phase of your dive.... when you are deepest (most wetsuit compression) and you have consumed significant gas (assume below minimum reserve in calculations to cover all contingencies). You are still carry weighting that you needed for earlier stages of the dive. At this stage, your BCD must supplement your buoyancy - providing the capability to achieve neutral or positive buoyancy.

Your BCD buoyancy capacity should also be sufficient to support that weight in toto (positive buoyancy). For instance, if you should remove the BCD at the surface - it needs to be able to float.

18lbs wing buoyancy is unlikely to be sufficient for any diver using a 7mm wetsuit or thicker (for some, a 5mm and thicker) and/or using steel or high capacity (more than 80cuft) cylinders.

Please explain further. I would assume that you would have the least buoyancy at the beginning of the bottom portion of the dive because the wet suit is compressed and the you still have most of your gas. For example a Al 80 has about a 5 lb swing going from full to almost empty.

 

[antattack]

The best practice for recreational diving is that your BCD needs to be able to do three things.
1. Your BCD should be able to float your tank (Full) and all your gear independently of the diver at the surface.
2. The BCD should also be able to float all your gear and YOU with your head above water in an upright position.
3. If you are using a compressible exposure suit, you need to be able to swim your gear up at depth. That means that if you are NOT using a dry suit, you will need about twice the reserve buoyancy used at your safety stop for a dive at the sport limit, assuming you are a reasonably strong swimmer. Overhead environment divers, you already know your requirements, so this is obviously simplified for recreational diving.

Now let's assume you are diving a dry suit, because that takes case #3 off the table for the moment, we'll come back to that one.

Most divers in any exposure suit are positively buoyant, so the first rule often determines the requirement.

The practical way to do this is to find a salt water pool, put all your gear on with a near empty tank and do a weight check, then take off all your gear and see if your BCD can float it. Done. But there are few salt water pools and fewer dive shops that will let you try that new BCD you really want to buy in a saltwater pool. So, let's try some math.

Here's how to calculate the requirement:

Add up the buoyancy of all your gear and you get the minimum lift of your gear. To do this, you will need to know your minimum weighting at a safety stop. But of course you have been logging all the gear you wear on every dive, so calculating that should be easy! My dry suit demands that I have a minimum of -22# with 500psi in my tank at 15fsw

With a bit of apprehension, I'll do my kit here in public to show you how

Let's see, my steel HP 100cf tank is about -10# when full (-2.5# when empty), -2# for my first and second stage, -1.5# for my aluminum backplate, -7# for my single tank adapter and keel weight, -1# for my jet fins, and -8# for my lead. I need a bit more than +28.5# of lift in my BCD- In real life, I use a 40# wing for my cold water adventures which gives me a bit of room to work with. My gear is safe without me, and I will be buoyant without my gear. The two requirements are met.

Now let's do a doubles rig, just to make sure it still works. The doubles rig has two steel 100cf tanks, so that part becomes -20# full and -5# empty. There is no STA and keel weight, but I add in tank bands and a manifold, for -7#. Add another set of regs (-4# now) and keep my Aluminum backplate -1.5#, and fins -1#, add -4# of lead and I'm ready to dive! So now I'm -37.5# with tanks full and -22.5# with tanks empty. I can get by with a 40# wing in my doubles using the same dry suit. It's a balanced rig, and I don't intend to do safety stops with empty tanks, so there is a bit of conservatism built in, but if I get a set of HP Steel 120 tanks, I'll need a higher capacity wing.

OK wet suit lovers, your turn.

You already know that you need to know your total balast requirements with near empty tanks at your safety stop. Let's say you have the same requirement as me in my wet suit, -22# at 15fsw. What happens at 130'?

At 15 fsw, your wet suit is compressed 1.5 bar at 15fsw. Remember boyles law? What happens at 5 bar? That's right, your wet suit will be compressed to near nothingness and loses just about all of it's buoyancy (you'll have about +2# at depth.)

So here we are at the bottom and we've lost +20#. In my singles kit, I have an extra 11.5# of lift and I might consider swimming up -8.5 with a completely full BCD. In reality, I know I could do this, but this would add un necessary task loading. Besides, nobody looks cool swimming around at 130' negatively buoyant. This is one of the reasons I dive a dry suit. I don't want to be cold at 130fsw, and I certainly don't want to be negatively buoyant, fighting to maintain trim.

The doubles kit is a non-starter in a wet suit at depth, I'd have to jump up to a +60# wing using a wet suit.

Wet suits require some additional planning if you will be diving deep.

I hope that was useful!

 

[DevonDiver]

Please explain further. I would assume that you would have the least buoyancy at the beginning of the bottom portion of the dive because the wet suit is compressed and the you still have most of your gas. For example a Al 80 has about a 5 lb swing going from full to almost empty.

My bad - typo. Thanks for pointing it out

 

[clownfishsydney]

Remember, a BCD advertised with 30 lbs lift probably does not have 30 lbs of lift. As explained by Michael Hollis (of Oceanic and Hollis equipment) at the trial of Gabe Watson, the lift is measured when there is no-one using the BCD and it is filled to capacity and then weights hooked onto it till it sinks. When put on a person, a considerable amount of the lift is lost due to less internal volume.

 

[alfie227]

thanks for all the replies everyone, feeling much clearer on the idea.
if real lift is not necessarily the same as advertised lift how large a safety margin would you suggest?

 

[SailNaked]

Why spend time calculating? put your wetsuit in the water and see how much weight it takes to sink it, do you float in the water? how much weight could you set on your stomach before you would go down with full lungs? does the BC float on its own? an aluminum 80 has 6# of breathable air.

just put it all on and with a full breath of air you should just float at eye level with an empty tank. if the tank is full then add 5# after bouyancy check.

do it at the dock before you go down on first dive.

oops did not answer your question: you need enough lift to get your buddy up if their bc does not work, but that should not be more than a few pounds unless they are really weighted badly. remeber the goal is to be weighted so that there is only 5# of lift in your BC at the begining and 0 at the end of your dive (with an Alum 80#)

 

[DevonDiver]

Remember, a BCD advertised with 30 lbs lift probably does not have 30 lbs of lift. As explained by Michael Hollis (of Oceanic and Hollis equipment) at the trial of Gabe Watson, the lift is measured when there is no-one using the BCD and it is filled to capacity and then weights hooked onto it till it sinks. When put on a person, a considerable amount of the lift is lost due to less internal volume.

That'd just be a badly designed BCD then...

 

[boat sju]

Are you factoring in if one or the other has a wing failure and one of them needs to get the other off the bottom in a controlled manner? I recommend 35 lb to 40. lb.

ams,

No I'm not factoring in a wing failure. You can only take redundancy/backup systems and "factors of safety" so far. A total spontaneous wing failure is hard for me to fathom. A wing failure shouldn't be catastrophic in a properly weighted diver anyway and you should always have the ability to ditch weight in a true emergency.

boat

 

[Pin83]

Hi Alfie,

if you're doubling-up 7mm wetsuits to stay warm, it is very likely you will end up in a drysuit down the track. Give serious thought to a backplate and wing...better value than a vest, and there's an almost unanimous love of them on here for a reason.

If for some reason you're against a plate, at least go for a Zeagle Ranger or DiveRite Transpac, so when you decide you wish you'd bought a plate, you can keep your wing and it's only $200 for a plate and some webbing.

As far as wing sizing goes, AntAttack's advice is solid. You don't want a doubles wing on a single tank really, but I happily dive a 44lb Zeagle Ranger wing on a singles plate all the time, and it's never taco'd on me (I like to have plenty of potential bouancy when I'm DMing, just in case...)

Pin