Weight-integrated BCD with trim pockets and positive buoyancy?
- Thread starter seeker242
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Not looking for an argument. Responding to this:
Given what the OP actually asked, the only time he would be neutral (with an empty BC) is at the end of the dive, at the safety stop. Deeper (while wearing neoprene)? He'll be negative. Earlier in the dive? He'll be negative. Deeper and just started? He'll be very negative. So, I think the responses predicated on him being neutral are not actually addressing the actual question asked.
OP
seeker242
Contributor
Yes, that is the very definition of positive buoyancy and I'm well aware of the definition. However, my question was what is it exactly that causes you to weigh less than the water you displace? If the persons body is naturally negativity buoyant, and the tank is negatively buoyant, then it can't be their body or the tank that causes it. So then it's just the deflated BCD itself? The hoses? The wetsuit? The salt water? Etc, etc?
archer1960
Contributor
Very few people have a body composition that makes them negatively buoyant, especially in salt water. The tank may be positive or negative depending on the tank and its contents. A wetsuit is probably always positive even at depth, though maybe only slightly when you're deep enough.
As in weight ditching aside? Lungs are the biggest thing in the body. Fatty tissue: brain, boobs, spare tire is lighter than water, muscle and bone's heavier. Air bubbles trapped in neoprene provide more or less lift depending on the size & thickness of the suit (and also depth since they get compressed). Last time I dived in a jacket BCD I wore 9lbs of lead, in a BP&W: 2lbs. So the BCD can add a few pounds. Compressed air inside the cylinder is dense and weighs more, so full tank is heavier than water. An empty tank is basically an air bubble in metal casing and its weight depends on the metal. Al80 is a few pounds lighter than water. So basically "all of the above".
What Archer said. If you're wearing a wetsuit, it will probably provide enough buoyancy to get positive, if you ditch enough weight. A dry suit definitely would.
However, the more important thing to know is that, generally, the experienced and knowledgeable divers on here (not that I am one of those, mind you - I've just read this multiple times) say that there is no Recreational diving scenario where you should ever need to ditch weights at all (other than possibly when you're on the surface, to maintain positive buoyancy). If you are weighted properly and have a BC or wing of the right lift capacity, the wing will be able to lift you off the bottom. If you were to have a complete, 100% totally catastrophic failure of your BC or wing, then you should be able to swim yourself up to the surface. That latter depends on the presumption that you are properly weighted. If you are carrying 10 # of weight more than what you actually need, then you might not be able to swim it up and so would need to be able to ditch some.
The idea of being properly weighted, again, for recreational diving, ranges from diving in just a pair of shorts with an AL80 in salt water, to diving in a 7mm suit with a single steel tank in fresh water. With shorts and an AL80, "correct" weighting would yield little or no weights at all - and you should be able to swim that up from any depth. With a 7mm suit and a steel tank, you might have a fair amount of weight on to be able to sink that 7mm suit. And, at depth, the suit will compress, losing buoyancy. At the deepest recreational depths, a thick wetsuit could compress enough to lose somewhere around 75% of its buoyancy. If you're at the start of your dive, your tank is full and so it is the most negative that it's going to be. So, if you had a complete BC or wing failure, at 130' (the max depth for recreational diving), right at the start of your dive, that would be your worst case. At that point, you would be negatively buoyant by the amount your suit compressed plus the weight of the gas in your cylinder (e.g. for an AL80, 5 to 6 pounds). Fortunately, if you had that worst case scenario, there would be a couple of things working in your favor. One, you would not have to swim up very far for your suit to start decompressing and regaining some of its buoyancy. If you were, for example, 20 pounds negative at 130', you would not have to swim that 20 pounds all the way to the surface. Remember, because we're assuming you're correctly weighted, that means when you get to 15', you should be neutral with an almost empty tank and an empty BC. That means, with a full AL80, at 15' you'll only be 5 - 6 # negative. So, in this example, you'll gradually go from 20 # negative to 5 # negative as you swim up.
The problem you always have to keep in mind is if you are correctly weighted and you drop weights when you are 100' down (and wearing a wetsuit that is compressed because of the depth), as you ascend your wetsuit is going to become more positively buoyant and (again, if you were correctly weighted to begin with) you are going to become positively buoyant overall (unless you have a full tank and only ditched 5 or less pounds) and end up corking right to the surface, possibly inducing Decompression Sickness due to a too-rapid ascent.
Also, if that worst case scenario really did come to pass and your BC is completely useless, you can hold down the purge button on your octo to dump air. The more air you dump, the lighter your tank will get, also helping you to swim up. Wasting air in an emergency situation may sound counterintuitive, but the option is still something to keep in mind. EDIT: As a LAST resort!
The next thing working in your favor is that a 100% failure of your BC is pretty darn unlikely (according to what I've read). If it does have a failure, it will most likely still hold some air - just not as much as it's supposed to. So, it will still give you some lift, in most failure scenarios. Especially if you maintain some self-awareness and can orient your body to keep the hole in the BC at the lowest point.
The last thing working in your favor (if you're striving to be a self-sufficient diver) is that you should have a buddy who can help you get to the surface, if your BC really does have a complete failure.
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bryanmc57
Contributor
Ummmm yeah... I think I'd rather have a tank full of gas if I had an issue at depth than a 3.5lb lift bag disguised as an aluminum 80. BTW Stuart, an aluminum 80 is only ~2 lb negative when full.
Stuart, your posts are usually thoughtful, if a bit long. And I do admire the thoroughness with which you examine many concepts in diving, not being willing to just accept things on face value.
But this should not be posted in a forum where new divers are looking for advice. It is a very dangerous suggestion.
Yes, by completely dumping a full AL80, you will drop 6 lbs of negative buoyancy - I'm assuming that the scenario that you envision would involve less gas than that. It would be extremely unusual for a diver to be unable to swim up their rig due to wing failure where 2-4 lbs will make the difference. And in that very rare situation, we are almost certainly talking about a new diver, who should be the last person to think about dumping breathing gas as ballast.
Also, remember that intentionally free-flowing a regulator at depth can cause it to stick open, which can completely empty an 80 in less than 3 minutes.
This is a recipe for a panic spiral and a disaster. Don't do it. Even as a "last ditch effort", better to sit on the bottom, breathe and think things through - you will either find a buddy to help or eventually get rid of that gas weight anyway...
OK, I will play.
My BCD has totally failed. I am somewhat negative since I have a pony and I weight myself for not having a pony in case I have to donate. I am diving in NC.
I could swim myself up but adding on leg cramps to make this more interesting I could do one of several things.
- Inflate SMB and use it to slowly help raise myself like using a lift bag but not attaching myself to it. I do have an exhaust valve down low on it.
-pass off the AL 19 pony to my buddy since it is slung.
-Figure out where the leak is and orient myself so the leak is down and the BCD holds some more air,
-Swim back to the anchor line and proceed up it. There is usually an anchor line in NC diving. If I am coming up from 100 ft in a 7 mil I will gain some buoyancy as I rise.
-since lead is cheaper than a new pony if no buddy around I could also keep the pony and drop a little weight from one of my two integrated weight pockets. I might do this also in some of the other scenarios once I reach the surface as needed to make staying at surface easy. Depends on wet suit. I have practiced taking pockets out and putting them back in under water.
My BCD has totally failed. I am somewhat negative since I have a pony and I weight myself for not having a pony in case I have to donate. I am diving in NC.
I could swim myself up but adding on leg cramps to make this more interesting I could do one of several things.
- Inflate SMB and use it to slowly help raise myself like using a lift bag but not attaching myself to it. I do have an exhaust valve down low on it.
-pass off the AL 19 pony to my buddy since it is slung.
-Figure out where the leak is and orient myself so the leak is down and the BCD holds some more air,
-Swim back to the anchor line and proceed up it. There is usually an anchor line in NC diving. If I am coming up from 100 ft in a 7 mil I will gain some buoyancy as I rise.
-since lead is cheaper than a new pony if no buddy around I could also keep the pony and drop a little weight from one of my two integrated weight pockets. I might do this also in some of the other scenarios once I reach the surface as needed to make staying at surface easy. Depends on wet suit. I have practiced taking pockets out and putting them back in under water.
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