Weight-integrated BCD with trim pockets and positive buoyancy?

[seeker242]

So I just picked an aqualung axiom BCD. It was the one I trained in and it was good so I got one. It has 2 releasable weight integrated pockets and 2 trim pockets near the tank band, of non-releasable weight. But I didn't use either because I just had a weight belt at the time. I plan on using them now and I was reading the manual and it has this warning about the trim pockets.

WARNING: The non-releasable weight pockets are intended strictly for containing nonreleasable
weight, used in addition to releasable weight. Do not fill either pocket with
weight unless you are certain you can achieve positive buoyancy at depth by releasing
your weight pouches or weight belt while your BC is completely deflated.

My question is theoretically what exactly is going to create the positive buoyancy if your BC is completely deflated and you drop the releasable weight? Especially if you are wearing a full tank like an AL80, which is negatively buoyant? Is it just the buoyancy of your body? The buoyancy of the deflated BC? It seems different people have different body buoyancy characteristics depending on their weight, body fat %, muscle mass, etc. With some being naturally N buoyant and some naturally P buoyant. I haven't tested this weight configuration out in the water yet but I'm planning to. I'm just theoretically curious as to where the buoyancy is going to come from according to the above warning? Especially if you happen to be naturally negatively buoyant body type? Two other factors that come to mind would be salt water and shorty or full wetsuit. Although, I don't know how much of an effect they would have.

 

[Dive Right In Scuba 2]

The wearing of ballast is usually to offset the buoyancy of your exposure protection, body mass, cylinder(in the case of an AL80 that is below...say 500psi), etc. With that in mind you should check the amount of weight you need and don't overweight yourself. So in theory, you should be able to dump a rather small amount of weight to achieve neutral buoyancy.

So basically, that warning says to not overweight yourself.

 

[RJP]

How much weight did you use in your OW course? Consider that, assuming it was the correct amount of weight to achieve neutral buoyancy, you would be positively buoyant if you ditched half of it.

 

[SFLScubapro]

First: Good choice on the Axiom: I love mine for the Wrapture feature that puts the weight on your hips instead your back. I love that (I have other sets as well, but that one has a warm spot in my gear rack) one. As to why you'd be neg or positively buoyant, the trim weights help to evenly distribute the weight off your hips to around the cylinder, i.e. the heaviest portion of your gear. However, you usually only put a few pounds in that area to help you keep the weight evenly distributed. Your weight pockets will have the majority of weight; if you take a course in perfect buoyancy, you'll learn that you balance weight to just about eye level in salt water. So, if you are weighted properly, when you dump (hopefully, you never have to) your weights, you are positively buoyant. Additionally, as noted by another commentator, AL80s go positive as they empty out; you probably notice this towards the end of the dive you have to dump air from your BC to stay neutral. Bottom line: Don't put too much weight in the trim weight pouches and all will be good. Have fun with the Axiom.

MS

 

[Neilwood]

YMMV but over here I would be wary of putting all your ballast in one place ie the ditch-able BC pockets.

If I dive in a wetsuit here (albeit a 7mm), I am hugely positively buoyant to the extent that I needed 30+lbs of ballast to offset it (and the buoyancy of the BC). Putting all of it in the BC means that in the event of a doff/don situation I would have difficulty in holding onto the BC as I would be so much more buoyant than it - one slip and I would be shooting upwards away from it at a rate of knots.

What makes sense here is either a harness (my preferred option with ditch-able weight in it's pockets) or belt weighted to offset the suit and the BC weighted to offset its buoyancy so both are (relatively) neutrally buoyant but if I need to drop weight I have two options.

 

[seeker242]

Thanks all. I used 14 lbs and that was good. 2 x 4lb and 3 x 2lb . I did float at eye level at surface, was able to stay at safety stop with minimal trouble with near empty aluminum tank, etc. I'm just curious as to what causes positive buoyancy when you drop the weight? Not really a practical question per say, more just curious about buoyancy in general . I understand you would be positively buoyant if you are not over-weighted with non releasable weights and drop the releasable weights, I'm just curious as to why exactly that is the case. I guess it's just a combination of the body in salt water?

 

[stuartv]

How much weight did you use in your OW course? Consider that, assuming it was the correct amount of weight to achieve neutral buoyancy, you would be positively buoyant if you ditched half of it.

Can you explain that?

Ditching 10 pounds of weight will make you 10 pounds less negative, whether you're at 15 feet or 115 feet, right? But, your actual buoyancy could be quite a bit different at those two depths, depending on what exposure protection you're wearing. Assuming you're correctly weighted, if you dive to 115' in a 7mm suit, when you reach max depth at the start of the dive, I don't understand how you can know that ditching half your weight would make you positively buoyant. Imagine you have 20 # of weight to be neutral at 15' with 500 psi in the tank and an empty BC. If you descend to 100' at the start of your dive it seems like you could easily lose 10 # of buoyancy from suit crush. Plus you're an additional 5 # negative from the gas you're carrying. So, you're 15 # negative. Ditching half your weight (i.e. 10#) is not going to make you positive (which is what the OP asked), right? Am I just making up numbers that would never actually happen in the real world?

 

[archer1960]

Thanks all. I used 14 lbs and that was good. 2 x 4lb and 3 x 2lb . I did float at eye level at surface, was able to stay at safety stop with minimal trouble with near empty aluminum tank, etc. I'm just curious as to what causes positive buoyancy when you drop the weight? Not really a practical question per say, more just curious about buoyancy in general . I understand you would be positively buoyant if you are not over-weighted with non releasable weights and drop the releasable weights, I'm just curious as to why exactly that is the case. I guess it's just a combination of the body in salt water?

It's pretty simple: if you are near neutral buoyancy at depth, and you dump weight, you quickly become positively buoyant.

 

[RJP]

I'm just curious as to what causes positive buoyancy when you drop the weight?

Really? You might need to go back to Chapter 1 of your OW manual.

If you are weighed such that you weigh the same as the water you displace, that's "neutrally buoyancy."

If, at that point, you ditch weight you will now weigh LESS than the water you displace. That's the very definition of "positive buoyancy."

---------- Post added September 15th, 2015 at 10:37 AM ----------

Can you explain that?

Can't make it any simpler. Sorry.

 

[stuartv]

It's pretty simple: if you are near neutral buoyancy at depth, and you dump weight, you quickly become positively buoyant.

The OP did originally specify "BC is completely deflated." Your "if" is adding a presumption that the OP specifically (by implication) excluded.

The OP is a new diver. The possibility exists that the new diver would be correctly weighted and start the dive by hitting the deflate button and dumping all the air from their BC to descend. Then maybe get a little distracted on the way down (say, focusing on equalizing to relieve the growing pain in their ears instead of watching their depth gauge and controlling their descent) and find themselves lawn darting towards the bottom. If said diver suddenly realizes they've dropped way deeper than intended and they start to wig out, "drop half your weight to achieve positive buoyancy" seems like it could be both completely incorrect and the wrong response anyway.