Quiz - Skills & Environment - Overweighted Diver

[Angelo Farina]

Angelo,

capo - head / volta -turn / capovolta: somersault.

Was the capovolta maneuver akin to what we call a surface dive in the skin diver course?

Addressing your other point:

Let's assume there is a point during your descent when your bodily and carried weight, the buoyancy of your tank, and the compression of your suit balance out to make you perfectly neutral. Let's assume this point happens at 20 meters.

What happens next? You continue breathing, so your tank will get lighter. You can continue to go deeper, but your suit can compress only so far. Half of the possible compression took place in the first 10 meters of your dive. And half of the possible compression remaining after that took place in the next ten meters of your dive. After that point of perfect weighting, going deeper will increase the rate at which you add buoyancy to your tank, but the rate at which your wetsuit compresses will decrease. The continued compression of your wetsuit won't keep up with your increased air consumption. At any time after that point, you will be underweighted.

I never said that the diving profile must be linear for keeping the buoyancy constant... of course as you go deeper, the air consumption will increase, so the tank will empty faster.
Let's assume that with the tank full, after the "capovolta", you reach 10m kicking down, and you are neutral. This is what we were taught to be a perfect weighting. At this point, as soon as you feel too light, you swim down, keeping the buoyancy constant. The suit compresses with almost inverse proportionality with the absolute pressure, as what is compressed is the gas trapped in the cells, which follows approximately the Boyle gas law. Hence, going deeper, there will always be a depth where you are neutral.
At the end of the dive you are neutral possibly at 30 meters, or more. As you start swimming up, you become positive, and this was nice, you could stop kicking and leave the buoyancy to bring you up "slowly" (at the time the maximum ascent speed was 18 m/min).
All this is considered quite unsafe nowadays, but in 1975 this was a standard diving profile.
And for correcting some deviation from perfect neutral buoyancy, we were taught to use our lungs...

 

[Scraps]

Angelo,
I believe you executed the dive profile you described, but I think your lung/diaphragm control was a bigger factor than the compression of your wetsuit.

They buoyancy of an aluminum 80 tank goes from -0.82 kg full to +1.82 kg empty, and that buoyancy gain occurs fastest at the depths you describe. Nearly all 3 mm wetsuits have an inherent buoyancy of less than 1 kg, only part of which is lost as one dives deeper, and most of the loss occurs in the first 10 meters.

I don't see how your buoyancy loss due to wetsuit compression can keep up with your tank's buoyancy gain.

 

[Angelo Farina]

Angelo,
I believe you executed the dive profile you described, but I think your lung/diaphragm control was a bigger factor than the compression of your wetsuit.

They buoyancy of an aluminum 80 tank goes from -0.82 kg full to +1.82 kg empty, and that buoyancy gain occurs fastest at the depths you describe. Nearly all 3 mm wetsuits have an inherent buoyancy of less than 1 kg, only part of which is lost as one dives deeper, and most of the loss occurs in the first 10 meters.

I don't see how your buoyancy loss due to wetsuit compression can keep up with your tank's buoyancy gain.

3mm ??? The standard was wearing a 6.5mm here, and doubled on chest for the two-pieces suit. Total buoyancy of the Rubatex suits was more than 6 kg. At 30m, the remaining buoyancy was around 1.5 kg (4 bars). So going from 0 to 30 m you loose 4.5 kg. Going from 10m to 30m you still loose 3 kg. Which was more or less the weight of air having been consumed...
And yes, we were using lungs for compensating deviations of buoyancy form neutral of +/- 1 kg. The point is that, with proper weighting, and not having to worry about deco - safety stop, it was perfectly possible to dive without BCD staying almost perfectly neutral for most of the dive.

Last trick.
When deco was planned, at the beginning of the dive we were wearing an additional weight belt, typically with 4 kg. After reaching the anchor, at 10 or 15 m depth, we were attaching this additional belt to the rope of the anchor (which helps to keep it down, ensuring better anchoring).
At time of surfacing, we did retrieve the additional belt at the anchor, close and pack the umbrella-type anchor, and go up the rope, making the required deco stops in slightly negative buoyancy...
I did this for a couple of years, then I built my my first DIY BCD. But this is another story...

 

[Steve_C]

At time of surfacing, we did retrieve the additional belt at the anchor, close and pack the umbrella-type anchor, and go up the rope, making the required deco stops in slightly negative buoyancy...
I did this for a couple of years,.

Ha, that is cheating for this question. It assumes that your weight carried is constant through out the dive.

 

[TMHeimer]

If I really studied what you two guys are saying I think I could follow it. Not sure if it matters. One way or another your tank is losing air, so your amount of air in the BC (or drysuit--I've never owned one) must be adjusted.
I don't think being neutral and being exactly properly weighted are necessarily the same thing.
You can be severely overweight and still make yourself neutral.
The question is just trying to say overweight is bad.

 

[johndiver999]

Angelo,
I believe you executed the dive profile you described, but I think your lung/diaphragm control was a bigger factor than the compression of your wetsuit.

They buoyancy of an aluminum 80 tank goes from -0.82 kg full to +1.82 kg empty, and that buoyancy gain occurs fastest at the depths you describe. Nearly all 3 mm wetsuits have an inherent buoyancy of less than 1 kg, only part of which is lost as one dives deeper, and most of the loss occurs in the first 10 meters.

I don't see how your buoyancy loss due to wetsuit compression can keep up with your tank's buoyancy gain.

I'm not sure where you came up with the idea that a 3 mm suit has a maximum swing in buoyancy of 1 kg, but I have seen significantly larger swings. I'm confident that it can be a lot more than 1 kg and I would guess it could be on the order of around 8 lbs or so - but that is somewhat of a guess.

Also, not trying to take your comments out of context, but a thick, multi-layer 7 mm suit can have a buoyancy shift of maybe 15 lbs, possibly more. So with some exposure suits, the swing from the suit can be much more than the tank.

 

[Johnoly]

The best SB smile came last year when someone asked how they could carry 53 pounds of lead because they were diving a dry suit. LMAO!!

 

[Scraps]

johndiver999,

I found an article from 2009 with the inherent buoyancy of a bunch of different 3 mm models.

The actual thickness and inherent buoyancy of any particular wetsuit is immaterial to my point.

Whatever the thickness of the suit--I don't care if it's 15 mm thick and has 10 kg of inherent buoyancy--if you start your dive positively buoyant and swim down until you reach a depth at which the compression of your suit makes you neutral, that will be the moment at which the race begins between a tank that grows increasingly positively buoyant and a suit whose rate buoyancy loss will be diminishing.

In fact, if you have an extremely thick suit, you will be so deep when you hit that neutral moment that your air consumption will be highly accelerated and your remaining wetsuit compression will be down to 15 or 20 percent of its potential at the surface. The rates won't be equal and won't offset.

You will still be exactly correctly weighted at only one moment, which is what this sidebar is about.

 

[wetb4igetinthewater]

What I find often missing in the discussion of the problem of overweighting, for new divers especially, is the ability to control one's depth when the diver isn't aware their depth has changed: Why overweighting is such a problem, especially for open water students | Facebook

 

[Ripley]

P.S. All "Face mask on your forehead" REALLY means is you are likely to lose your face mask.

But..but... it looks so cool! Even cooler is reversing it with the strap on the forehead and mask at the backside.
The aquatic version of the reversed baseball cap.

j/k.
It amazes me to see how many divers actually put their mask on the forehead / back of the head.
Did it once during one of the pool trainings. Once! Mentor waved his hand towards me (I thought by accident) and down went the mask. "Go and get it..." Then I knew it wasn't an accident but on purpose. Lesson learned.