Descent difficulty, even overweighted

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I was taught to use a full cylinder (rather than a cylinder containing only 500 psig).
If I'm at the surface with a full tank and able to easily ascend/descend with breath alone, then at the safety stop I will be -2.5 lb in my 3mm wetsuit due to compression. When I return to stop depth after consuming 5 lbs of air (AL80), I will be +2.5 lb... struggling to stay down and begging for a CO2 headache with shallow breathing. With a large capacity cylinder I will be +6 lb. Heck, add shorts & rashguard, I will be more than 8 lbs positive. 😳
 
FWIW, it's fine to test/adjust with a full tank as long as you also add the gas weight you'll consume as a final adjustment.
 
Well, I often descend swimming head down, and ascend swimming head up. As a previous free diver, this looks normal to me.
If descending the legs stay up, that is just perfect: the optimal attitude for swimming down using a good kick...
This insistence of always staying horizontal is quite disturbing to me: one should stay in the position which minimizes drag , allowing to swim with minimum effort.
If you are swimming horizontally, then of course your body should be horizontal. But when swimming up or down, the body should be vertical, either head up or head down.
In my view a BCD is not an elevator, it is just a device to be used for correcting gross buoyancy variations due to the suit compressing with depth and the tank becoming lighter when empty.
Going down or up should be done swimming, while keeping an almost neutral buoyancy thanks to the BCD (or using a low-compressible suit and proper weighting, which makes the usage of the BCD substantially superfluous).
^^^this^^^
 
@rx7diver interesting approach. What is the advantage of being able to float on the surface with a full tank and an empty BC?

And, what in the hell circumstances led you to surface swimming 3 miles in fresh water???

Not knocking your idea, I'm genuinely curious.
It's not his idea.
That's how you supposed to do it. In the PADI and SSI OW manual it says you're not supposed to sink with an empty bc. You exhale to start the dive.
People think they need to sink once they empty the bc and that's one reason why many dive with too much weight.
 
In the PADI and SSI OW manual it says you're not supposed to sink with an empty bc.
Of critical importance is how much of a breath to have when doing this. A "medium" breath is typically enough to offset the non-reserve air in an AL80, but is quite imprecise and varies wildly from person to person. It also won't work for higher capacity tanks. I prefer to use the recommendation in @rsingler's fantastic buoyancy calculator: weight for end of normal exhale (easy to find) and then add lead to offset the non-reserve air weight in whatever cylinder you have (12.5 cuft per pound).
 
These discussions keep happening because PADI's "float at eye level" training doesn't come with the rationale behind it. That methodology is for the population that PADI is most oriented to: the occasional diver in a 3-5mm vacation wetsuit. If you're in cold water, everything changes due to your neoprene.

Where you float with an empty bc, and how big a breath you retain, and full or empty tank, ALL relate to only two requirements: being able to float at the surface with a failed bcd, and being neutral at the 10-15ft stop with no air in your bcd. Everything else devolves from this.

Why do the requirements of the buoyancy tests that folks are taught vary? Because the neoprene varies.
Example 1): 3-5 mm wetsuit. At 15 feet, that wetsuit will lose ~5lb of buoyancy due to suit compression. If you finish your dive with min gas, and need to hold your safety stop with a failed bcd (or no air in it), you have to weight for that suit compression. At the surface, where you do your buoyancy check, you are 5# light due to suit expansion. If you weight yourself to float at eye level with a 500 psi tank, you are supporting approx 4 lb of skull weight above the surface, and will be approximately neutral at the safety stop. Especially since most divers have more than an "end expiratory" lung volume during the test due to a little stress.
Where it gets complicated is that OW courses NEVER give you a 500 psi tank to test this for 3mm. So you're 6# heavier, and won't even float awash without full lungs. Stupid to not discuss the theory. What I teach is to weight just as above (but with a full tank), then add weight for the gas you'll breathe. Typically 5-6 lb. Problem with this on vacation with a new setup is that you have to be prepared to jump off the boat and do your quick weight check with a full tank, and then quickly add 6 more pounds, all the while your buddies have descended and left you behind. No wonder it never works out right.

Example 2): 7mm wetsuit or maybe a Farmer John. At 15 feet, that wetsuit may lose 12# of buoyancy due to compression. To do a surface buoyancy check that will make you neutral at the safety stop you have to account for 12# of buoyancy that you won't have at 15' at the end of your dive with a failed bcd. If you start with a full tank, there's 6-8 lb that you won't have at the end of the dive. If you float at eye level, there's another 4# less than neutral. If you have slightly more air in your lungs, that gives you the extra buoyancy to make the test valid, and account for suit compression.
At the end of a dive, you are weighted for the buoyancy of your neoprene AT 10-15 FT, so you don't need bcd air to hold your stop, and you don't need extra weight for the near-empty tank. It all balances out.

But when the test is performed at the surface and PADI doesn't teach the theory of suit expansion, we get six different methods, and they're all correct.
Different neoprene, different tests.

Now for the end of the dive with the failed bcd. With thick neoprene, it's easy. You're 10-12# light at the surface and can float easily. For the vacation diver in 3mm who just passed his/her OW, you're at eye level being 4-5# light. A big breath and you're easily floating on your back. But in a panic, that may not seem good enough. Hence, ditchable weight.

Coincidentally, this relates to the OP's problem. If you're correctly weighted in heavy neoprene, even with a full tank you're 4lb light at the surface at the beginning of the dive, unless your lungs are empty and your head is just awash (which is not the way divers like to rest). So unless you exhale more than four lb of buoyancy and hold that until suit compression takes over, you won't sink. Starting vertical, to compress those legs and squeeze bubbles up is a good start. Exhale hard, don't panic, wait 15 sec for negative buoyancy to take over, and slowly rotate to horizontal as you continue your descent. Don't take a huge breath close to the surface. Problem solved.

With a dry suit, it gets a little more complicated, but not much, since you always keep the same (relative) volume of air in the suit if you use it for your buoyancy control. The problem with drysuits is that at the start you may need to carry 6-12# more buoyancy for your full tank(s). If you don't, you'll have some squeeze at the end trying to stay neutral. Alternatively, start your dive with 3-6 liters of air in the bcd and bleed that off as you use gas. It gets complicated as teaching varies re: which device is the buoyancy adjuster under way.
 
People dove without a BC for many years: Enter the water from shore pulling a substantial diver's float (maybe an inflated inner tube) that flies a diver's flag, and surface swim (snorkel power) to the spot where the dive will begin.

If you're winded from the swim out, then rest up, holding onto the float. (Remember, you're a bit negatively buoyant!) Maybe, rehydrate by drinking the water or Gatorade that is strapped to your float. Catch your breath, drop your heart rate.

Switch to your regulator and do a surface dive and kick down to depth, swim around awhile down there, and then slowly return to the surface.

At the surface, if you're tired, then rest up, holding onto the float. Maybe, rehydrate by drinking the water or Gatorade that is strapped to your float. Eventually, surface swim (snorkel power) back to the exit point and, um, exit.

The weighting approach I describe upthread is how you might do things if you're diving in MO or AR lakes, and you're wearing a two-piece 0.25 inch (6.5 mm) farmer John/Jane wetsuit with hood and booties and a steel 72 (for example). A BC really isn't necessary (although it is nice to have).

It's a shame that some people regard this approach to diving as "strange" or "impossible."

It's a shame that some people will never actually try this and experience how much fun--how liberating--diving this way actually is.

It's not complicated. Not necessary to overthink it.

P.S. Remember: If you get into trouble at the surface, then drop your weight belt to establish positive buoyancy (hopefully)!

rx7diver
 
Where it gets complicated is that OW courses NEVER give you a 500 psi tank to test this for 3mm.
What do you mean?
The students breath down their tanks.

The floating at eye level is a starting point.
I feel like you're making this way more complicated that it needs to be. You guess what they need, than on the first dive you check if they're not too heavy/light on the surface with the full tank and at the end of the dive you check how much gas they have in their bc at 5m. And done. Works the same with every type of suit.
 
It's not his idea.
That's how you supposed to do it. In the PADI and SSI OW manual it says you're not supposed to sink with an empty bc. You exhale to start the dive.
People think they need to sink once they empty the bc and that's one reason why many dive with too much weight.
They call it elevator diving.
Dump air to sink, inflate BC to rise. Keep adding weight until this phenomenon is achievable. It’s all great until your BC craps out and you realize you’re 20lbs overweighted, woops! All of this feet first in an upright position, and so much weight that the air cell is constantly well inflated. This causes a whole separate problem of air bubble management that is an exclusive scuba problem brought on by the miracle of the BC, and more to the point, the abuse of the modern BC.
This is where scuba training took a left turn away from it’s roots in freediving/skindiving.
 
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