Buoyancy 1.0 - the sudden fly away effect

[261311]

My only concern is however is it looks like everyone is over weighted, if you're at the proper weighting/ level and you're neutrally buoyant I'd assume you have to "swim" down, correct? I actually did "try" going without a weight belt but I didn't sink (makes sense given the purpose isn't to actually sink you) but the instructor and DM assist were teaching decent in a column (slowly controlled sinking) by venting air in their BCD which, if my BCD is airless and I'm neut buoyant on the surface- well, now I'm confused. It seems some initial over weighting is needed, or is that few extra pounds to compensate for the tank usage the precise thing you need to just get you started granted water pressure will just naturally help you after you're under (which the BCD counter acts at depth?)? It's funny how 2/5 in and I still feel this ignorant about BC. The book doesn't exactly explain much rather than the IMPORTANCE of it or the displacement of an object, not how that changes as its place in water is modified.

 

[Quero]

Let's start at the beginning, shall we? When you do a weight check, you do it with a full breath of air in your lungs. If you float at eye level while holding a full breath, guess what happens when you release the air you're holding (when you exhale)? You sink! This is, of course, because the air in your lungs buoys you up some.

When you're weighted for neutral buoyancy with a fully charged tank, you will gradually become positively buoyant as you consume the air in your tank and the tank gets lighter. When this happens, no amount of blowing out will get you back down if you start to float away simply because the positive buoyancy of the tank + your natural buoyancy + the buoyancy of your wetsuit are greater than the negative buoyancy of the weight you are carrying (which is, I'm sure you realize, why you weren't able to get off the surface with no weight at all). This buoyancy swing that results from the use of the air in the tank is why the materials specify that you should do the eye-level weight check with a nearly empty tank. Therefore, when properly weighted, we are always somewhat negatively buoyant early in the dive. (Notice that I wrote "properly weighted" rather than "overweighted." Being negatively buoyant does not necessarily imply that you are overweighted.)

During pool training, however, grabbing a weight stashed in some easy-to-reach place is simple enough that I think it's perfectly practical to be weighted for neutral buoyancy regardless of how full the tank is. It's already a challenge to stay neutral in the shallow water of a pool; I see no reason to make it even harder by weighting students to compensate for a light tank when their tanks are full--that can wait until the open water dives.

 

[Peter Guy]

Zack -- you are way over thinking/over analyzing this. Being "negative" is different from being "overweighted."

How much weight do you need? Enough to sink at the start of the dive and enough to stay down at the end of the dive. The key is the second part -- enough weight to stay down (i.e., able to do a safety stop or whatever you might want to call it) at the end of your dive in very shallow water. By the end of the dive you've "lost" at least several pounds of weight that helped you sink at the start of the dive -- that is, the weight of the air you've breathed. That might be 3 pounds, that might be as much as 10 or more pounds. But whatever it is, you need to be able to compensate for that loss of weight so that you are neutral (or almost neutral) in shallow water at the end of the dive.

How does one become neutral? Well, you have several air bladders (at the very least your lungs and BCD and perhaps a dry suit) into which you can put air to counter your negativity from your gear. Most people don't want to have totally empty lungs (!) so you'll need enough weight to counter the air in them. It's OK to have a totally empty BCD/wing in very shallow water so you might not need weight to counter that (but it may be hard to have a totally empty BCD and if you have a "traditional" BCD, it will be somewhat buoyant all on its own so you'll need weight to keep it neutral). And if you are diving dry, you may well want to have some air in your suit so that you stay warm which means you'll need to have weight to counter that.

While perhaps too much information, there is no perfect answer -- you need as much weight at the start of your dive to be able to be neutral at the end of your dive -- and how much weight you need to be neutral at the end of your dive depends on your gear and your personal choices!

Are you "overweighted" at the start of a dive? No, you have the weight you need at the end of the dive which means you need to "compensate" for that weight with your BCD at the beginning of the dive.

 

[dumpsterDiver]

The solution to the "fly away" problem is simply screaming "SHEEEEET!". If at any time you begin to lose control and begin floating up, a very rapid and complete exhalation should be enough to stop your ascent (if you are paying attention). THEN, you can locate the dump valve and reduce some air in the BC. Finding the dump and operating it takes much longer than exhalation (particularly if you are learning the gear).

If you pay attention, you should never get so bouyant, that a super fast exhalation (don't inhale until you dumped air) will fix the problem.

Incidentally, I have no idea what quero is talking about when she says she can't exhale fully when tense. There is no way I can feel any sort of effect like that.

 

[lowviz]

...//...can't exhale fully when tense. There is no way I can feel any sort of effect like that.

-I took it to mean really, really, new newbies. You know, the kind that belch on the surface.

 

[261311]

I know what quero is talking about with tensing. Take a full breath of air, tense up your muscles (abs, diaphragm mostly) exhale as much as you can and hold your breath. Untense your abs and you should be able to breathe further. It's funny because I didnt think it would be that significant.

As per buoyancy: see, I knew that I had to start the dive with a few extra lbs but I don't think it would cause me such heavy over exertion when I'm all kitted up. In the pool with an empty bcd if I'm not kicking up a storm and wading with my arms I sink like a rock, even if I were properly weighted I'd expect a more gradual slow decent (basically what I get now if I'm only finning upwards [yes I'm still sinking while kicking] and not wading with my arms). I'm just assuming proper weighting allows for control while being still so that you have direct control of where you are at all times (what I'm assuming is the concept of neutrality).

 

[lowviz]

...//...proper weighting allows for control while being still...//...

"Allows for" is a good phrase. Proper weighting does not guarantee neutrality. It will all become easier as you begin to trust your gear.

Just remember: "Trust, but Verify."

 

[DevonDiver]

It seems some initial over weighting is needed, or is that few extra pounds to compensate for the tank usage the precise thing you need to just get you started...

Yep, the air in your tank at the beginning of a dive ensures that you can sink easily. As you begin to sink , that is further amplified by the compression of your exposure protection (wetsuit); creating more negative buoyancy. You add air to your BCD to compensate for both factors.

As the dive progresses, you consume air from the cylinder; in doing so, you become more buoyant. You gradually reduce air volume in your BCD to reflect that buoyancy change.

As you ascend at the end of the dive, your exposure protection (wetsuit) expands to its original (un-compressed) thickness. You reduce air volume in your BCD to reflect that buoyancy change also.

As you reach your safety stop, you are at your most buoyant. Your cylinder is most 'light' and your exposure protection is most 'floaty'. THAT is the stage where you need your maximal weighting/ballast.

Put simply: At the beginning of a dive, the diver benefits from being slightly negative; meaning that dumping air from the BCD and exhaling air from the lungs is sufficient to initiate a descent. Once initiated, the descent becomes faster, as it progresses, due to suit compression. To then get neutral, divers then have to add air to the BCD to balance the loss of exposure suit buoyancy and the weight of air in their tank.

At the end of the dive, if properly weighted, you might have to 'swim' back down to descend. However, the need to descend at the end of a dive is quite rare

 

[TSandM]

latestartfish, do I correct read that you were using 28 pounds of weight in a freshwater swimming pool, wearing a BC and a bathing suit and no wetsuit at all? Unless you weigh about 400 pounds, that's a ridiculous amount of weight. Yes, weight requirements vary from person to person, and heavy people need more -- but most folks using a steel tank in fresh water with a bathing suit are already overweighted without additional lead, and even with an aluminum tank, you are just talking about adding a couple of pounds. I have never heard of anyone having to carry 28 pounds in the pool without the use of a thick wetsuit or dry suit, and if you were using that much weight, you were so overweighted that buoyancy control would be a pipe dream.

Not only do you need more pool time, you need pool time with somebody who knows what they are doing.

I agree with you -- if your single day in the pool was as confusing and unproductive as you describe, you are nowhere near ready to go to open water, and any instructor who would countenance that is not paying attention to you.

Please talk to your shop. Please ask for more time, and explain your concerns -- tell the shop owner that you were diving with 28 pounds of lead in the pool. You need better help.

 

[lowviz]

261311,
Looks like the OP bailed. This is for you:

I haven’t been able to shake a hypothesis that seems to explain quite a lot about the “sturm and drang” of dive education.

A dive professional, when given a new diver, finds attending to “Job One” as far too time-consuming. “We need to press on or we will never finish this class.”

Buoyancy and trim, movement and position, stable platform, basic control, call it what you wish, but it is all the same thing. It is “Job One”. “Job One” is optimizing one’s gear so that only lack of skill remains as the variable that determines the student’s postion in the water. Please re-read the last sentence. You could quit here if you are in a rush.

You can’t fight physics, I have tried. Up until quite recently, when I stopped all motion and tried to hover, I took a sharp shoulder-roll to a turtle position. The only answer to such an improper setup was constant motion. So “Job One” would have been to solve this serious problem first. Seems obvious, but never happened. “A beginning is the time for taking the most delicate care that the balances are correct.” Does anybody actually do that? There are a few exceptions, but I seriously doubt that ANY agency does it as a core principle. Come prepared or suffer the consequences.

I made it to beginning tech before someone actually attempted to properly address my issue. I made some progress, but was again left on my own to complete the job due to real class time constraints. Rigging and un-rigging, playing with weights and placement, re-routing hoses, reconfiguring –all this while paying attention to the instructor? No, not at all possible. It takes time, so a compromise is always struck. You lose.

Given a class of four to six students, properly weighting and trimming out all the same will seriously tax the course’s time frame. Do the math, there isn’t time for the most honestly committed dive instructor. So the dive pros do the best that they can, overweight the newbies a tad and thus enjoy a dramatic reduction in “needy” divers. The unintended message being sent is that buoyancy and trim are actually an advanced concept, they are great to master, but we just don’t have time to go into that just yet. For now, it is rinse and repeat. Next course covers buoyancy a bit better, care to sign up?

Some agencies elevate buoyancy and trim to their prime standard. Great! So somebody actually offers a progression that attends to “Job One”first. Sounded good to me! Time to do some reasearch. Problem. A disconnect occurs when one compares the published course outlines to a meaningful sampling of student reviews.

Consensus: “You really should have been mentored before taking this class.” What??? OK, I read this as: You had better find somebody to get your trim and buoyancy squared away before you get a real ego beating while you watch the previously tutored and the naturals all make a fool of you while TRYING TO LEARN TRIM AND BUOYANCY! Catch 22. This is particularly hard on crossover dive instructors, they should have had this buoyancy thing squared away long ago. I can only imagine the feeling, -and I’m just a student. Sorry, I can easily get that feeling by taking a math course two levels above where I should be. Such is the mark of exclusivity, not inclusivity. Confused. Are you guy culling for only the best with funding from wannabees? Looks that way to me. Agencies should teach to published standards and students owe it to themselves to find courses that directly address their weaknesses at a level that is appropriate to their needs. look for a 1-to-1 association.

So back to“Job One”. It seems to be pretty much up to the student. Which implies that the student (you, sir) figure out a way to provide immediate feedback as to position and trim. That is how the human mind is wired for easiest learning. For me, it was smoke and mirrors. Actually, no smoke, just mirrors. After realizing that you are on your own, and much more so than anyone will admit, work at it for a while and you will get it squared away -as I did.

I find it truly amazing how damn simple diving is when you can just hang there.

Anybody could teach you anything...