Avoiding Slipping into Negative Buoyancy During an Ascent

[WKenny]

I have encountered problem on occcasions with venting too much gas from my wing too early on some ascents and slipping into negative buoyancy which makes the ascent more difficult. I'm trying to get a better conceptual understanding of how large a volume of gas I should be retaining in my wing, (and dry suit), during the ascent to avoid slipping into negative buoyancy.

I'm diving in cold water, (the Great Lakes), with a DUI TLS 350 trilaminant material dry suit, a BARE "Hi- Loft" thinsulate undergarment, a doubles rig with two 85 cu ft, low pressure Worthington steel tanks, and an 80 cu ft aluminium deco stage tank, and no "extra weight" beyond a few miscellaneous things like a small hand-held light, small shears, and a DSMB with a small spool. That rig is about 30 pounds negatively buoyant at the end of a normal dive with a reserve of about 1/3 gas in the double tanks, and the deco tank is full, determined as follows:

1) Negative buoyancy of the doubles tank when empty, (-0.7 lbs each), so @ -1.5 lbs
2) Weight of the gas with the tanks about 1/3 full @ -4.5 lbs
-(the capacity if each tank is about 82 cu ft, so total capacity of both
tanks is about 164 cu ft;
-(1/3 of 164 cu ft is about 55 cu ft)
-(55 cu ft of gas times .08 pounds per cu ft = @ 4.5 lbs of gas.
3) Aluminium BP, harness, wing, 2 regs, SPG, tank, manifold & bands @ -20 lbs
4) Negative bouyancy of full 80 cu ft aluminium deco tank. @ -3.5 lbs
TOTAL Negative Buoyancy of the tanks/rig on a normal ascent: about -30.0 lbs

I am about 6 ft, 2 inches tall, and I weigh about 210 lbs. I don't know how I could get the rig much lighter. I'm not carrying any extraneous weight as I see it.

Before starting an ascent, I check my buoyancy and try to get neutrally buoyant with my lungs about half full of gas. I ascend in a horizontal position. I inhale to establish slight positive buoyancy to start the ascend. As soon as I feel upward movement, I vent a small puff of gas from the wing, and exhale to move back toward neutral buoyancy. I conduct the ascent generally by repeating this procedure, unless I slip into negative buoyancy which throws the ascent out of whack. The dry suit pretty much self vents as the depth gets shallower.

What is the approximate volume of gas that I should have in the wing, (and dry suit), to be neutrally buoyant when I start the ascent at a depth of 100 feet? Using the old adage "A pint is a pound", (i.e. 1 U.S. pint of water weighs about 1 lb), should I have about 30 pints of gas in the wing, (and dry suit), to offset the 30 pound negative buoyancy of those tanks and rig? 8 U.S. pints equals 1 gallon, so 30 pints is equivalent to about 3.75 gallons. Really? That seems like a great deal of gas to have in the wing, (and dry suit) for buoyancy control. Am I analyzing this correctly, or am I missing something?

Then as I ascend and that gas expands, do I need to vent it so that I maintain about that same volume, 3.75 gallons of gas, during the entire ascent to remain close to general neutrally buoyancy during the ascent? At the surface, wouldn't the 30 pints of gas, (i.e. 3.75 gallons), still be necessary to offset the 30 pounds of negative buoyancy of the rig for neutral buoyancy at the surface?

I understand that I should vent a series of small puffs of gas from the wing "early and often" during the ascent to maintain a balance of venting sufficient gas to avoid an uncontrolled ascent, while not venting too much gas too soon which can cause me to slip back into negative buoyancy. However, it is the total volume of gas that I need to retain in the wing, (and dry suit), during the ascent that has me somewhat confused. With a rig that is about 30 pounds negatively buoyant as described above, do I reallly need to retain about 3.75 gallons of gas in the wing, (and dry suit), over the course of the entire ascent to avoid slipping into negative buoyancy during the ascent?

If I really do need that much gas in the wing, (and dry suit), for proper buoyancy control, then I need to be diligent about not venting too much gas too soon on the ascent. But having that much gas in the wing seems like it might be courting disaster if I ever failed to vent it quickly enough to prevent it from expanding too much and causing an out-of-control ascent.


Thank you for your advice and assistance with this issue.

 

[dumpsterDiver]

No one can give you the answer and the way you pose the question leads me to beleive that you really don't understand the process. a horizontal ascent is by definition a bouyant ascent (since you can not kick up if you are horizontal). You do not need to vent early and often, in fact you may be able to ascend 20 feet or more (from 100) without venting any gas.. since the expansion of gas is slow at this depth.

In any rate nobody thinks about the volume of gas they are venting in quantiative terms... You are trying to balance expanding air and an ascent rate and the variability from the breathing cycle. Not to sound critical, but does a kid learning to ride a bike ask "how far do i turn the handlebar to the right, if i feel like I am loosing my balance?". The answer is "a little" and you will have to feel it.

What you should do, is try to do an ascent where you rise up a little when you inhale and completely STOP when you exhale. You have to carefiully watch particles in the water column and cheat and watch the computer depth too.

The whole process is much easier to learn if you ascend vertically.. then you just kick up super gently and if you screw up and dump a little more air then necessary, you just kick up a little harder for 10 seconds and when you rise 8 feet, you should be back to neutral...Using your fins to compensate for over dumping is much easier to learn and less likely to get a yo yo ascent then adding air back to the BC.

 

[TSandM]

I read your very thoughtful post, but one thing really stuck out for me. You state that your backplate, regs, tank, manifold and bands adds up to 20 lbs negative. I think that's a big overestimate.

The way I calculate it, you have two tanks that are 1.5 negative when empty, 4.5 lbs of gas, an aluminum backplate (about -1), two regs, manifold and bands (about -5 altogether) and a bunch of d-rings and other hardware (about -2). I would guess that your entire setup is somewhere around 15 lbs negative (which goes along with the fact that I use 31 lbs with a single tank, and about 12 with doubles).

I like to run my suit fairly loose, so that I can stay warm, but with my double 85s, I do have to run gas in my wing. I do my ascents exactly as you describe -- inhale, begin to float up, then exhale and see if I stop -- if I do, I inhale again and start the process over, and if I don't, I vent. My dry suit will often auto-vent, so it's only the wing I have to empty. It's small volumes at a time, and it did take me a fair while of practice to learn exactly how much, to avoid the dreaded "yo-yo" syndrome. It's really not possible to describe how you do it -- you just have to muddle through until it becomes second nature.

 

[spectrum]

Assuming you have a good weight check under your belt we can assume you are not handicapped by an excessive amount of air in the BC or suit. Weight Check.

Were you ever any good at this in a wetsuit? That could isolate you problem to suit venting, especially if the prior proficiency was with a cold water wetsuit. Venting could be a bad valve, wrong setting or you failing to present the valve at the apex.

After that, like they say it's practice. Start with heads up ascents. Once they are making sense start horizontal and eventually go most of the way horizontal.

Pete

 

[DivemasterDennis]

My contribution to this issue may be simplistic and obvious, but I'll weigh in with it. Add and vent air from you bcd or wing in SMALL bursts. When I am working with students at all levels on buoyancy, one of their biggest problems is due to "changing buoyancy" too radically through adding and venting air. As applied to ascents, remember that a safe ascent is a slow ascent, ideally 30 feet a minute, never more than 60 feet a minute. That is slow. You have a lot of time to "fine tune" buoyancy as you ascend. Use it. Fine tune buoyancy as you ascend by small unit venting of air, and at a more advanced level, use of breathing control- control, not breath holding. Dumpsterdiver's analogy to righting an out of balance bicycle is a good one. Corrections are small and constant. So too in buoyancy control, including buoyancy control on ascents.
DivemasterDennis

 

[RJP]

Sounds to me like you're consciously or unconsciously too worried about a run-away ascent and are over compensating by remaining "too negative" at some level. If you can manage to establish and maintain neutral buoyancy at depth there's no gear or skill problem to managing it little-by-little on the way up.

 

[Agility]

I do not remember where I read it, it might have been in "The Six Skills", holding the inflator horizonally when pressing the deflation button will keep enough air in the wing to stay neutral. This works well for me in a wetsuit, I do not have enough experience in my drysuit to evaluate this procedure in this combination.