What happened - rapid ascent

[Bob DBF]

All tanks have the same buoyancy change per cubic foot of air.

It doesn't matter if your tank is made from steel, aluminum or kryptonite. If you have used up 80 cubic feet of air, it (and you) will be 6.45 pounds more buoyant than when you started.

flots.

Where can I pick up those kryptonite tanks, I hear you can take another 10# off your weight belt.

Now the original question is how can one be positively buoyant in 40' of ocean wearing a 7mm wetsuit. My contention is that in order to be under weighted enough to be neutral at 40' with an empty BC, it would be extremely hard if not impossible to submerge in the first place, and you would notice an issue. I don't believe the increase of buoyancy from the tank would offset the loss of buoyancy in the wetsuit and whatever weight he was wearing with an empty BC at 40'. I'd be going over that BC with a fine toothed comb to make sure that didn't happen again. Until you get it figured out, stay shallow and away from NDL.



Bob
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There is no problem that can't be solved with a liberal application of sex, tequila, money, duct tape, or high explosives, not necessarily in that order.

 

[trblake]

This is a very good discussion. I was diving last week in Jamaica with my wife; she was having trouble with dumping air from her bc because she was too horizontal in the water and needed to be more vertical as the air was not escaping. Thanks for bringing this up.

 

[Qasar]

Honestly I don't see the problem here.
It happens many times: if you are underweighted at one point you go up... Who has never saw that?
Weighting is a big issue and there is a very good (and very long) thread on it somewhere on this forum http://www.scubaboard.com/forums/new-divers-those-considering-diving/295390-weight-not-weight.html.

For me I dive with 5, 3 mm or none wetsuit depending on the water temperature and in two different ocean Gulf and Indian ocean where the salinity is not the same, and I have done a matrix with all my experiences as an easy reference.
Of course, you will find people advocating the perfectly neutral buoyancy, but in fact I would assume that if you have a doubt, one or two kg more, put you on the safe side. It's better to stay 1 or 2 mn less (increase AC) than coming up more and more quickly in an uncontrolled ascent.

 

[Agility]

Being overweighted is certainly bad but being underweighted may be dangerous, too.
When I go to a new destination I consult my logbook looking for the weighting info in comparable exposure protection and adjust for tank configuration (steel/alu/size). Then I take 2 kg more than I think I need on my first dive and do a weight check at the end of this dive.

 

[The Kraken]

One other thing to remember to keep in consideration, your wet suit.

A 7mm has lots of bubbles inside, these bubbles collapse as you descent. Starting your dive with a full tank you may not notice this, but as you complete your dive and ascend, the bubbles expand and add a bit of buoyancy you may not have expected.

Safe dives . . . . .
Safer ascents . . .

the K

 

[pasley]

All tanks have the same buoyancy change per cubic foot of air.

It doesn't matter if your tank is made from steel, aluminum or kryptonite. If you have used up 80 cubic feet of air, it (and you) will be 6.45 pounds more buoyant than when you started.

flots.

Yes, all tanks become MORE buoyant during the dive. But the tank does make a difference in how much weight you need at the end of the dive.

We agree, as you use air the tank will become lighter due to the loss of the weight of the air. The difference is that at the end of the dive, which as you point out you must weight for at the beginning and plan for the loos of the weight of the air, AL tanks will float and Steel, tend to sink. See the chart for your particular tank. ALUMIUM 80 tanks will be about 4 pounds positive when empty (it will float) and a steel 80 tank will be 2-4 pounds negative (it will sink) when totally empty (note some tanks AL and STEEL are NEUTRAL when empty). See Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan for the numbers on this. EXAMPLE Catalina S80 AL tank is -1.8 pounds FULL and a +4 empty. STEEL PST E7 80 is -8.5 FULL and -2.5 EMPTY. So with a Steel tank of the same size you need 4-6 pounds LESS weight at the END of the dive. And the end of the dive is the most important part because that is where you are lightest due to the loss of the weight of the air consumed.

Definition: Buoyant - object that floats in the present medium (air, fresh water, salt water, salad oil etc). Buoyancy is obtained when an object displaces more weight in the medium than the object weighs. Example a cubic foot object weighing 64 pounds or less floats in salt water. In the true strict definition of the word, an object is either buoyant or it is not. (It either floats or it doesn't). In SCUBA we talk about POSITIVE, NEUTRAL and NEGATIVE Buoyancy. Technically these are not correct terms but are generally accepted to make talking about Buoyancy easier. POSITIVE: Floats, Neutral - neither floats or sinks, it's buoyancy characteristics are equal to the medium in which it rest. Negative - sinks.

BACK TO THE OPs TOPIC: The issue here is the OP was improperly weighted at the beginning of the dive by an estimated 2 pounds of weight. Towards the end of the dive, having consumed 3-4 pounds of air weight his tank moved from being heavier (negative 1.8 pounds) than the weight of the water it displaced to lighter (positive 4 pounds) than the weight of the water it displaced (it floats). In my limited experience, an AL80 will start to get noticeably "light" at about 1000 PSI and if you are not properly weighted to account for the tanks positive buoyancy characteristics at the end of the dive, it will start to pull you to the surface.

OP: Review the below Videos and your books to get the weight right. Remember in scuba when you change anything, your weighting needs to be re-checked for any changes (something I forgot when on dive 3 of the day I took a buddies empty camera case down on a dive to check for leaks. That empty air filled case made me underweighted). Define Anything: You gain or lose weight or change dimensions (change fat to mussel or the other way around), change wetsuit (or it ages), new regulator, BCD, carrying a camera etc.
These videos break it down into small steps of you, tank, bcd etc. You can save time by just going to the last video and doing a check with everything.
AL80 Cylinder Buoyancy Full Vs Empty Illustration: http://www.youtube.com/watch?v=XvHJEuOXszc&feature=player_embedded#t=8
AL80 Cylinder Buoyancy weighting: http://www.youtube.com/watch?v=PG_fhMMV5Kw
BCD GEAR Buoyancy Check: http://www.youtube.com/watch?v=_8wiRKbdIXw
How to do a Buoyancy Check for you and your wetsuit only: http://www.youtube.com/watch?v=2oWMAPbqrGY

Put it all together - Buoyancy check for You and your gear with tank: http://www.youtube.com/watch?v=sd0cHmOXb9U Best to do the test with 500 PSI, but if you do it with a full tank, you will need to add weight to compensate to the air you will consume during the dive and the empty buoyancy characteristics of the tank. Don't forget that in Salt water you need more weight than in freshwater. Note, salt water swimming pools typically do NOT replicate the buoyancy characteristics of the ocean.

Also consider your body position and where the air bubble is located. Use the air dump closest to the surface at the time.

 

[Steve_C]

All tanks become more positively buoyant as the weight of air in them is decreased by you breathing it.

Finally somebody said the right thing. It does not matter whether your tanks are steel, aluminum, concrete, or titanium. Using air reduces your starting weight and thus you gain some lift. The tank itself may not be buoyant but your overall configuration will gain some lift and that is what counts.

My BCD will dump air when I am horizontal and I tend to stay horizonal until I get close to the surface. I have had a couple of rental BCDs that I did not feel comfortable in at first until I realized that I had to get an upward tilt before they would dump the rest of the air.

 

[pasley]

...You have guessed by now there is no POWER DEFLATOR only a power INFLATOR. And thats not a stupid question. hope that answers how it operates.

Well, it appears that now there really is a power deflator.

http://www.tusa.com/us-en/Tusa/BCJs/A.P.A._-_ACTIVE_PURGE_ASSIST TUSA’s Active Purge Assist system is a revolutionary BCJ accessory technology. This pneumatically assisted purge system and newly designed OPEV (Over Pressure Exhaust Valve) offers significantly enhanced flow rates over a standard inflator system, which allows for immediate purge and inflation response.

Additionally, the A.P.A. is incredibly streamlined reducing drag and bulkiness. The system is designed for low-maintenance and easy care reducing inflator malfunctions and is standard on select TUSA BCJ’s or available as an add-on replacement accessory.

Don't know why anyone would need it. IMHO this is rather like buying a "power lead ball dropper" to help the lead balls get out of a bag when you open the bottom of the bag. As another poster said, a Solution in search of a problem. No idea how it works either or why anyone would A: Need it, B: want to add another button to their BCD hose, C: Add another failure point D: Add anything that will use air to operate or E: Spend the extra money. You are under water, the water is pressing on the airbladder of the BCD. The air is under pressure. You open a vent that is closest to the surface and it is going to come out under pressure. Why add anything else to "help" that? Don't know. But maybe TUSA has something? The market will say in time.