Redundant Bouancy

[ArcticDiver]

...Buoyancy compensation is just that, compensation for the loss of buoyancy of the wet suit. But now, even instructors teach being overweighted so that it is not necessary to swim down to the bottom. ...

Appreciate your experienced perspective.

But this part of your answer raised a question in my mind. Maybe you could expand a bit?

If a person has to swim down to the bottom at the start of a dive wouldn't they be too light at the end of the dive? This could make a deco stop or a "safety stop" a bit chancy, eh?

Or, is there a technique from the past that has gotten lost and would take care of this situation?

 

[vablackwater]

Due to diving in the zero vis I just ordered a new wrist computer, consol is now backup. I found I really, really, really don't like having a consul. I want my computer and compass right next to each other. So far that is my biggest lesson in diving in zero vis water.

not to be a stickler, but if you are diving 0 vis and have relatively few dives... whatever divemaster is at the quarry (if required) needs to make a better judgement call. if you are diving in conditions where you can't keep track of your buddy, you are probably diving out of your experiance/ training level. 0 vis/ blackwater diving is a whole other can of worms, and there are ways to minimize the risks.

 

[budgy]

I'd generally agree if your too under ballasted, to sink with a full cylinder and need to swim down, your not going to be able to stay down, to make a safety stop. Other times it's okay, as its only a little air stuck in the dry-suit and or someone hasn't fully emptied their bcd. I generally dive with 300 bar steel cylinder, it doesn't float when empty. I've tried the steel 15litre faber 232bar but I found them floaty and I'm to old and lazy to carry the extra ballast. I tried that redundancy equipment idea, but I've found a single cylinder is enough, my reg is reliable and I use it regularly, after many 100 rd's of hours I've never found a need to carry 2. Solo I don't need an octopus, dry suit buoyancy with a shoulder dump works fine, and a back back is cheaper and more convenient than a bcd or wing. Hundreds of clam divers go diving this way. The wing and pony have been lying unused in the garage along with spare regulators + 2nd stages, reels, smb's, slates and other dangalies I was once told I should never dive without. If you feel you need all the extras its okay and if any of you are on my side of the pond, come around and relieve me of some of them.

 

[John C. Ratliff]

Appreciate your experienced perspective.

But this part of your answer raised a question in my mind. Maybe you could expand a bit?

If a person has to swim down to the bottom at the start of a dive wouldn't they be too light at the end of the dive? This could make a deco stop or a "safety stop" a bit chancy, eh?

Or, is there a technique from the past that has gotten lost and would take care of this situation?

ArcticDiver,

First, thanks for the PM. I've been off posting on the Bike Forums, and having intensive discussions about a fatal bike accident there, so I've neglected these posts.

Actually, there is a technique that I'm afraid is in danger of being lost. In the buoyancy check (which is also not being done anymore by some), with all air out of the BC, and wearing a wet suit (dry suits are a different matter), with the original dive tanks (71.2 cubic foot tank filled at 2475 psi at its greatest), the diver should be neutral at the surface at the beginning of the dive on a full breath. By neutral, I mean floating in a verticle position with only your eyes and the top of the head out of the water. This allows you to sink easily (release air from your lungs and swim down), ensures that you are several pounds heavy at ten feet ("safety" stop), even at the end of a dive, and makes the whole dive experience more like flying than swimming, because you are weightless throughout the dive.

Many new tanks are different, as they have a much greater change in buoyancy, and greater negative buoyancy. So you must compensate with the weights you have for the tank's negative buoyancy, and remember to be heavy at the beginning of the dive because you typically gain seven or so pounds of buoyancy due to the tank's increased buoyancy with decreased pressure. This is why I still prefer the older tanks, ones before the current "low pressure" tanks. Now, "low pressure" tanks are 3000 psig, and High Pressure tanks are what? 3800 psig, 4000 psig, 4400 psig? My low pressure tanks are my twin 45s, which are 1800 psig. They therefore do not increase in buoyancy greatly, whereas today's gain greatly in buoyancy.

So the above buoyancy check has become more or less obsolete, in instructional agencies minds. If you want to experience this, get an old steel 71.2 cubic foot tank, and a backpack, and dive it without a wet suit. You will see that at the beginning of the dive, you will be just a bit heavy (heavier than the above buoyancy check, perhaps, depending upon your build). This tank is slightly negative (a pound or so), empty in salt water, and just a bit heavier in fresh water. The buoyancy characteristics were designed for a diver without a wet suit or a BC (it was designed before BCs existed). Divers in that era were more streamlined, and swam easier, than today's divers with the huge BCs and harnesses, and four hoses coming out of one single hose regulator (they actually used to be single hose regulators).

That's the technique, and some of the history behind it.

SeaRat

 

[ArcticDiver]

Thanks for the info. If I understand this is really an equipment specific technique. As such unless the tank and the rest of the kit have different characteristics it will have to be modified.

So, with the probably most common tank today, AL80, one would have to carry extra weight to offset the positive buoyancy at the end of the dive. Therefore, at the beginning of the dive one would either need large lungs or a bit of air in the BC.

Thanks for the useful info.

 

[John C. Ratliff]

I use the technique all the time with my 3000 psig AL80s (I have two). I have to add about three pounds to my weight belt though.

John

 

[ArcticDiver]

I use the technique all the time with my 3000 psig AL80s (I have two). I have to add about three pounds to my weight belt though.

John

What I meant was that controlling buoyancy with no additional weight as you first mentioned was equipment dependent. With tanks that have end-of-dive positive buoyancy a person would need to carry enough weight to offset that amount of buoyancy.

With that in mind, actually we use the same technique. I can usually keep myself on the surface with no air in my BC by careful breathing. Then, to go down I just exhale. I use a bit more weight than you. But the principle is the same. One has to somehow have enough negative buoyancy on board to handle the positive buoyancy at the end of the dive.

I adjust for sea state, etc. to be sure I can do the controlled stops the dive plan calls for. Sometimes at the beginning of the dive I need to swim down, especially if I'm wearing a lot of neoprene, or I have a lot of retained air in my lungs from heavy exertion on the surface. But those, to my mind, are just intelligent adjustments to meet the circumstances. The principle is unchanged.

Thanks for the insights.

 

[John C. Ratliff]

You're welcome! I think we are on the same track. One thing to realize is that I have a 6 liter lung vital capacity, so when I exhale, I can loose a bit of buoyancy. Others may experience it a bit differently.

The other variable is the tanks that are being used. The older steel tanks had a pressure rating of between 1800 psig and 2475 psig. These tanks don't gain as much buoyancy as the newer HP tanks. Scubapro, for instance, in their 1986 catelog stated the following buoyancy characteristics for their tanks:

95.1 cubic foot tank
Working Pressure: 2400 psig + 10% overfill
Buoyancy (calculated nominal w/o valve in sea water):
-6.2 pounds full (+10%)
+1.5 pounds empty
Swing: 7.7 pounds

75.8 cubic foot tank
Working Pressure: 2400 psig + 10% overfill
Buoyancy (calculated nominal w/o valve in sea water):
-4.6 pounds full (+10%)
+1.5 pounds empty
Swing: 6.1 pounds

71.4 cubic foot tank
Working Pressure: 3000 psig + 10% overfill
Buoyancy (calculated nominal w/o valve in sea water):
-10.3 pounds full (+10%)
-4.6 pounds empty
Swing: 5.7 pounds

95.1 cubic foot tank
Working Pressure: 3000 psig + 10% overfill (3300 psig)
Buoyancy (calculated nominal w/o valve in sea water):
-7.61 pounds full (+10%)
-2.25 pounds empty
Swing: 5.36 pounds

Even in 1986, there was a lot of variation. I have never used any of these tanks. My tanks were USD 71.2 cubic foot tanks, and vary 5.5 pounds:

Metal___VOLUME___SERVICE___LENGTH___DIAMETER___EMPTY WT.___BUOYANCY___BUOYANCY
steel___71___2,250___25.0___6.8___29.5___3.5___-2.0
See: http://vintagescuba.proboards2.com/index.cgi?action=display&board=general&thread=1139681701&page=3

The cylinders (at least some of them) were calculated to float when empty (but with the regulator attached, probably did not). But if I start out with a full breath at neutral buoyancy, I will be about neutral at 10 feet on a near-empty tank.

Going back to my original statement about my vital capacity of 6 liters, we can make a calculation:

6 liters = 0.211888 cubic feet

According to the US Navy Diving Manual, March 1970, "For example, we can say that the density of pure (fresh) water is 62.4 pounds per cubic foot. The density of average sea water is 64 pounds per cubic foot because of the added weight of the salts which are dissolved in it." (pg 26)

So my vital capacity is 0.211888 cubic feet, times 64 pounds per cubic foot equals 13.6 pounds. That means that I can compensate for a lot of loss or gain in buoyancy with my lungs alone. Now, other people have different vital capacities. In 1970, the US Navy said that the average was four to five liters for an adult male. From Google's conversion program (simply type in "4 liters in cubic feet" and hit the enter key), we see that this is "4 liters = 0.141258667 cubic feet", times 64 pounds per cubic foot for sea water, and we get 9.04 pounds. So we really do have some control over this. The newer very high pressure tanks may go over this nine pounds, and that can create problems. But the older steel tanks, especially, are pretty easy to use.

John

 

[captain]

Great post John. I have a friend who had recently become certified and I took him out on his first post certification dive. I was diving my Aqua Master with no BC and he asked how would I control my bouyancy so I proceeded to demonstrate how I used my lungs as a BC. He said that was never mentioned in class that your lungs could be usde for bouyancy control. He had about 15 pounds of lead on his belt and I had him go down to none (fresh water no wet suit). He couldn't believe it.

 

[TheRedHead]

So true, Captain. Most new divers don't have a clue about proper weighting. The instructors plant them on the bottom and never tell them that they don't need to dive overweighted. They even teach negative ascents.