Sunday, June 6, 2010

Tips on Getting Perfect Buoyancy

Get Perfect Buoyancy
From Scuba Diving Magazine

Relax. Anxiety usually reveals itself in constant sculling with the hands and minor finning. In the normal, face-down diving attitude, this usually results in upward thrust. That upward thrust has to be balanced by additional lead on the weight belt, which is one reason why inexperienced divers are so often over-weighted. They think they are neutrally buoyant because they maintain a constant depth, but in fact they are constantly swimming upward.
Master the three essential skills every diver must have, and learn 16 tips for a lifetime of better diving.

What separates the truly masterful diver from the bubble-blowing hacks? Three skills that every diver can master.

Lower Air Consumption
It comes down to two commandments: "Be efficient"--do less work so you need less air (because you use air to burn calories).
And "don't waste"--don't use more air than you need. Do this:

1. Breathe deeply and slowly. Each breath means work. To inhale, you have to thingy open a demand valve in your second stage and pull gas down your throat and into your lungs. Each inch along the way means friction. Each corner that gas turns means turbulence. Both increase the work of breathing. More bad news: Friction and turbulence increase with depth.

So don't force it. Try for a long, steady inhale until your lungs are full, then a long, steady exhale until they are empty. A complete exhale also reduces the amount of "dead" air that remains in your lungs.

2. Pause after inhaling. Your instructor told you to "breathe normally," but what he meant was not to hold your breath. In fact, the most efficient breathing pattern under water is not the same as your dry-land pattern. Instead of your familiar inhale--exhale--pause--inhale--exhale--pause pattern, try for inhale--pause--exhale--inhale--pause--exhale.

Gas transfer, the process where oxygen enters your bloodstream and carbon dioxide leaves it, happens only when your lungs are full or nearly so. Pausing with your lungs full rather than empty gives time for more of that gas transfer to take place. Rapid shallow breathing, by contrast, just dumps air into the water without extracting as much benefit from it.

Isn't this "holding your breath?" Doesn't that pause risk lung over-expansion and embolism if you are ascending? Not if you keep your throat open. Hold the air in by holding your chest expansion, but don't close the epiglottis in the back of your throat. That way, expanding air still has a path to escape.

3. Fin efficiently. Propel yourself with the smallest amount of energy possible, so you have to use the least air. Short strokes with the full leg nearly straight transfer the most muscle power into forward movement.

Go slow. Doubling your speed requires about four times as much energy. In fact, you should make all your movements--moving your arms and turning your head as well as finning--in slow motion to conserve energy and therefore air.

Minimize drag. Streamline yourself so you have to push less water aside as you swim forward. Get rid of accessories, or clip them close. Keep your arms at your sides--paddling with your hands wastes more energy than it's worth. Don't drag your feet--try for a horizontal position so your fins go through the hole in the water made by your head.

4. Improve your aerobic conditioning. Diving can be surprisingly strenuous because water is so much more dense than air. Swimming in full scuba gear at only one mile per hour requires about 13 times the exertion of sitting at rest and about the same amount as you'd need for a pretty fast run.

Even much lower levels of exertion will cause your breathing rate to rise. How much it rises and how soon depends mostly on your aerobic conditioning. The breathing rate of a diver in better condition will increase less when the workload goes up, so he will use less air.

5. Improve your equipment. Are there small leaks from your tank yoke? Your SPG? Does your octopus free-flow often? The amount of air being lost may seem small, but it's constant, so it adds up. Replace O-rings or otherwise fix the leaks.

Even a leaky mask can waste air, not only the air you waste to clear it, but the stress the problem causes, which probably elevates your breathing rate.

6. Stay warm. Simply put, cold divers use as much as 20 percent more air than warm divers. Why? A cold diver's body is using more energy to stay warm. And to make more energy, your body needs more oxygen, and therefore, more air.

But Never Do This
Never "skip breathe," holding your breath forcibly by closing your throat, as if you were snorkeling and diving below the surface. When you've closed your epiglottis, your lungs and throat become like a tied balloon. Your lungs' small air sacs (the alveoli) are vulnerable to rupture if you ascend even a small amount.

Better Buoyancy
One of the key traits of an experienced diver, good buoyancy control is necessary to avoid damage to the coral we all pay so much to see, and to prevent uncontrolled ascents. It also allows you to relax and to reduce your air consumption. Do this:

1. Make small changes. And then wait for them to take effect. Are you sinking? Add a short burst of air to your BC and wait five or 10 seconds to see what happens. Even if you added exactly the right amount of air, it can take that long for the extra buoyancy to arrest your downward momentum and bring you to a stop.

One of the most common mistakes is to keep adding air to your BC until you begin to feel a change. By that time, you have almost certainly added too much air to your BC and will have to dump much of it. If you keep dumping until you again feel that you're not rising, you will have dumped too much. You can continue to yo-yo around perfect buoyancy all day.

Obviously, you have to use some judgment here. If you are very heavy and sinking fast, you will have to act more aggressively to stop your descent. That's even more true if you are ascending rapidly. But most of the time, most divers miss neutral buoyancy because they over-correct.

2. Minimize your weighting. Too much lead on your weight belt means you'll carry too much air in your BC to neutralize it. That would not be a big problem if your depth remained constant, but it doesn't. When you ascend, the air in your BC expands and becomes more buoyant, accelerating your ascent. When you descend, the air compresses and becomes less buoyant, accelerating your descent.
Neutral buoyancy is like the flat spot on a hilltop where a ball will not roll. Once it's pushed from that spot in either direction, however, it gains speed. Too much lead on your weight belt, and therefore too much air in your BC, makes the flat spot smaller and the slope on each side steeper.

The correct amount of weight is that which makes you neutral at 15 feet with an empty BC and nearly empty tank--so that you can comfortably hover at your safety stop. That means you'll have to be about four or five pounds heavy at the beginning of the dive. (The difference is the weight of air you use during the dive.)

3. Use your lungs. Once you've found neutral buoyancy, don't mess with it. Instead of adding air to your BC to make temporary changes--to hop over a boulder in your path, for example--add air to your lungs. Breathing from lungs mostly full can give you as much as two pounds more buoyancy than breathing from mostly empty lungs. Don't hold your breath by closing your throat, however. Maintain the greater lung volume with your chest muscles.

4. Anticipate buoyancy changes. Your buoyancy will increase the moment you start ascending, so begin venting your BC immediately. If you wait until you sense positive buoyancy, you will be playing catch-up and will probably find yourself over-correcting.

Likewise, you know that your BC and your exposure suit will compress as you descend, making you more negative. Add small amounts of air to your BC soon after you begin your descent so you don't become too negative.

As you breathe down your tank, you will become four to six pounds more positive by the end of your dive. Anticipate this, and vent small amounts of air from your BC to adjust for it.

5. Relax. Anxiety usually reveals itself in constant sculling with the hands and minor finning. In the normal, face-down diving attitude, this usually results in upward thrust. That upward thrust has to be balanced by additional lead on the weight belt, which is one reason why inexperienced divers are so often over-weighted. They think they are neutrally buoyant because they maintain a constant depth, but in fact they are constantly swimming upward.

You will never know if you are in fact neutrally buoyant until you can completely relax in the water. One test is called the "Buddha hover": sit cross-legged holding your fin tips (that keeps both hands and feet occupied). If you're neutral, you can ascend or descend by changing your lung inflation.

But Never Do This
In trying to minimize your weight, never remove so much lead that you cannot stay neutral at 15 feet at the end of your dive. That means you should be four to five pounds heavy at the beginning and should sink slowly even while inhaling.

There are even times when it would be wise to be a couple of pounds heavier than that. For example, when using a thick wetsuit for the first time, the neoprene itself will cause large buoyancy changes and if you are not careful in venting your BC as you ascend, you may need the extra lead to bring your ascent under control.

Precise Navigation
The common fear of getting lost under water probably stems from inexperience in finding your own way. Too often we play "follow the leader" when we dive, so we don't develop our navigational skills. Next time, go off on your own (or with your buddy) and try this:

1. Map it in your mind. The key to underwater navigation is having a map, either tangible or mental, of the dive site, and constantly positioning yourself on that map. A real map, drawn on a slate, can be a valuable training tool, but before long you can probably do without it. Specific details are what make a map memorable. Form an image not only of the general shape of the dive site, but of maximum and minimum depths, the steepness of the bottom, the location of prominent features like rock outcrops and ledges, the direction of the sun (you can usually see it from under water) and so on.

The divemaster's briefing should give you considerable information for your map. Don't be reluctant to ask questions. As you move through your dive, imagine yourself moving across your map.

2. Plan a route through the site. It's very easy to get lost if you just follow your nose. Instead, decide in advance what route you will take and lay out that route on your mental map.

The easiest route to follow is probably an out-and-back pattern, because you can return along a somewhat familiar path. That need not be boring, however, because most things do look different from the other side. And there's still room for spontaneity: You can leave your planned route to investigate something off to the side as long as you remember how to get back. Another way to vary an out-and-back pattern is to return at a slightly shallower depth.

3. Identify a chain of landmarks. As you dive, don't think of the passing scenery as a steadily unrolling scroll. Instead, think of it in segments linked by recognizable landmarks. From here to that rock that looks like a parrot's beak, for example, and from the rock to that Pepsi can. The landmarks should be close enough together that you can see from one to the next.

The best landmarks are those that don't belong in the scene, like that Pepsi can. At some sites it may be acceptable to make a landmark by stacking a few small rocks, but use some judgment; no one wants to see a pristine scene littered by man-made objects. In any case, it's probably best to knock down the pile on your return trip. (And pick up the Pepsi can.)

4. Look behind you. As you follow your route through the dive site, pause frequently to look behind you. That will give you an idea of what the route will look like on your return. Be sure to look back at each landmark after you've left it. It's surprising how a distinctive object can look totally different or even be invisible from the other side.

5. Use your instruments. Most dive sites do not lend themselves to following the straight lines of compass courses for very long. But a compass can help.

One way is if you lay out your map (either written or mental) with north at the top. At any point in the dive, you can use your compass to face north and orient what you see to the map. Is the reef to your left? Another use is to note the compass course you took from the dive boat to the reef, assuming you made a transit with no obvious landmarks. The reciprocal course, from the same takeoff point, will bring you back to the boat.

Your depth gauge is a navigational instrument too. For example, note the depth of the anchor before you leave it. Suppose it was 40 feet. On your return route, if you follow the 40-foot contour along the bottom, you'll find the anchor (assuming no great tide change).

But Never Do This
In making landmarks, never break or mark anything living or dead.

No comments:

Post a Comment