December 2006

By John Francis

So, you found a sweet deal on a two-dive charter to the Oriskany, the aircraft carrier-turned-artificial reef off Pensacola, Fla. You and your dive buddy want to dive it, but you wonder if you can handle the depth. You've made some dives in the 60-foot range, even one to 80 feet. Now you both have advanced open-water cards. Sounds like that qualifies you for any recreational dive to 130 feet, which just about includes the flight deck of the Oriskany.

Actually, it's a little like a teenager who gets his driver's license and thinks he's qualified to top out his dad's Z3. He might make it around Deadman's Curve, and you might come back from the Oriskany's flight deck on your first dive below 80 feet, but neither of you can say you were smart to try it.

The risks are different, but 130 miles per hour and 130 feet have this much in common: Danger approaches quickly. Underwater, it's your air supply that goes fast but, unlike the power poles whipping by Dad's car, it's stealthy about it. Unless you keep close watch on your gauges, you may have no clue how fast the digits are changing. You may not even realize how deep you are; the water is so clear and warm at many tropical dive sites that 130 looks and feels like 30 feet. Add the blissed-out effect of nitrogen narcosis, and your first clue of danger may be when you suck on your reg and not much comes out. Though there are other dangers in deep water, running out of air is undoubtedly the most serious one.

Rehearsal Dives

The key to making the leap from 60 feet to 130 feet is not to leap. Instead, increase your maximum depth gradually, in measured steps. Many instructors would say you should not push the depth envelope by more than 10 feet per dive. In other words, dive to your current maximum depth, say 80 feet, until you're comfortable and relaxed. When you're ready to try a deeper dive go to only 90 feet.

Increments of 10 feet may seem like baby steps. It would take five of them, after all, to get from 80 feet to 130. But you're entering unexplored territory. First, you don't really know how much your air consumption will increase down there. Physics alone will multiply it by five at 130 feet compared to the surface, but there are other factors that can increase it more. You'll be a little nervous, probably more nervous than you realize, and that will induce you to take shallow, frequent and wasteful breaths. Air at depth is denser, so you'll work harder sucking it into your lungs and pushing it out again. Fatigue and carbon dioxide also push up your breathing rate and waste air. After that, you're in a feedback loop: shallow, wasteful breathing causes shallower, more wasteful breathing.

Second, your nervousness gets worse when breathing seems difficult. Anxiety and panic can follow without any apparent reason for it. You can convince yourself you're out of air when you're not.

Third, you're entering the depth range where nitrogen narcosis appears. It's a serious risk. It may take the classic form of euphoria, but it may reinforce anxiety instead. In either case, nitrogen narcosis can cause you to ignore all the warning signs of danger--like the rapidly sinking needle on your pressure gauge.

Deep Planning

Diving deep calls for more serious dive planning than many of us have grown accustomed to. "Dive computers have made many of us lazy about planning," says PADI Course Director Linda Van Velson. "We get in the bad habit of relying on the computer to tell us what to do." Rather than reacting to it, you should know in advance how much no-deco time you will have at 90 feet and how long your air supply will last and decide in advance when you will turn the dive. You want a contingency plan too. "If you want to dive to 90 feet," says Van Velson, "have a plan for 100 and 110 just in case, so if you bump 100 feet you know how long you can stay."

What will limit the dive, no-deco time or air supply? It depends on the diver, the computer and the depth. Who will get there first, you or your buddy? Naturally, if either of you reaches his limit, both need to turn the dive. You'll want more than 500 psi in reserve before turning a deep dive, but how much more? Some divers use a "rule of thirds" that is modified from cave diving's rule of the same name. The rule calls for using one-third of your air for the descent and the swim away from the ascent point, one-third for the swim back to the ascent point and the other third for the ascent and any delays that you may encounter. That would mean turning the dive at 2,000 psi with a 3,000-psi tank and ascending with at least 1,000 psi left in your tank. Another approach is to allow 100 psi for each 10 feet of ascent plus a reserve of, say, 500 psi, so you'd turn a 90-foot dive at 1,400 psi.

You also want to reserve more than one minute of no-deco time. You don't want to bump the limit accidentally and take on a deco stop obligation when you might not have enough air to fulfill it.

Dive Day

On the day you hope to push your personal depth limit by 10 feet, eliminate as many other dive risks as you can. Be well-rested and well-hydrated. Fatigue predisposes you to DCS, nitrogen narcosis and--probably more important--carelessness, anxiety and panic, so plan on an early night before the morning of your dive. Dehydration also increases the DCS risk, and leads to fatigue sooner, so limit your intake of alcohol and caffeine starting the night before the dive. Drink lots of water--about two extra quarts during the day.

Be well into the green zone of your computer too so you have margin for error. Normally, that means making the deep dive the first of the day.

What are dive conditions today? You might want badly to go 10 feet deeper than you have before, but if the vis is bad, the water is cold or the current is strong, maybe this isn't the day for something new.

When gearing up, check your own and your buddy's configuration more carefully than usual. Before stepping off, go over the dive plan again, then look each other in the eye: Does this still feel good? Either buddy can abort the dive for both at any point, and shouldn't hesitate to do so for any reason.

In the Water

Watch your buoyancy as you descend. Your buoyancy will fluctuate more with greater depth as the air in your BC and the bubbles in your wetsuit compress. Unless you watch it, it is easy for your descent rate to accelerate so much that it can be hard to stop at your planned depth. You can then waste considerable air inflating and venting your BC as you search for neutral buoyancy. One of your goals on these rehearsal dives is to minimize your weighting. Excess lead requires excess air in your BC to float it, and that causes excess buoyancy fluctuation and more risk of losing control.

Watch your breathing rate. Slow, deep breathing is especially important during a deep dive because the air you're breathing is much denser. As the air stream turns corners and passes restrictions in its journey from your tank to your lungs, friction causes turbulence, which restricts the air flow and increases your breathing effort. (Think of heavy traffic on the freeway.) Denser air and fast-moving air both increase the amount of turbulence. Denser air also means more molecules of the stuff that your regulator has to process with each breath. Rapid breathing at depth can exceed the flow rate of many good, well-maintained regulators--again, increasing your work of breathing. So as the air becomes denser with depth, it's important to keep the speed of the air stream down--to breathe slowly. That conserves both air and energy.

If you catch yourself breathing faster than normal, stop finning and hold on to something like a mooring line if you can. Take a slow, deep breath, trying to completely fill your lungs. Then exhale slowly, trying to completely empty your lungs. Do it again, until you relax.

Check your gauges more often than you're used to on shallower dives because readouts for depth, pressure and no-deco time change more quickly at depth. At the same time, the margin for error is smaller and the consequences are more serious.

Check your buddy more frequently than normal too because you need to defend against nitrogen narcosis. Look for signs of anxiety, like rapid breathing, jerky movements and a "wide-eyed" look. Is your OK sign returned quickly and calmly? Check your buddy's gauges from time to time, and expect him to check yours. A combined gauge and buddy check on a regular schedule helps keep both of you together and on the dive plan.

"Dive the plan" is one of those overworked mantras, but it's especially important on a deep dive where the hazards are greater and the options are fewer. Departing from the plan with a spur-of-the-moment judgment can be more dangerous at depth because not knowing what comes next can easily lead to anxiety.

One exception: Never be reluctant to begin your ascent earlier than you had planned, whether because conditions are different from what you expected or you've used air faster than you thought you would or because either of you feels uneasy. Any member of a buddy team can always turn the dive for any reason at all.

Going Up

The most dangerous part of a deep dive is undoubtedly the ascent. Your ascent rate will increase almost imperceptibly at first and accelerate as you go up. Until you're accustomed to it, it's easy to lose control. The ascent is also when buddies often become separated. If one of you leaves the bottom a second or so before the other, the distance between you tends to increase. The way to avoid separation is to make eye contact before you start up and watch each other all the way.

To help control your ascent rate, make a serious effort to locate and use a line--a mooring line or the boat's anchor line. Watching the line pass in front of your eyes will give you a good measure of your speed, and grabbing it allows you to stop instantly.

Ready to start up? First, write your depth, air pressure, no-deco time remaining and dive time on your slate. Learning how much air and time you'll actually use ascending from this depth will help in future dive planning. Now get neutral, face your buddy and simultaneously start your ascents by kicking upward, not by adding air to your BCs. Neutral buoyancy will be a moving target anyway; don't make it harder to find by abandoning it voluntarily. Start venting air from your BC almost as soon as you start moving up.

Uh-oh

At the standard, recommended ascent rate of 30 feet per minute, your ascent will take three minutes from 90 feet. That's a long time. What if you stayed longer than you intended at your maximum depth? Are you sure you will have enough air? If you have any doubt that your remaining air will get you to the surface, make the bottom half of your ascent the faster half. Bubble growth is much faster near the surface, so it's most important to go slow there. In a low-air situation, you might start up following your smallest bubbles--that's about 60 feet per minute, then slow down at 50 or 60 feet. In any case, try to stop briefly there, if only to re-establish control of your buoyancy.

Now that you're out of deep water, your air supply will go slower and you should be able to maintain 30 feet per minute to your safety stop depth at about 15 feet. Extend your safety stop beyond the recommended three minutes if you have enough air. There's no point taking 500 psi back to the boat when you could reduce your DCS risk by breathing some of it here.

After the dive, rest. Even if you haven't violated no-deco limits, you have taken on a large nitrogen load and unpredicted DCS is always possible. Avoid exercise for an hour or so because it promotes bubble formation. And drink plenty of water. Moist tissues exchange gas more quickly, and a well-hydrated blood supply is thinner so it circulates more quickly.