Deco dive

We've learned that every dive is a decompression dive. You on-gas; you off-gas. Surface with too much of the former and not enough of the latter: You're bent. Safety stops are a decompression stop. Slow ascents are a decompression technique. You're a decompression diver, like it or not.

We like it. There's no reason that with proper training, skills and equipment recreational divers can't employ this legitimate technique for maximizing hard-earned diving dollars. Especially considering the next reason:

We're wired. Virtually all of today's dive computers provide the information you need to plan and execute simple forms of decompression diving. Some computers do it with graphic displays and flashing icons. Some talk us through it.

We're stupid. Boiled down, decompression is a relatively simple procedure. You ascend to a given depth and hang out for a period of time prescribed by your computer, then go up another 10 feet and zone out again. Try not to fall asleep.

But don't be fooled: At some point, a few curve balls will get tossed your way and the best laid plans take a turn south. That's why the key to responsible decompression dives is planning and preparing for the things that can go wrong.

What's Wrong With This Picture?

Most divers are never taught proper deco technique in traditional diving courses. And that's fine since planned decompression is not part of open-water training. Safety stops receive strong mention, but watching many recreational divers do safety stops is like trying to follow a yo-yo in action.

But with planned decompression, stops are factored into the overall dive profile. If you decide to take your 30-foot stop at 20 or 15 feet, the odds are you may get to experience the reality of decompression sickness firsthand.

Planning:

Method #1: Use dive tables to predict a dive profile that includes required decompression stops. This allows advance planning for "bottom gas" (what you will need to breathe during the working portion of the dive) and "deco gas" (which you'll need to complete the decompression). Given this scenario, the diver can easily compute a schedule that will allow him to carry enough gas supply for both portions of the dive and allow a contingency factor.

A diver breathing standard compressed air can use that gas for both phases of the dive. Indeed, most tables published for scuba diving assume that any deco will be made on the same stuff that the diver was breathing while he was swimming about on the reef or around a wreck.

However, most divers planning decompression today look for an additional safety edge by using pure oxygen or a nitrox mix to increase the efficiency of off-gassing. These divers carry a smaller cylinder of "deco gas" to switch to, usually around the 20-foot stop, or they use one provided by the dive boat.

Method #2: Plan the dive and the decompression as it occurs. This is done with dive computers and allows a much greater window of flexibility to the dive plan since no rigid profile is set in advance. The governing factor in this type of plan is to allow enough gas to complete whatever decompression may be required. There are endless variations to gas volume management that can be calculated by the diver under water, including the classic cave diving "rule of thirds" (one-third of your gas for the descent and dive, one-third for the ascent and decompression, and one-third for reserve).

Method #3: Divers using secondary deco cylinders have an extra edge on the folks who want to decompress on the remaining air in their primary tanks. Not only will they benefit from the higher oxygen mix, they will also generally have from 20 to 50 cubic feet of additional gas to breathe during the hang.

Tips for Avoiding the Chamber

So you completed your dive and are ready to head up to begin the decompression indicated. Here are some basic do's and dont's.

• Know your computer. If you haven't thoroughly familiarized yourself with your computer's decompression functions, don't even think about trying a deco dive just to see what happens. Some units will default if no-decompression profiles are exceeded. Others will scramble if you venture beyond a certain depth limit. Still others have deco models so conservative that you will spend more time at 10 feet than you spent in line at the DMV.

• Monitor ascent rate. Your ascent is part of your total decompression profile. If it's supposed to take you two minutes to come up to your first stop, don't do it in 30 seconds. Likewise, don't come up unreasonably slow as this will only increase nitrogen gas loading during the deeper part of the ascent. Your computer should remind you if you ascend too quickly for its deco model by either flashing a warning on its display or beeping at you with an audible signal.

• Control your buoyancy. Stay in control by dumping excess air during the ascent, and stop completely at least 10 feet below the first stop to ensure you can hover comfortably. Dive with extra weight to counteract the positive buoyancy of tanks as they are empty.

• Stay static. Preferably, your first experiences should include some type of static support like an anchor line or deco bar. This also gives you a firm handhold if current is present or if the dive boat is swinging on its mooring.

• Keep it simple. Initially, no decompression should be planned with more than one stop until you are completely comfortable with your ability to maintain depth, predict breathing rate, and manage your gauges.

• Do it all. Never cut a decompression stop short. Spend all the time required at the depth prescribed. It's OK to extend the last stop. You can consider that part of your "safety stop."

• Assume the position. If you're in a vertical position at your stop, measure the designated stop depth at the chest area. Ideally, try to maintain a horizontal position during the stops as this will subject the body equally to the surrounding pressure.

• Chill. Decompression should be a period of relaxation. Mild exercise is fine and benefits outgassing, but activities should be limited to easy swimming while maintaining depth. Arm windmills or bicycling the legs while holding onto a line or bar also can improve circulation.

• Don't chill. On long hangs, even warm tropical water can edge you toward hypothermia. You'll often need to be more warmly dressed as a decompression diver.

Deco Diving — The Real Risks

Stage-decompression divers are forced to stay below a virtual ceiling or face a high risk of DCS. Therefore, they must carry additional equipment that allows them to solve problems at depth, because surfacing is not an acceptable option.

Risk — Dealing with this gear on a dive boat results in a higher risk of physical injury.

Risk — The extra gear results in a larger profile while diving, creating an increased risk of overexertion due to swimming resistance.

Risk — The additional equipment creates a greater number of potential failure points in the gas delivery system, increasing the potential of a failure.

Risk — Because stage-decompression divers cannot rise above the decompression ceiling, should they miss the ascent/descent line, their risk of being carried away from the boat in a current is greater.

Risk — Stage-decompression diving also tends to result in longer in-water exposure times, greatly increasing the chance of hypothermia.

Risk — The extended exposure time also equates to increasing the odds of experiencing a problem while under water.

Bottom Line — Stage-decompression diving requires training and experience beyond the norm. The only way to truly learn to properly execute stage-decompression dives is to seek out a quality training program from an experienced, knowledgeable instructor. While there are increased risks involved, done properly the risks of stage-decompression diving are manageable within an acceptable level.