|Decompression-stop diving allows divers to safely go beyond traditional sport diving depth and time limits. Photography by Jesse Cancelmo/Seapics.com|
Admit it: You've been tempted to go deeper and stay longer than the so-called no-decompression limits allow. Maybe there's a wreck that's calling your name from the other side of 130 feet. Or maybe you just want more bottom time at, say, 100 feet than the tables or your computer will allow.
If the no-decompression limits are cramping your diving ambitions, it's time to take your dive skills to the next level with certification in staged-decompression diving.
Risk and Reward, Myths and Reality
A staged-decompression diving course--sometimes called deep air, decompression procedures or extended-range diving--is more than your key to greater depths and longer bottom times. It's an entry-level technical diving course and therefore different from advanced recreational training. When you tackle decompression diving, you're accepting more risk and greater responsibility for your safety than you ever have before.
The good news: According to statistics from the Divers Alert Network (DAN), properly trained and equipped divers engaged in staged-decompression diving actually have a lower rate of decompression sickness (DCS) than divers following the no-decompression tables.
The key words here are "properly trained and equipped." Attempting decompression diving without the right tools and training is extremely risky and accounts for the overwhelming majority of DCS cases related to decompression diving.
In addition to the need to stay under water for five or 10 extra minutes, maybe more, staged-decompression diving exposes you to other risks, from running out of breathing gas to currents to hypothermia. For these and other reasons, you really should take a training course from a qualified instructor before attempting decompression diving.
Obviously, we can't substitute for such a course here. Don't think we're even trying. Consider this an overview of the skills needed for safe deco-stop dives--not a license to make them.
The unusual risks of deco diving require unusual precautions. Among them:
- CAREFUL PLANNING. For example, you have to know, before you dive, that you will have enough breathing gas for the dive, for all the decompression you might need, and for surprises. Do you know how to predict your gas consumption?
- MORE EQUIPMENT. In particular, more redundancy so you can deal with an equipment failure without surfacing. What would you do if, say, your computer failed before you began your decompression?
- BETTER SKILLS. Buoyancy control and ascent rate control are especially important because errors can multiply decompression obligations quickly. Some tech diving instructors use the term "precision diving." Can you hover at 20 feet (not 19, not 21) for five minutes without holding on to a line?
- A DIFFERENT ATTITUDE. Compare the recreational diving term "dive buddies" to the tech term "dive team" to get a taste of the difference. The emphasis has to be less on getting your personal jollies, more on the satisfactions of successful teamwork. Do you like to follow rules?
Planning a Deco Stop Dive
As most divers know, it's easy enough to run out of gas on a no-stop dive within recreational limits. It's easier still when your dive is deeper or longer and you have to add many minutes of decompression time.
Obviously, planning your gas supply is an important part of planning a deco-stop dive, and you can't decide how much gas you'll need until you know what your depth, bottom time and ascent time will be. The whole dive has to be planned, and the plan has to be followed. Immediately you run into unknowns:
- WHAT WILL YOUR DIVE PROFILE BE? Most recreational dive computers do not give adequate planning information for dives requiring deco stops. Calculating stops requires the use of specialized tables.
- WHAT'S YOUR SURFACE GAS CONSUMPTION? The only way to find out is by experiment and calculation. Actually, it's a good idea for all divers who are pushing their no-stop limits to know this--you don't have to be in deco-stop mode to run out of air.
- STEP 1: Go in the water with full scuba gear and an 80-cubic-foot, 3,000-psi tank.
- STEP 2: Staying at 33 feet, swim at a moderate pace and measure the time it takes to use 750 psi. That's one-fourth of the tank, or 20 cubic feet.
- STEP 3: Surface ambient pressure is half the pressure at 33 feet, so divide the 20 in half: 10 cubic feet.
- STEP 4: Divide 10 cubic feet by the time it took to use 750 psi. If it took 12 minutes, your surface air consumption is 10/12 or 0.83 cubic feet per minute.
- DOING THE MATH. In theory, you can now multiply your surface gas consumption number by a simple formula and know your consumption at any depth. (Multiply by: depth in feet divided by 33, plus 1.)
However: Your actual gas consumption will change. It will go down when you are resting and (more likely) it will go up when you are working hard or experience cold. Also, your dive profile gave you maximum and stop depths and times at depths, but you must also account for the descent and ascents surrounding the stops.
Oh, and be careful. As you may remember from math class, mistakes are no excuse. The answer is still wrong, and in deco diving the wrong answer can hurt you.
Not simple, is it? And there's more to planning your gas supply: You want to include extra gas to allow for an emergency--a third more is a typical reserve. Assuming you're diving with a buddy, you want to include even more so you can help him; your gas supply might have to decompress two divers.
Then there's the whole question of oxygen toxicity if you're using a nitrox mix and stay too long, or go too deep. It's another potential killer, and too complex to get into here.
The only safe way to learn these skills--and what's more important, practice them under supervision--is to get certified as a decompression diver. The class will also expose you to proprietary planning charts, and to software that allows you to quickly compare several dive profiles for deco obligation and gas consumption and generate custom decompression schedules. (And it does the math correctly.)
But there is a lot more to planning a deco-stop dive than deciding on a profile and estimating the gas supply. For example, how would you handle a serious equipment failure? A lost ascent line? A missed deco stop? In no-stop diving, the solution to a problem is almost always to return to the surface as slowly as circumstances permit. But the risk of DCS that is inherent in a deco-stop dive means most problems should be solved under water. Procedures for situations like air sharing and buddy separation need to be planned and practiced.
Equipping Yourself for Deco-Stop Diving
|Multiple tanks ensure these deco divers have sufficient breathing gas. Photography by Jesse Cancelmo/Seapics.com|
Safe diving with a decompression obligation means more and different equipment than is necessary for most sport dives. One reason is evident from the discussion about gas consumption: You may need more, and larger, tanks to complete a deco-stop dive. Another reason is your need to stay at, say, 20 feet for another five minutes whether all your equipment is working properly or not; you'll want backup gear.
How to best rig your equipment for deco-stop diving is another issue. You don't want to waste time fumbling for the backup reg, for example. Ease of access is especially important when nitrogen narcosis or visibility may be a problem.
Equipment choices and rigging options are complex issues. Debate over them among tech divers generates as much heat as it does light, is ongoing and may be unending. Don't expect the final word here, just some of the options and trends.
- CYLINDERS. Every staged-deco dive requires a backup gas supply with enough volume to complete the dive and all planned decompression stops, plus a reserve of 30 percent to 50 percent. Single primary tanks with a small pony bottle of 30 cubic feet are common in entry-level deco courses and for simple decompression profiles. However, as depths increase beyond the 150-foot range, so does the complexity of the dive and the need for greater volumes of breathing gas. For deeper and more complex profiles, it's common to use identical tanks side by side as the primary air supply with two, three or even four more tanks clipped beneath a diver's arms. High-capacity steel cylinders tend to be the bottle of choice for deeper and longer excursions. They offer additional gas volumes and negative buoyancy when empty, which eliminates the need for additional weighting.
- REGULATORS. A backup second stage on one first stage is not enough. A first stage failure is highly unlikely, but it can happen. You'll want at least two completely redundant regulators and both should be top quality for low work of breathing.
- BUOYANCY COMPENSATOR. For basic staged-deco dives or entry-level classes, you'll need a technical BC with professional-grade D-rings sturdy enough to accommodate the rigging of secondary tanks. In order to handle the extreme negative buoyancy of the multiple steel tanks required for more complex profiles, tech divers favor backplate-and-harness BCs with interchangeable bladders. These BCs allow the diver to customize the amount of lift and attach a redundant bladder. Some divers also rely on a dry suit for redundant buoyancy.
- INSTRUMENTS. The old saying, "Plan the dive and dive the plan" is never more true than in decompression diving. Your decompression dive plan (and two or three alternative profiles--just in case) is laid out in advance from tables or dive planning software and outlined clearly on a slate. Relatively simple decompression profiles on air or nitrox can be monitored by dive computers, but you should always have either a backup computer or depth gauge and timer. On more complex decompression profiles, tech divers follow the more conservative table-based dive plans with depth gauges and timers--computers are relegated to backup status.
- OTHER STUFF. A lift bag and reel let you create your own ascent line in case you have to drift with the current. Add signaling equipment for when you surface and more exposure protection for the long minutes hanging to decompress. Add a backup knife, maybe a backup mask, a backup ... . Or maybe not. The weight, the drag in the water and the mental stress caused by keeping track of too much gear can make a diver less safe, not more. The rule of thumb, says Mike Ange, Rodale's Scuba Diving's technical editor, is "minimalist--none of the gear you don't need, but all of the gear you do need."
Deco-Stop Dive Skills
Many of the basic skills of sport diving become even more important in deco-stop mode. For example:
- BUOYANCY CONTROL. You are carrying a large load of nitrogen and riding close to the edge of bubble formation. Accidental descents and ascents, even of small amounts, can have DCS consequences. Correcting them also wastes time and breathing gas. Your buoyancy control will be complicated if you are carrying more than the usual 80 cubic feet of gas because it means more buoyancy shift between full tanks and empty. (Each 15 cubic feet of gas weighs about one pound.)
- ASCENT RATE CONTROL. Your computer or table assumes a certain ascent rate as part of its decompression calculation. Ascending too fast has the same effect as cutting short a stop: It increases your risk of DCS.
- BREATHING CONTROL. Efficient breathing is necessary when your gas supply is already marginal. Rapid shallow breathing not only wastes gas, it slows your off-loading of nitrogen and increases your carbon dioxide levels. High carbon dioxide levels seem to increase the effects of nitrogen narcosis. Inefficient breathing is tiring too, leaving you less able to cope with an emergency.
- SELF-CONTROL. Problem-solving when the risks are greater requires controlling your emotions and analyzing your options rationally. You need to have learned and rehearsed skills for dealing with every conceivable emergency. Self-control tends to come with experience. Most technical training agencies require you to have at least 25 to 30 logged dives and advanced open-water certification before taking their entry-level tech training. The requirement for more advanced training jumps to 100 dives--or more.
- SELF-AWARENESS. How do you feel right now? There are those days when you just shouldn't be pushing the limits. You need to learn to monitor your fitness, energy and anxiety levels because if something doesn't feel right, it probably isn't.
- COMMUNICATION SKILLS. Team diving requires more frequent and precise communication than is typical between no-stop buddies. Exercising these skills is complicated by the unusual task-loading that comes with lots of equipment and with specific dive profile obligations. Another complication is the higher likelihood of nitrogen narcosis at depths below 100 feet. Both complications tend to erase recently acquired skills, so the skills need to be drilled until they're second-nature.
How To Make an Emergency Deco Stop
OK, you didn't plan this exactly, but now your computer is beeping at you and the screen is in deco mode. How do you make the deco stops correctly?
A good first step (now, before you go diving) might be to reread the pages in your computer owner's manual about deco mode so you understand what the gizmo is saying. Failing that, remember this:
- ASCEND NORMALLY NOW. Don't dawdle at the bottom--your deco obligation may be increasing rapidly. But don't rush the ascent either. Watch your ascent rate indicator and ascend at a normal speed.
- A STOP DEPTH IS A CEILING. Besides current depth, the computer will display another depth (probably 10 or 20 feet) and, on some models, a time. That's your first (or only) deco stop. It is a ceiling, not a floor. Stop ascending before you reach it and stay below the deco depth by a foot or so for the time indicated, or until the stop indication is cleared from the screen. If you accidentally wander above the stop depth by only a foot, your computer will get very upset. If you stay there for a few minutes, most computers will stop subtracting time from your deco obligation, which can dramatically increase your required time at the stop. Even worse, some will lock into gauge mode, giving you no decompression information at all.
- DEEPER IS OK. You can be several feet below the ceiling without affecting your stop time. More than that and the computer will probably subtract deco time at a slower rate, but not as much as if you were 1 foot above the ceiling. If a 10-foot stop is called for but swell or boat traffic makes that difficult, make your stop at 13 or 15 feet.
- HANG HORIZONTAL. The U.S. Navy thinks a horizontal position is best because your entire body is at the same depth and pressure. If that's difficult, put your chest close to (but not above) the stop ceiling.
- TAKE IT EASY. Try to hang off an anchor line or rock, or cruise the reef slowly, expending as little effort as possible. Just enough exercise to keep the blood flowing is fine.
- WATCH YOUR COMPUTER. The stop time will count down, but it may stop decreasing or even increase if you wander shallower or deeper. Don't leave the stop until the countdown reaches zero. At this point, it will either signal a shallower stop and time or revert to normal, no-decompression mode. Remember to ascend to the shallower stop (or to a safety stop or to the surface) at a normal, slow rate.
- ADD A SAFETY STOP. If your gas supply permits, add three to five minutes to your last 10-foot stop. Or just stay at 10 feet until you reach the last 100 psi in your tank. Do not descend to 15 feet for the safety stop. If you are using nitrox, it even helps to breathe down the tank on the surface because nitrox has more oxygen than air.
Get Training First. Please.
Every training agency, every instructor, everyone with a soapbox to stand on is insistent that you should not try deco-stop diving on your own, that you should take the proper training course first. They're right: a hands-on training course is important not so much for the information offered (because you can find most of it in books) as for the drill.
In an emergency, people forget first what they learned last and practiced least. That's going to be the book you just read on deco-stop diving or the warning your buddy gave you just before entering the water.
With a decompression procedures course, as with any diving course, the most important part of the class is actual practice of the skills, over and over, under the eye of someone who can correct you when you're doing it wrong. Repetition may reach the point of boredom, but the instructor knows that the repetition could save your life.
In an entry-level decompression diving class or for simple deco-stop profiles, you'll need:
REDUNDANT SECOND-STAGE REGULATORS. A common tech diving configuration uses a seven-foot hose (looped under the right arm, across the chest and around the neck) on the primary second stage. In an air-sharing situation, the diver passes this hose and breathes off the backup second stage, which is secured by a bungee cord or surgical tube "necklace."
TECH BC. A technical BC with D-rings sturdy enough to accommodate the rigging of stage tanks is vital. For adequate buoyancy, many deco divers use a backplate harness with interchangeable bladders.
STAGE TANK. A 30-cubic-foot pony bottle with its own top-quality regulator (hose and second stage secured to tank with rubber bands) and submersible pressure gauge. This tank is attached to the BC's D-rings by snapbolts.
REDUNDANT INSTRUMENTS. As a backup to his dive computer, this diver's console includes a depth gauge with a digital bottom timer. In order to streamline, some divers replace the console with a simple submersible pressure gauge and carry a backup depth gauge and timer in a BC pocket.
SLATE. Worn on the wrist or carried in a BC pocket, it includes the dive and decompression plans--plus two or three backup profiles.
The Myth of No-Decompression Limits
Whether you ever attempt staged-decompression diving or not, it's important to understand that all dives are decompression dives. The so-called no-decompression limits calculated by recreational dive computers and tables assume a slow ascent rate (a rolling deco stop) and recommend a "safety stop" (just a deco stop by another name) at 15 feet for three to five minutes.
The discipline of staged-decompression diving follows the same principle of allowing the body time to offgas nitrogen, but takes it to a more precise level. Proper decompression procedures allow a diver to calculate the gas supply he'll need, and the additional bottom time he'll enjoy by adding stops.