Speed kills. After turning on the air, the second most critical skill for safe scuba diving is turning off the speed on ascent – which is followed by an important third action of continuous breathing.
Scuba diving is a safe, enjoyable activity typically practiced as a slow paced sport – lots of floating, drifting and easy kicking – until divers are ready to return to the surface, that is. They seem to step on the gas and race upward then.
Beginning divers learn about Boyle’s Law and how an air bubble at depth expands as it rises. Every diver sees the proof of Boyle’s Law as pinpoint sized bubbles at depth grow to dinner plate sized bubbles by the time they break the surface. Classroom training presents the concepts of nitrogen absorption, bends and decompression tables. Some diving classes have more human physiology information than students were ever exposed to in their school careers. This information about the human body and how it processes air inhaled at depth, when combined with Boyle’s Law, is powerful information and tends to leave a distinct impression on new divers. As these divers leave the classroom and get wet, the fun and ease of diving let them forget those powerful physics and physiology lessons.
A safe ascent is unbelievably slow. Some might describe the safest ascent as agonizingly slow. The old rules of thumb were 60 feet per minute or as slow as your smallest exhaled bubbles. Both are too fast. The newer dive computers utilize much slower rates. Some are based on 30 to 40 feet per minute ascent rates. Others apply a 40 foot per minute rate for depths from 100 to 60 feet, then change to 20 feet per minute from 60 feet to the surface. Certification agencies and others recommend maintaining an ascent rate not exceeding 30 feet per minute and to slow this rate in the last 60 feet of your ascent.
To appreciate how incredibly slow 20 feet per minute is, mark off a 20 foot distance. Then walk that length in what you estimate to be one minute (no looking at your watch). You will probably cover the distance in much less time. If your foot is 12 inches long, you would need to pause for three seconds each time you placed one foot in front of the other. Try it.
Have you been ascending too fast? Probably. Following are some techniques for slowing your ascent.
USING A LINE
It is difficult to judge distance when swimming through a vertical water column. An ascent line provides a visual reference for judging distance and can act as a brake to slow ascent. Ascent lines could be the boat’s anchorline, a separate, weighted line from the boat or a line attached to a surface float such as a surfmat or inner tube, U/W divers approach the line and use it as a visual reference on ascent or grab the line and move up hand-over-hand to assure a slow, even ascent.
Ascent-descent-decompression lines are names commonly used interchangeably in recreational diving. In scientific, specialty or commercial diving the names may refer to three different lines, each used for a distinct purpose.
On a large resort dive boat, ascent lines can seem like the freeway at rush hour. Consideration of fellow divers goes a long way toward goodwill on the ascent line. When you approach the line, have your buddy in sight and begin your ascent together. If a current is running, hang on the flow side of the line, letting your body stream out in the current. This prevents you from crashing into others as you twist around the line.
Try to avoid corkscrewing. If you rotate around the line looking at the view, checking out your buddy or trying to shoot a last few frames of film, chances are you will start bumping into other divers.
Try to allow space between yourself and others above and below so you are not kicking or crashing into them. If a diver is carrying camera or video gear, give him/her extra space. It is courteous and much easier than the confrontation and expense that results when you put your tank bottom through a camera lens. This author’s philosophy is that the bull in the china shop pays for the damage.
Since everyone, in theory, is ascending at the same slow rate, don’t pass on the line just for the sake of getting up faster. Certainly, if divers are pausing for an extra decompression stop, you may need to pass them to keep your ascent time appropriate.
Once on the line, there is a tendency to steal last glances at the terrain below. By looking down, you may restrict your airway and the outflow of exhaled air. Remember the old rule about looking up when ascending? It still applies on the ascent line. Looking up will help pace your ascent, you can see what is happening with others above you on the line and you can plan your release from the line to swim to the ladder. Looking up also opens your airway for maximum exhaling and nitrogen outgassing.
Once holding the ascent line, it is easy to monitor your dive computer or gauges. Check your rate of ascent. Watch for stop/slow messages from the computer. Rising on the ascent line is a calm time, when you have stopped taking pictures or shooting video. You are tuned into your buddy for signs of any problems. Buddies should stay on the line together. If you leave the line for any reason, be certain to communicate with your buddy before you do. One definition of instant terror is when your buddy is next to you on the line one minute and gone the next.
While ascending you will probably need to reduce your buoyancy. If there is air in your BC it will expand as you rise and you will be more buoyant. Release little puffs of air from your buoyancy compensator to maintain neutral buoyancy. If you start to hang on the line in a bottom up attitude owing to a nearly empty scuba cylinder and increasing positive buoyancy, release air from your BC until you regain a vertical orientation. If you have added air to your BC at depth, expect to release air as you ascend. Once at the surface you can partially reinflate your BC if positive flotation is needed for a surface swim.
When ascending, stop between 10 and 20 feet for three minutes. This extra safety stop is insurance decompression time to give your body a few more minutes to outgas. Once you drop off the line and begin swimming back to the boat, take care to stay above 20 feet.
ASCENDING WITHOUT A LINE
Inland lake dives are usually shore dives. Some states require divers to tow a float with a dive flag. If diving in these areas you can use the float’s tow line as an ascent line. In other states, however, you can enter from the shore and disappear underwater. Common practice is to use the gentle sloping bottom terrain as a guide. A dive profile might begin with a perpendicular to shore swim to maximum depth, followed by a zigzag or angled swim to shore. The gentle bottom contour becomes the guide for a gradual ascent at the dive’s conclusion.
At other times on a shore or boat dive you might want to surface mid-dive to check position. The tendency is to pop to the surface, verify your location and quickly descend. This ascent should be slow and controlled, the same as your final ascent. Bubbles expand by the same physics laws no matter when you ascend.
Sometimes you may start uncontrollably ascending owing to substantial positive buoyancy. The classic instructions in this situation are to 1) try to release/bleed the BC or suit by using dump valves, oral inflators or power deflator mechanisms, 2) flare out to increase drag on your body and 3) exhale continuously.
The safest ascents are controlled, planned, slow movements from depth toward the surface. Safe divers have sufficient air to slowly make their way up a line, while continually practicing buoyancy control. This ascent is incredibly slow – not to exceed 30 feet per minute, while traversing from 60 feet toward the surface. Divers are encouraged to stop between 10 and 20 feet for a three minute safety stop on all dives. Those using dive computers are reminded to monitor their units and follow their instructions to slow ascent, pause or proceed.
Diving is a safe, enjoyable sport. Learn to use the last few minutes of each dive to slowly and gradually leave your undersea adventures, inching your way toward the surface.