Bill struggled to pull the spike that was embedded in the old shipwreck. He banged vigorously, but he grew short of breath and felt increasingly anxious. He tried to pull a deep breath off his rebreather loop, but he couldn't get enough to breathe even though he could hear the bypass valve kicking in to replenish the loop. He exhaled several times through his nose to purge the loop and replenish it with fresh nitrox before making a rapid but controlled ascent to his decompression stop. On board the dive boat, he sucked from the emergency oxygen bottle as the tightness in his chest grew more pronounced. Suddenly, Bill's vision failed and he went completely blind.
Bill was an active scuba instructor in his 40s, in good health, with more than a decade of diving experience. He had logged thousands of dives, and especially enjoyed the challenge of scouring historic wreck sites for lost artifacts.
In order to maximize his bottom time on this dive, Bill used a semi-closed-circuit rebreather capable of delivering up to three hours of bottom time with a 30-cubic-foot cylinder. As a safety precaution, he also carried a bailout tank with enough gas for his short decompression stops. After descending to 145 feet and swimming along the 100-year-old wreck for a few minutes, Bill found an intact construction spike that he figured was the founder's spike. These contain identifying data about the builders of the ship, and Bill desperately wanted to recover this one. Unfortunately, it was embedded in the wood buried beneath the seafloor, likely frozen in place by limestone deposits.
Bill exerted himself heavily in the deep water, trying to free the spike. His rebreather's bypass valve kicked in repeatedly, constantly pushing gas from his small supply into the breathing loop. He knew he was overbreathing the system, but he was determined to recover the spike. When a serious shortness of breath and anxiety hit Bill, he remembered his training. He gave up his efforts and began an ascent to the surface. Because of his time at depth, Bill's dive plan required a short decompression stop at 10 feet. He leveled out at his stop, and continued purging the loop by exhaling the gas through his nose and replenishing it with fresh nitrox from the rebreather's small cylinder. This is how rebreather divers are trained to eliminate suspect gas from the breathing circuit.
As Bill did this, his anxiety and disorientation got worse. He felt perpetually short of breath and his chest felt tight and constricted. Finally, he cleared his computers and made it to the surface. He struggled aboard the boat, sat down and immediately started breathing from an 80 percent oxygen supply. His shortness of breath continued, and suddenly his vision faded to black. After several minutes, his vision came back into focus and he began to feel better. He returned home the following day without any injuries.
Bill was using a semi-closed-circuit rebreather (SCR). A SCR recycles the gas a diver exhales by filtering out biological waste products, mainly carbon dioxide, with a chemical compound called a scrubber. As the air recirculates, the oxygen level goes down as it's metabolized in the diver's lungs, so the rebreather replenishes the oxygen in the loop by continuously flowing a small amount of nitrox into a bag called the inhalation bag, or counter lung. When divers learn to use a SCR, they learn a formula that allows them to estimate what the oxygen content in the loop will be based on the gas in the tank, the flow rate of the rebreather and the depth of the dive. To dive these units safely, divers have to make two calculations: First, they estimate what nitrox blend they will actually inhale during the dive. Second, like any nitrox dive, they calculate the maximum operating depth for the nitrox mix used to replenish the loop. Oxygen becomes poisonous at elevated pressures, so the nitrox mixture is limited by the partial pressure of oxygen at the planned depth and the duration of the dive, called the oxygen dose.
To safely dive a rebreather, the diver must use a mix that does not exceed the safe oxygen dose, but provides enough oxygen to breathe safely in shallower water. If the oxygen content in the breathing loop drops too low, a diver may lose consciousness and drown. If it's too high, the diver can experience oxygen toxicity, symptoms for which include shortness of breath, anxiety, visual disorientations and convulsions that can also lead to drowning. An analysis of Bill's equipment after this dive would show that he used a 34 percent nitrox mix at 145 feet.
Under normal breathing conditions, the oxygen content in Bill's breathing loop was substantially lower than the oxygen content in the tank. However, when Bill overbreathed the loop and purged it, causing the bypass valve to add a large supply of fresh gas to the breathing loop, the percentage of oxygen in the breathing loop likely rose to about the same as in the tank. Bill failed to allow for this eventuality. He only calculated the minimum oxygen content he could expect in the breathing loop without figuring a maximum operating depth calculation. So, when Bill engaged his bypass valve repetitively, he breathed a much higher percentage of oxygen than was safe at this depth, and experienced oxygen toxicity. As Bill ascended, he noticed the beeping of the oxygen alarm on his computer, but he had missed this alarm at depth. And instead of actually completing the calculations for a safe dive as a backup, Bill had merely relied on his dive computer.
Bill was lucky. It is exceedingly rare for divers to survive bouts with oxygen toxicity. Even though Bill knew how to calculate the percentage of oxygen to use on a rebreather dive, he had taken a shortcut, relying on a piece of technology to replace both his training and common sense. It was a decision that nearly cost him his life. His luck was subsidized by common sense--many divers die of "china fever," a desire to obtain an artifact at nearly any cost. When Bill found he was having issues, he wisely chose to abort his dive and leave the artifact behind.
Lessons For Life
Diving procedures are designed for maximum safety. Shortcuts kill.
always calculate your maximum operating depth with either a formula or a dive table prior to each dive. And the maximum operating depth should be clearly labeled on any nitrox cylinder before it is used.
replace experience, training and common sense. Technology can fail or be misused, and good training and experience can provide a valuable safety net.
is worth your life. If your safety is at risk, leave it behind.