Photography by Chris Jaffe

November 2003

By Michael Ange

Photography by Chris Jaffe

Bill swam forward, enjoying the silence of his rebreather along with the relaxing, weightless sensation of being under water. The freshwater lake in his hometown wasn't exactly a diving paradise, but it was close, convenient and wet. Besides, with his new closed-circuit rebreather, he could move among the catfish and bream with surprising stealth. In all his years of diving, Bill had never felt so relaxed under water. A sense of euphoria enveloped him as he swam forward, and he never noticed his field of vision getting narrower and narrower. That sense of euphoria was the last thing that Bill would ever feel before dozing off, permanently.

The Diver

Bill was a professional in his early 40s who had reached a point in his diving career where he felt like he'd seen it all--been there, done that, got all the T-shirts. Diving was starting to bore him, until he discovered the cutting-edge technology of closed-circuit rebreathers.

This silent, bubble-free technology appealed to Bill, an underwater photographer who relished the opportunity to get closer to marine life. The units were expensive, but not above his means, so Bill purchased a top-of-the-line closed-circuit model and earned the proper certifications. Excited about diving again, he couldn't wait to start diving on his own with his new toy.

The Dive

Shortly after completing his rebreather certification, Bill went to a local dive site to gain some more experience with his rebreather unit. Following his training, he meticulously set up the unit and completed all of his safety checks. As he approached dive time, his excitement and enthusiasm began to grow.

Bill entered the chilly water, taking several deep breaths and listening for the hiss of the demand valve that fed fresh air into the breathing loop. The sound of the valve inflating told him the mechanical system was functioning properly. Satisfied, he swam away from the entry point and descended to his dive depth of around 40 feet.

The Accident

Unfortunately, in his excitement to dive, Bill failed to turn on the rebreather's computer. His high-end system relied on a computerized network of electronic sensors to monitor and control the oxygen level of his breathing loop. Without the computer control, there were also no alarms, no bells, no warnings of any type as the oxygen level began to drop.

Bill gained a false sense of security from the sound of the demand valve at the start of his dive. What he heard was the initial pressurization of the breathing bag with fresh gas, air in this case.

By design, the demand valve would have mechanically added gas to the system in order to adjust to the water pressure as Bill descended. But without the computer to read and react to the oxygen sensors, the rebreather unit could not know how much fresh air to add to the breathing loop in response to Bill's consumption of oxygen.

Once he reached a static depth, the system stopped compensating for pressure. The oxygen Bill was consuming was not being replaced, so he was getting less and less oxygen with every recycled breath. When the oxygen content fell below 16 percent, Bill would have felt the first sense of euphoria. As the oxygen in the loop dropped even more, he would have slipped into a deep hypoxic sleep--still breathing until the oxygen level was too low to support life.

When the search and recovery team found Bill's body, there was no indication that he had ever realized the problem. His gear was still in place, and the mouthpiece of the rebreather was still firmly in his mouth. There was ample gas remaining in both of the unit's cylinders.

Analysis

Photography by Chris Jaffe Closed-circuit rebreathers are very sophisticated pieces of equipment. Unlike open-circuit scuba regulators or even semi-closed-circuit rebreathers, closed-circuit rebreathers require meticulous attention to detail, rigorous maintenance and the completion of a checklist before every dive.

Based on what we know about this accident, Bill followed all the procedures necessary for the safe use of his unit except one. He failed to verify the operation of the electronic control module.

Once he reached a static depth, the pressure demand for diluent gas ceased. Without the computer to monitor his breathing loop and to add gas in response to his oxygen consumption, Bill had no way of knowing that he was depleting the oxygen in the system with every breath.

When investigators examined his gear, the control module was still in the pre-dive mode. If he had at least checked the module display upon descent, he would have realized the mistake and could have aborted the dive safely. In this case, a dive buddy may have also prevented this accident by ensuring that Bill check the readout before entering the water.

Lessons for Life

• The best dive training in the world is worthless if you fail to use it. Advanced forms of diving equipment like closed-circuit rebreathers require a strict adherence to checklists and safety procedures. Though his training properly covered all pre-dive procedures, Bill skipped the last step in his excitement to dive.

• All divers should verify the function of their gauges and dive computers soon after entering the water. Failure to notice malfunctions or other issues early in the dive can lead to catastrophe. This is especially true for advanced equipment like rebreathers.

• Solo diving is a specialized form of diving requiring advanced training and experience. Though Bill had a lot of dive experience, he did not take the proper safety precautions for a solo rebreather dive.