The integrity of scuba tanks is an essential safety measure to prevent diving accidents. In Switzerland, scuba tanks must be pressure tested every five years and visually inspected every two and a half years. Often people only think about preventing bursts, but two diving accidents in France show that there are other important reasons for regular inspections.
Case 1
In the first case, an experienced middle-aged diver and his teenage son, both with no previous illnesses, tested a new buoyancy control device in their swimming pool. They used a scuba tank with a pressure of 60 bar. The son was the first to try out the equipment. While still on the surface, he lost consciousness after a few breaths from the regulator. After the regulator was removed, he quickly regained consciousness. The father then tested the same equipment again (sic!). He also lost consciousness within a minute, but also recovered quickly after the regulator was removed. Both were taken to hospital for examination, where no medical problems were found.
Subsequent analysis of the breathing gas by a specialized marine laboratory in Toulon revealed almost no oxygen content, but dangerous levels of carbon monoxide, hydrogen and volatile organic compounds. The moisture content of the breathing air was extremely high. When the bottle was opened, 300 ml of a brown, viscous liquid was discovered and the inner walls showed considerable corrosion.
Case 2
In the second case, a 51-year-old experienced diving instructor with over 1000 dives noticed that his diving tank only had a pressure of 100 bar - too little for the planned dive. He decided to switch to a new tank with 140 bar. Shortly after beginning his descent, he lost consciousness. He was found at a depth of 14 meters without a regulator in his mouth by his diving partner, who brought him to the surface. There, the unconscious diver was not breathing, whereupon his partner performed mouth-to-mouth resuscitation. He regained consciousness and began breathing again. He was hospitalized for further examination.
The medical examination revealed signs of barotrauma to the face, eyes, ears and paranasal sinuses as well as the effects of aspiration of seawater while unconscious.
Analysis of the breathing gas revealed an oxygen content of only 8%, accompanied by high levels of hydrogen and volatile organic compounds as well as a significantly increased moisture content. For test purposes, the scuba tank was refilled after the accident. Over a period of eight weeks, repeated gas analyses showed a progressive decline in the oxygen level. When the cylinder was opened, 250 ml of a brown, viscous liquid with a high salt content was discovered and the inner walls were heavily corroded.
Immediate loss of consciousness at the surface or during descent always suggest a problem with the breathing gas.
Discussion
Loss of consciousness while diving is rare, and the exact cause often remains unclear. Common causes are heart problems such as perfusion issues of the heart muscle, cardiac arrhythmias and immersion pulmonary edema. Immediate loss of consciousness after breathing from a scuba tank indicates a problem with the breathing gas. Although hypoxia (oxygen deficiency) due to tank corrosion is rare, it remains a serious risk if tanks are poorly maintained.
When moisture enters the tank, it reacts with the steel walls and leads to oxidation. During this process, the oxygen in the water dissolves and reacts with the iron, creating rust (iron oxide) and depleting the available oxygen. The overall chemical reaction is:
4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → Fe₂O₃ + 3H₂O
Over time, this reaction reduces the oxygen content of the breathing gas, making it hypoxic and unsuitable for breathing. In addition, hydrogen and organic gas compounds are often produced as a by-product, which further impairs the gas quality.
Conclusion
Corrosion can not only lead to accidents due to cylinder bursts, but also to oxygen deficiency. Regular tank checks are therefore essential. If there is any uncertainty regarding the quality of the breathing gas, the oxygen content of the breathing gas must be analyzed before the dive.
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