Snippets from the Research world – Scuba tank maintenance: it’s not all about pressure and strength!

Most divers know that scuba tanks should be periodically tested using visual checks (inside and out) and hydrostatic pressure tests.  The potential of moisture inside a cylinder to damage the structure due to rusting are well known and understood.  It may therefore be tempting to think that any diving cylinder reading 150-200 bar (or lower!) is relatively safe to use if nothing else is available.

A paper recently published in Diving and Hyperbaric Medicine by Arnaud Druelle and colleagues dramatically demonstrates why that is absolutely not the case!  They report two separate instances where divers have been rapidly rendered unconscious due to breathing gas mixes unable to sustain life, primarily due to internal cylinder corrosion.

• The first instance  involved a father and son testing a BCD in a pool using an old cylinder with 60 bar air.  The son breathed from the regulator with his head above water and rapidly lost consciousness, fully recovering without resuscitation shortly after removing the regulator. Unperturbed, the father repeated the ‘experiment’ and also lost consciousness before also making a full recovery on removal of the regulator.  On testing the oxygen content of the cylinder was measured at less than 1% (air is 21% oxygen).
• The second instance involved a diver using a ‘safety scuba tank’ found on a dive boat after realising his own tank held insufficient air for him to complete his planned dive.  He started his descent and was retrieved at 14m having been found unconscious without his regulator in his mouth.  The diver was lifted to the surface, revived using mouth-to-mouth and made a full recovery.  On analysis of the ‘air’ in the tank the oxygen level was measured at 8% despite the cylinder having been topped up to 180bar with air since the incident.

The main issue in both cases was hypoxia caused by the oxygen in the air being ‘used up’ in the chemical reaction (oxidation) that converts steel (iron) to rust (iron oxide).  These incidents clearly show that the ability of a scuba cylinder to contain pressure is no indication of the quality of the gas inside.

Divers should be acutely aware that untested cylinders harbour greater threats to life than simply exploding if the cylinders are not tested and in date!

Druelle A, Daubresse L, Mullot JU, Streit H, Louge P. Hypoxic loss of consciousness in air diving: two cases of mixtures made hypoxic by oxidation of the scuba diving cylinder. Diving and Hyperbaric Medicine. 2023 December 20;53(4):356−359. doi: 10.28920/dhm53.4.356-359. PMID: 3