OSHA in the U.S. - and various other governmental agencies around the world - classify all pressurised containers as hazardous material.
This section explains the importance of following recommended scuba tank filling guidelines and why due diligence in cylinder safety helps to keep them safe and in proper working condition.
It's not a myth! You will find exploding scuba tanks in the news reports of recent scuba diving accidents.
Some of these fatal tragedies involve fires, loose valves, tank ruptures, blowouts, and mismatched valve to cylinder thread size.
Worse still, some case reports cite injuries and death occurring because of inferior rebreathers and oxygen cylinders.
Not only does poor scuba tank maintenance cause injury and fatalities, but scuba cylinder explosion also destroys property.
So, how do they reinforce scuba tank safety guidelines? In general, the regulatory authorities in most countries address the safety aspects of using high-pressure gas cylinders.
For the most part, regulation helps to protect cylinder filling-station operators, recreational and commercial scuba divers, as well as members of the public (e.g. innocent bystanders).
Important: The rules help to ensure cylinder inspectors have received authorised function-specific training. Plus, better awareness and education helps to ensure refilling stations and divers reduce these unfortunate statistics.
Most divers understand (and expect) that there are legal requirements relating to the design, manufacture, visual and hydrostatic inspections, and certification to stringent engineering standards.
Among others worldwide, the commonly recognised regulation of scuba cylinder integrity, and the competence of anyone handling dive bottles, falls under:
Furthermore, complying with international standards, and the required regulatory documentation, provides limited variations on cylinder safety margins.
As a result, there are several design parameters that users must operate within. The important scuba tank safety guidelines include:
Statistics issued by the Divers Alert Network (DAN) show that over 90% of scuba tank ruptures occur during the filling process. When a cylinder cracks or bursts, the force of the ensuing rupture can cause severe damage to life and property.
Using a cylinder beyond its allowable stress limits increases the potential for failure. Moreover, ensuring that it has never been abused or overfilled at some time in its life is almost impossible to guarantee.
So, what happens to a tank as a result of overfilling it on a regular basis (i.e. close to or exceeding the hydrostatic test pressure)? In fact, the resultant deformation leads to its inability to withstand the allowable working pressure.
But wait - there's more:
The situation gets even more dangerous if you expose an overfilled cylinder to excessive heat (e.g. in the back of a vehicle). Because summertime temperatures can reach 94° Celsius (200° Fahrenheit), the internal pressure will also increase 'exponentially'.
Check the scuba tank markings and you can see what material was used during its manufacture (e.g. aluminium, fiberglass, or steel).
Hence, overexposure to temperatures above 60° Celsius (around 140° Fahrenheit) may cause structural damage to the integrity of the metal.
Another consideration for safety is the thickness of the wall. Two typical causes of inadequate wall thicknesses are:
All scuba cylinders should be clean and fitted with the appropriate valve assembly (e.g. Deutsche Industrie Norm (DIN) or a yoke fitting).
Filling dirty tanks with oxygen-enriched breathing gas (more than 25% oxygen) can cause an explosion - due to the presence of an internal fire.
More than (1) million foot-pounds force (kinetic energy) exists inside the average dive bottle. You can equate this to a 3-ton truck traveling at 112 kilometres per hour (70 mph). In simple terms, the potential release of this force after a cylinder ruptures is the main reason for administering the strict scuba tank safety standards.
Pro Tip: It's easier to visualise mechanical damage on the outside of a cylinder, such as a fracture, split, dent, or gouge. But, it is very difficult to see damage caused by the incorrect matching of threaded connections, including structural changes, without proper analysis.
It is imperative that all inspections and tests carried out comply with the regional regulations. These simple steps can help improve cylinder safety:
Note: Another section explains how opening diving valves can be deadly and why some pressure gauges indicate the tank is around half-full at the time of a breathing difficulty.
Note: The short video [6:59 seconds] presented by Simply Scuba contains some dive cylinder safety tips - especially useful for beginners.