Most recreational scuba cylinders are filled with compressed air or a planned breathing-gas blend such as enriched-air nitrox. They are not generally filled with pure oxygen for ordinary recreational scuba dives. Breathing-gas choice depends on training, depth, equipment, and the dive plan.
What is compressed air?
Compressed air is filtered air stored at high pressure in a scuba cylinder. It contains roughly 21% oxygen, with most of the remainder nitrogen and small amounts of other gases. The exact composition and cylinder condition must meet applicable filling and service requirements.
What is nitrox?
Enriched-air nitrox contains a higher percentage of oxygen than ordinary air, commonly blends such as EAN32 or EAN36. It can change inert-gas exposure for a planned dive, but its oxygen content also imposes maximum operating depth and oxygen-exposure limits. Divers should use nitrox only with the appropriate training and gas-analysis procedures.
Why is pure oxygen not used for ordinary deep scuba diving?
At increased pressure, oxygen can become toxic to the central nervous system and cause seizures, which can be fatal underwater. Pure oxygen therefore has strict depth and use limits and is not a general-purpose breathing gas for conventional scuba diving. Specialized oxygen use, including some decompression applications, requires specific training, equipment, and procedures.
How do divers verify the gas?
- Use a reputable filling provider and cylinders that are properly inspected and maintained.
- For nitrox, analyse the oxygen fraction with a suitable analyser and confirm the label.
- Calculate and respect the maximum operating depth and oxygen exposure limits for the actual blend.
- Use only the breathing gas and equipment appropriate to your training and dive plan.
Bottom line
Ordinary recreational scuba cylinders generally contain compressed air or a planned breathing-gas blend, not pure oxygen. Gas composition and limits are safety-critical; never guess what is in a cylinder or use an unfamiliar blend without the required training.
What gas is inside a scuba tank?
Most recreational scuba cylinders used for ordinary air diving contain compressed air, not pure oxygen. Air is mostly nitrogen with roughly one-fifth oxygen, plus small amounts of other gases. Some trained divers use enriched-air nitrox, which contains a higher oxygen fraction, while technical diving may involve other carefully planned gas mixtures. The correct gas depends on the training, equipment, depth, and dive plan.
Why pure oxygen is not used for ordinary deep scuba
At increased pressure, oxygen can become toxic, and the depth at which this risk becomes unacceptable depends on oxygen partial pressure. Pure oxygen is therefore not a suitable breathing gas for typical open-circuit recreational dives at ordinary scuba depths. Specialised oxygen use has strict limits and requires specific training and procedures.
How to verify a cylinder before diving
Never identify a gas solely by cylinder colour or someone else’s word. Check the label, analyse the gas when required by training and local procedures, confirm the cylinder contents with the supplier, and match the gas to your computer and plan. If anything is unclear or inconsistent, do not use the cylinder until a qualified person resolves the discrepancy.