How scuba diving works.

What the word means

Scuba is an acronym for self-contained underwater breathing apparatus. Wikipedia (secondary) describes the defining feature as a breathing supply the diver carries, completely independent of any gas fed from the surface. Wikipedia says this gives the diver limited but variable time underwater: the supply is finite, and how long it lasts varies. Wikipedia credits the word itself to Christian J. Lambertsen, who used it in a patent he submitted in 1952.

Diving with a hose to the surface is a different activity. Wikipedia notes that most commercial diving uses surface-supplied equipment wherever that is practical, while scuba is used for recreation and for professional work such as scientific, military and public safety diving.

Two families of breathing set

Wikipedia groups scuba into two basic types, according to what happens to the gas a diver breathes out.

The equipment section explains the main parts of an open-circuit set: cylinders hold the gas, regulators deliver it, buoyancy control devices help the diver adjust buoyancy, and dive computers track the dive.

Pressure is the thing that changes

SSI states that pressure rises by roughly one extra atmosphere for every 10 m (33 ft) of depth. PADI's diving physics post (5 June 2026) puts it as 1 ATA for every 10 m of descent in sea water, and says Open Water Diver students learn how that pressure affects the body. The gas laws in PADI's post link pressure to the volume of a gas and to how much gas dissolves in a liquid, and PADI presents Boyle's law with Archimedes' principle as the basis of buoyancy. Those ideas are covered, by name and without calculations, on dive physics explained.

Recreational, technical and professional

Training organisations and writers split diving into broad kinds:

• Recreational diving is diving for its own sake, within limits each training agency publishes. SSI, in an undated article, calls 40 m (130 ft) the generally accepted maximum recreational depth. Each agency's own figures are on depth limits by certification.
• Technical diving is described by Wikipedia (secondary) as recreational scuba beyond the generally accepted recreational limits, with a greater risk of serious injury or death. One definition it gives is a dive where, at some point, the diver cannot ascend straight up to the surface. Agencies such as TDI, which says it was founded in 1994 to train divers beyond sport diving, and GUE, whose first courses in 1998 were in cave, technical and rebreather diving, teach it as separate training.
• Professional diving covers scientific, military, public safety and commercial work. This site covers recreational diving.

How people learn

Wikipedia says recreational training is delivered by instructors affiliated with certification organisations, and that most of those organisations require a minimum level of fitness and health. The entry-level course is a mix of theory and practice. PADI's 2018 blog post describes its Open Water Diver course as five confined water sessions and four open water dives, alongside knowledge development. SDI describes its Open Water Scuba Diver course in three parts: eLearning, pool training with an SDI instructor, and supervised open water dives.

The international standard for the training of recreational scuba divers is ISO 24801. Its first two parts separate two levels:

Supervised diver (ISO 24801-1, Level 1)

A level of training for diving under the supervision of a dive professional.

Autonomous diver (ISO 24801-2, Level 2)

The level at which a training organisation may issue a qualification to dive without a dive professional. Wikipedia (secondary) reads this as diving independently with a buddy.

The theory content listed for ISO 24801-2, according to Wikipedia (secondary), includes equipment, physics, decompression management and planning, medical and psychological problems, the environment and nitrox use. The ISO diver standards page covers the standard in more detail, and certification levels shows how each agency's courses map onto it.

What this section does not do

These pages describe what agencies, standards bodies and medical bodies publish. They do not teach skills, explain how to plan a dive or tell anyone whether they are fit to dive. Those things are taught by an instructor and decided, where medicine is concerned, by a physician. For words you meet along the way, see the glossary.