Scuba type: open-circuit gear
Scuba gear is carried by the diver. The main parts:
- Tank (cylinder): strong steel or aluminium bottle holding compressed air, often about 200 bar. A 12 L tank at 200 bar holds about 2400 L of air at surface pressure.
- Regulator: reduces the high tank pressure in two stages, so air comes out at the same pressure as the water around the diver, only when the diver breathes in.
- Pressure gauge: shows how much air is left. Depth gauge or dive computer: shows depth and time.
- BCD (buoyancy control device): a vest that holds air to help the diver float, hover or sink.
- Weights: lead on a belt or in the BCD to cancel buoyancy.
- Mask, fins, wetsuit or drysuit.
In open-circuit scuba, breathed-out air bubbles into the water. Rebreathers clean and reuse the breath gas, so they last longer and make no bubbles.
Helmet-type diving (surface-supplied)
The diver wears a hard helmet on a dry suit or heavy suit. Air is sent from a compressor or air bank on the boat through a hose.
- Umbilical: one bundle holding the air hose, a phone line, a depth-measuring tube and a strong lifeline.
- Weights and heavy boots keep the diver on the seabed for work.
- Tender: a person on deck who looks after the hose and lifeline.
- Standby diver and a chamber on the boat for emergencies.
Used for long, heavy jobs like harbour repair, salvage and underwater welding.
Full-face mask type
A full-face mask covers eyes, nose and mouth. Air comes from the surface hose or a tank. Good points: the diver can breathe through the nose or mouth, can speak by radio, and the face stays warm and dry. If the diver becomes unconscious, the mask keeps the airway protected. It is lighter and easier to put on than a helmet, so it is common in work like inspection and fishery jobs.
Other equipment
Divers also use torches (colour and light), knives or cutters for nets and ropes, surface marker buoys so the boat can see them, communication sets, underwater cutting and welding tools, scooters that pull the diver through the water, and a decompression (recompression) chamber on large jobs. Gear needs care: rinse in fresh water, check hoses, test the regulator, and service on schedule.
Try it
In the 3D, switch between Scuba, Helmet and Full-face. Say aloud which one needs a boat. At the beach, wear a snorkel mask and notice how fins make swimming easy. Do not use real scuba gear without a trained instructor.
Key formulas and definitions
- Air in tank (L at surface) = tank size (L) × pressure (bar)
- Air use at depth = surface use × absolute pressure
- Time = air available / use per minute
- Pre-dive check BWRAF: BCD, Weights, Releases, Air, Final check
Worked examples
1. A 12 L tank is filled to 200 bar. How much free air does it hold?
12 × 200 = 2400 L.
2. A diver uses 20 L/min at the surface. How long will the 2400 L last at 20 m (3 atm)?
Use at 20 m = 20 × 3 = 60 L/min. Time = 2400 / 60 = 40 minutes (a diver always leaves a reserve).
3. Which system would you choose to inspect a ship's hull for 3 hours while the supervisor talks to the diver, and why?
A surface-supplied full-face mask or helmet: air is not limited by a tank and the phone line allows talking.
Common mistakes
- Thinking the tank holds pure oxygen. It holds compressed air.
- Forgetting that a diver also needs weights; a wetsuit and BCD are buoyant.
- Thinking the regulator stores air. It only reduces pressure.
- Thinking helmet and full-face are only for scuba. They get air from the surface.