Pressure and ATA Basics for Scuba Divers

Almost every dive calculation starts with one fact: water adds pressure quickly. Once absolute pressure is clear, gas duration, PPO₂, buoyancy, and decompression formulas stop looking like unrelated rules.

Where underwater pressure comes from

At sea level, the atmosphere already applies about 1 ATA. Seawater adds approximately another atmosphere every 10 meters33 feet. That gives the planning formula: ambient pressure = depth ÷ 10 + 1 in metric, or depth ÷ 33 + 1 in imperial units.

Absolute and gauge pressure

Absolute pressure includes the atmosphere and is used for breathing-gas and decompression calculations. Gauge pressure measures pressure relative to the surrounding atmosphere; a cylinder gauge reading zero is not a vacuum. Mixing the two is the classic “forgot the +1” error.

Bar and ATA are numerically close enough for common teaching approximations, but psi is a different scale: 1 bar is 14.5038 psi. Use the dive unit converter when a fill, gauge, or plan uses an unfamiliar system.

A worked pressure ladder

At 30 m, EAN32 has PPO₂ = 0.32 × 4 = 1.28 ATA. A diver with an 18 L/min surface RMV consumes about 72 L/min from the cylinder. Both results use the same four-atmosphere multiplier.

Why pressure appears everywhere

• Gas planning: depth multiplies RMV and reduces cylinder duration.
• Nitrox: gas fraction × ambient pressure gives partial pressure and MOD.
• Buoyancy: flexible gas spaces shrink on descent and expand on ascent.
• Decompression: changing ambient pressure drives inert-gas uptake and release.

Apply the same foundation in the tank-duration guide and PPO₂ guide.

The diving-units guide separates exact conversions from planning shortcuts. Use the dive unit converter when a fill, gauge, or plan uses an unfamiliar system.