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 sea-level seawater planning formula: absolute pressure is the water-column pressure plus the one atmosphere already at the surface.

$P_{\text{abs}}$ is absolute ambient pressure in atmospheres absolute (ATA), and $d_{\text{m}}$$d_{\text{ft}}$ is depth in metersfeet of seawater. The 10 m33 ft divisor is a convention, not an exact physical constant: the US Navy Diving Manual treats 33 feet of seawater as one atmosphere, while metric practice defines 10 msw as 1 bar, which is about 0.987 atm. The $+1$ term adds the sea-level atmosphere.

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: 1 standard atmosphere is exactly 1.01325 bar. Psi is a different scale: 1 bar ≈ 14.5038 psi. Use the dive unit converter when a fill, gauge, or plan uses an unfamiliar system.

A worked pressure ladder

Take EAN32 and a surface breathing rate (RMV) of 18 L/min0.64 cu ft/min to 30 m99 ft of seawater.

1. Ambient pressure: $\frac{30}{10} + 1 = 4\ \text{ATA}$$\frac{99}{33} + 1 = 4\ \text{ATA}$
2. Oxygen partial pressure: $\text{PPO}_2 = 0.32 \times 4 = 1.28\ \text{ATA}$.
3. Gas use at depth: $18\ \text{L/min} \times 4 = 72\ \text{L/min}$$0.64\ \text{cu ft/min} \times 4 \approx 2.56\ \text{cu ft/min}$ drawn from the cylinder.

Both results use the same four-atmosphere multiplier. The metric and imperial inputs are each round numbers in their own system, so they are close but not exact conversions of each other.

Why pressure appears everywhere

• Gas planning: depth multiplies gas consumption 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.