Absolute Pressure — What It Is and Why It Matters
Absolute pressure is the total pressure at a given point, including atmospheric pressure. It is the sum of atmospheric pressure (1 ATA at sea level) plus the pressure from the water column above the diver. Absolute pressure is the value used in all decompression and gas physics calculations because dissolved gas behavior depends on total pressure, not just the water-column component.
Absolute pressure is expressed in ATA (atmospheres absolute), bar (1 ATA ≈ 1.013 bar), or PSI. At sea level and the surface, absolute pressure is 1 ATA. At 10 meters in seawater, it is 2 ATA. At 20 meters, 3 ATA. This linear relationship (for practical diving depths) makes calculations straightforward.
Frequently Asked Questions
Why do dive tables use absolute pressure instead of gauge pressure?
Because gas solubility and partial pressures depend on the total pressure acting on the gas, which includes atmospheric pressure. At the surface, you already have nitrogen dissolved at 1 ATA. Gauge pressure would ignore this baseline, leading to inaccurate decompression calculations.
What is 1 ATA in other units?
1 ATA equals approximately 1.013 bar, 14.7 PSI, or 101.325 kPa. In practical diving calculations, 1 ATA and 1 bar are often used interchangeably since the difference is negligible for decompression purposes.
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