ATM / ATA — What It Is and Why It Matters
ATA (atmospheres absolute) is a pressure unit used extensively in decompression theory and dive physics. One ATA equals the atmospheric pressure at sea level — approximately 1.013 bar or 14.7 PSI. In seawater, pressure increases by approximately 1 ATA for every 10 meters (33 feet) of depth, making calculations intuitive: 10m = 2 ATA, 20m = 3 ATA, 30m = 4 ATA.
ATA is the standard unit in dive table calculations, partial pressure formulas, and decompression algorithm documentation. The distinction between ATM (atmospheres, relative to surface) and ATA (atmospheres absolute, total pressure) matters: at 10 meters, the gauge pressure is 1 ATM but the absolute pressure is 2 ATA.
Frequently Asked Questions
Why do divers use ATA instead of just meters?
Because gas behavior depends on pressure, not depth alone. ATA directly tells you the pressure multiplier: at 3 ATA, gas volumes are one-third surface size and partial pressures are tripled. This makes physiology and gas calculations immediate without an extra conversion step.
How does altitude affect ATA calculations?
At altitude, surface atmospheric pressure is less than 1 ATA. Diving in a mountain lake at 2000 meters elevation starts at about 0.8 ATA, which means decompression limits are shorter because the relative pressure change per meter of depth is greater. Altitude diving requires adjusted tables or computer settings.
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