Ambient Pressure — What It Is and Why It Matters
Ambient pressure is the total pressure exerted on a diver by the surrounding environment — the combined weight of the atmosphere above and the water column overhead. At sea level, ambient pressure is 1 ATA (atmosphere absolute). Every 10 meters of seawater adds approximately 1 ATA, so at 30 meters the ambient pressure is approximately 4 ATA.
Ambient pressure is the master variable in diving physics. It determines gas volumes (Boyle's law), partial pressures (Dalton's law), gas solubility (Henry's law), buoyancy changes, and the decompression requirements calculated by your dive computer. Understanding pressure is understanding diving.
Frequently Asked Questions
What is the difference between ambient pressure and gauge pressure?
Ambient (or absolute) pressure includes atmospheric pressure plus water pressure. Gauge pressure reads zero at the surface and only measures the additional pressure from the water column. Your SPG reads gauge pressure (it reads zero when the tank is empty and open to air). Your dive computer uses absolute pressure for decompression calculations.
Does freshwater have the same pressure as saltwater at the same depth?
Not quite. Saltwater is denser than freshwater, so it exerts slightly more pressure per meter of depth. The difference is small — about 3% — but dive computers and tables account for it. Diving in a freshwater lake at 10 meters results in slightly less pressure than 10 meters in the ocean.
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