What's the Difference Between fsw, msw, ATA, Bar, and PSI?
Diving vocabulary mixes units freely: an American tank gauge reads in PSI, a rental reg in Cozumel reads in bar, a physiology textbook talks in atmospheres absolute, and dive tables are labeled in feet or meters of seawater. They are all describing the same physical quantity — pressure — from different angles. Here is how they fit together. (Our unit converter translates between all of them.)
Pressure is the underlying quantity
At the surface, the weight of the atmosphere presses on you at about one atmosphere — roughly 14.7 PSI, or just over 1 bar. Descend in the ocean and the water column adds pressure on top of that, increasing steadily with depth: roughly one additional atmosphere for every 33 feet (about 10 meters) of seawater. That relationship between depth and pressure drives essentially everything interesting in diving physiology — how gases compress, and how much inert gas dissolves into the body — as described in the National Library of Medicine's article on decompression sickness.
Bar and PSI: gauge units
Bar and PSI (pounds per square inch) are the units on pressure gauges. One bar is very close to one atmosphere, which makes metric mental math pleasant: each 10 msw of depth adds about 1 bar. PSI is the US customary equivalent; 1 bar is about 14.5 PSI. Tank pressures are quoted in these units — a common aluminum cylinder rated at 3,000 PSI is a roughly 207-bar tank. When a divemaster asks for your pressure and you answer in the wrong unit, this is the conversion you were missing.
ATA: atmospheres absolute
ATA counts total pressure in multiples of surface atmospheric pressure, including the atmosphere itself — that is what "absolute" means. The surface is 1 ATA; 33 fsw is 2 ATA; 66 fsw is 3 ATA. Textbooks and tables favor ATA because gas laws are simplest in absolute terms. The distinction between absolute and gauge pressure trips up many students: a gauge reading zero at the surface is reporting pressure above atmospheric, while ATA counts everything.
fsw and msw: depth as a pressure unit
Feet of seawater (fsw) and meters of seawater (msw) look like depth units but are formally pressure units, defined by convention: 1 msw is defined as 0.1 bar, and 1 fsw is about 0.0306 bar. In seawater the pressure conventions and your depth gauge agree closely, which is why nobody notices the distinction on an ordinary ocean dive. The distinction matters at the edges — fresh water is slightly less dense than seawater, and diving at altitude starts from less than 1 ATA of atmosphere, so a depth gauge and a true pressure measurement drift apart. Standards documents like the NOAA Diving Standards & Safety Manual express table depths in these seawater-pressure units precisely so the tables track pressure, which is what the body responds to.
Why the fluency matters
None of this arithmetic is hard, but doing it while gearing up on a rocking boat is a bad idea. Learn the handful of anchor points — 1 atm ≈ 1 bar ≈ 14.7 PSI, 33 fsw ≈ 10 msw ≈ 1 additional atmosphere — and let the converter handle the rest. And keep the units straight when reading anything technical: a table row labeled 60 fsw and a physiology passage about 2.8 ATA are talking about the same world in different dialects.