Which depth in feet of seawater is equivalent to 1 atmosphere of pressure?

• A. 66 fsw
• B. 33 fsw
• C. 10 fsw
• D. 44 fsw

Answer: (B)

Pressure increases with depth because the water column above adds to the atmospheric pressure at the surface. In seawater, about 1 atmosphere of additional pressure is produced for roughly every 33 feet of depth. So the depth at which the water column itself exerts 1 atmosphere of pressure is about 33 feet of seawater. At that depth, the total absolute pressure is about 2 atmospheres (1 atm from air at the surface plus ~1 atm from the water). The other depths correspond to greater or lesser water pressure: 66 fsw is about 2 atmospheres of water pressure, 10 fsw is well under 1 atm of water pressure, and 44 fsw is around 1.3 atmospheres of water pressure. Therefore, 33 fsw is the depth equivalent to 1 atmosphere of pressure from the water column.