Hydrostatic pressure is the force exerted by the weight of the water column pushing down on a submerged object. Because water is an incompressible fluid with a constant density, this downward force increases uniformly and predictably with depth. The pressure experienced at any point is directly proportional to the height and density of the liquid above it. This constant relationship means the environment 100 feet below the surface is vastly different from the surface.
Understanding Pressure Increase with Depth
The rate at which pressure increases underwater is determined by the fluid’s density. Saltwater is denser than freshwater, resulting in a slightly greater force over the same distance. For the ocean, the standard rate of pressure increase is approximately 0.445 pounds per square inch (PSI) per foot of depth. Freshwater produces a slightly lower increase of about 0.432 PSI per foot.
This steady rate is often described using atmospheres (ATM), a unit equal to the standard air pressure at sea level (14.7 PSI). In saltwater, 33 feet of depth creates one full atmosphere of pressure. This means that for every 33 feet a diver descends, the pressure increases by one additional ATM.
Determining Absolute Pressure at 100 Feet
To determine the absolute pressure at 100 feet, two distinct values must be combined: the gauge pressure from the water column and the atmospheric pressure above the surface. Gauge pressure is the pressure exerted by the water itself, calculated by multiplying depth by the rate of increase. Using the saltwater rate (0.445 PSI per foot), the gauge pressure at 100 feet is 44.5 PSI (100 x 0.445).
The weight of the atmosphere (14.7 PSI at sea level) is transmitted directly to the submerged depth and must be added to the gauge pressure. The total force, or absolute pressure, is 44.5 PSI plus 14.7 PSI, equaling 59.2 PSI. Expressed in atmospheres, the total absolute pressure experienced is approximately 4.03 ATM, representing just over four times the pressure felt at the surface.
Real-World Impact of Pressure on Objects
The substantial increase in absolute pressure at 100 feet has direct physical consequences on both gas and solid structures. The most notable effect on gas is described by Boyle’s Law, which states that pressure and volume are inversely proportional. As pressure increases from 1 ATM at the surface to over 4 ATM at 100 feet, the volume of any trapped gas must decrease by a factor of four. This volume reduction causes air spaces within objects, such as sealed containers or diving equipment, to compress significantly.
This principle means that unsupported structures, such as hollow or thin-walled submersibles, must be engineered to withstand a crushing force of nearly 60 pounds on every square inch of their surface. The structural integrity of deep-sea equipment must account for this massive external load.