What is the formula for converting absolute pressure (D) in fsw?

To understand the conversion of absolute pressure in feet of seawater (fsw), it's important to recognize what absolute pressure is composed of. Absolute pressure at a depth in water is the sum of the atmospheric pressure and the hydrostatic pressure exerted by the water above that point.

In the context of diving, the hydrostatic pressure increases with depth. The general rule is that for every foot of seawater, there is approximately 0.433 psi of pressure. Thus, at a depth of D fsw, the hydrostatic pressure can be calculated using the relationship that each foot contributes about 0.433 psi.

To convert this depth in fsw to absolute pressure, typically the atmospheric pressure (which is about 14.7 psi at sea level) is added to the hydrostatic pressure. The atmospheric pressure creates a baseline structural force on all objects at sea level, which must be taken into account when determining the total (absolute) pressure felt underwater.

Thus, the absolute pressure can be represented as:

Absolute Pressure = Atmospheric Pressure + Hydrostatic Pressure

= 14.7 psi + (D fsw × 0.433 psi/fsw)

Since 33 feet of seawater corresponds closely to 14.7 psi (because 1