In which depth area does the greatest expansion or compression per foot occur?

The greatest expansion or compression per foot occurs in the first 33 feet of depth due to the significant changes in pressure that divers experience. This range encompasses the transition from the surface pressure at 0 feet to the increased pressure of 33 feet, where the weight of the water above begins to make a noticeable difference.

At sea level, divers experience a pressure of 1 atmosphere (atm). As they descend, the pressure increases by approximately 0.4 atm for each additional foot of seawater, reaching about 2 atm at 33 feet. This results in a total pressure change of nearly 1 atm within this first 33-foot interval, which is significant when considering how gases behave under different pressures. This rapid change greatly affects the expansion of gases in a diver's body and equipment, such as in their lungs and buoyancy control devices.

In contrast, other depth intervals like 66 to 33 feet, 99 to 66 feet, or 132 to 99 feet have less dramatic increases in pressure per foot traveled, making them less critical for understanding expansion and compression dynamics. At those depths, divers are already experiencing higher pressures, meaning changes to gas volume become less pronounced compared to the initial descent from the surface to the first