What type of pressure is defined as gauge (water) plus atmospheric (air) pressure?

The correct answer is absolute pressure, which is defined as the total pressure exerted on a system, including both the atmospheric pressure and any gauge pressure measured. In the context of diving, this means that absolute pressure accounts for the pressure of the surrounding water plus the atmospheric pressure acting on the surface.

When a diver descends underwater, the gauge pressure increases due to the weight of the water above them. This is measured by a gauge, reflecting only the pressure exerted by the water column. However, to understand the actual pressure that the diver experiences at a given depth, it is essential to add the atmospheric pressure at the surface, which is typically considered to be about 14.7 psi at sea level.

For instance, if a diver is 33 feet (approximately 10 meters) underwater, the gauge pressure would be around 14.7 psi, which corresponds to the pressure from the water column alone. Adding this to the atmospheric pressure brings the total to approximately 29.4 psi; hence, absolute pressure is the sum of these two components. This measurement is vital for various calculations related to diving, including understanding buoyancy, decompression, and the physiological effects of pressure changes on the body.