Water is 700 to 800 times denser than air depending on its salinity. This is the primary factor that causes changes to a diver’s physiology, and makes it necessary to follow the required depth and time guidelines in order to avoid decompression illnesses.
Pressure is defined as the continual physical force applied against an object by something in contact with it. When discussing pressure under water, we must distinguish between the pressure applied to a fluid, and pressure applied to a solid. Pressure applied to a fluid is transmitted in all directions with the same intensity. In contrast, pressure applied to a solid is transmitted linearly, flowing a direct path from the point of application.
Divers are not crushed at depth by the surrounding pressure because the pressure is distributed uniformly around the body, exerting the same pressure at each point.
The human body is composed of liquids, solids and gases, with liquids and solids being the majority. Because of this, the pressure exerted on the liquid and solid portions of our bodies when diving is counterbalanced. Our lungs, and other air spaces are equalized to the surrounding pressure by breathing from the regulator, which delivers gas at a pressure equal to that which surrounds us, called ambient pressure.
All objects on the planet are subjected to Atmospheric Pressure. Atmospheric Pressure is created by the weight of the air that surrounds the Earth’s surface. It affects every inanimate and living object. Hydrostatic Pressure is the pressure exerted by the weight of water surrounding an object. It only affects objects that are in the water, and differs based on the water’s temperature and salinity. Ambient Pressure, also called Absolute Pressure, is the total amount of pressure affecting an object. Under water, Ambient Pressure is the sum of the atmospheric and hydrostatic pressure combined. On the surface, Ambient Pressure is the same as Atmospheric Pressure.
Atmospheric pressure at sea level is defined as the weight of a one square centimeter (or one square inch) column of air extending from sea level to the top of the atmosphere. The weight of this column is 1.033 kg/cm2, kilograms per square centimeter (or 14.7 psi, pounds per square inch), which is equal to 1 bar or 1 atm, and 760 millimeters of mercury (mmHg) / 29.92 inches of mercury (inHg). Also, because this is the weight of one atmosphere, the atmosphere (atm) unit of measure is often used in conjunction with diving and is synonymous with bar. Altitudes higher than sea level will have a lower atmospheric pressure because there is less air in the square centimeter/square inch column that rises to the top of the atmosphere. The lower atmospheric pressure at high altitudes affects divers and consequently, dive plans. Changes brought on by high altitude dives — dives conducted at an altitude of 300 meters / 1000 feet or higher, will be discussed later in the chapter.
Hydrostatic pressure is determined by calculating the weight of the water above an object. For example, a one square centimeter/one square inch column of seawater that extends 10 meters / 33 feet in length weighs the same amount as one atmosphere of air at sea level. This means that a diver adds one atmosphere of pressure for every 10 meters / 33 feet he descends. As we previously discussed, freshwater is less dense than seawater; therefore, it takes a greater volume of freshwater to match the weight of one atmosphere of seawater. A one square centimeter/one square inch column of freshwater 10.3 meters / 34 feet) in length is equivalent to one atmosphere of seawater. While it is not a great amount more, it is enough to notably affect the physiological changes in our bodies while diving.