               Physical Laws There are many laws governing the physics and chemistry of scuba diving. A few of the basic principles are described below. Pressure: Pressure is defined as the amount of force per unit area. At sea level we are exposed to 14.7 pounds of pressure per square inch, or one atmosphere. Seawater is a lot denser then air and so a diver is subject to increased pressures as he/she dives to great depths. A column of ten metres of seawater exerts a pressure of 1 atmosphere. This means that a diver at 20 metres is exposed to 3 atm of pressure (2 atm for the 20 metres depth, and 1 atm from the sea level atmospheric pressure). Due to this pressure a diver needs to breath pressurized air, as the lungs are not capable of expanding enough to intake air that is not under pressure. Boyle's Law: "If the temperature is kept constant, the volume of a given mass of a gas is inversely proportional to the absolute pressure" PV=nRT For scuba diving this means that as a diver descends the air in the divers cavities decreases in volume as the pressure increases. At a depth of 10 m the same volume of gas will occupy only half the volume it would at sea level. Partial Pressure: "The total pressure exerted by a mixture of gases is the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume" Henry's Law: "The amount of gas that will dissolve in a liquid at a given temperature is almost directly proportional to the partial pressure of that gas"