Page created by Heather C. Hopkins
Last updated 11/25/2009
First, what is pressure in physics terms? Pressure is a force acting on a unit of area, or, Pressure = Force/Area. Atmospheric pressure is caused by the weight of the atmosphere, which is different at various altitudes. Basically, divers know that pressure increases as depth (underwater) increases. Similarly, pressure decreases at higher altitudes.
Hydrostratic pressure is the force acting due to fluids at rest (or static). Immersed divers experience increased pressure due to the force of water acting upon them. This is also known as compression.
Decompression, the opposite of compression, is when a diver experiences decreasing pressure as he or she ascends towards the surface of the water. In this case, air expands and fills a larger volume. Compression or decompression can cause damage in a person's body, due to unequal air pressures. This damage is known as barotrauma (see dangers section for more information).
The process that occurs during compression and decompression is portrayed by Boyle's Law. This law is expressed as PiVi=PfVf, and describes pressure as being inversely proportional to volume when temperature is constant. Boyle's Law explains why divers will feel a "squeeze" or discomfort in their ears as they descend. The squeeze is caused by an unbalanced, greater amount of pressure outside an air space (the ears, for example). It is necessary to reduce the discomfort in the ears by adding air to the space with various equalization techniques. Boyle's Law also explains why a diver should never hold their breath underwater, because air expands during ascent, and if the air has no way of being released, the pressure may burst air sacs in the lungs (see dangers section for more information).
Illustration of Boyle's Law, courtesy of