Boyle's Law

Inversely proportional means when one value decreases the other increases. Boyle’s Law tells us when pressure increases the gas volume decreases or when the pressure decreases the volume increases. Described another way, double the pressure and the volume would be half,  or decrease the pressure by half and the volume is doubled.

This gas law explains the inverse relationship of air and depth in SCUBA. Air has weight. The weight of the atmosphere exerts a pressure of 14.7 psi of force at sea level, referred to as 1 atmosphere of pressure or 1 atm. This is also the pressure exerted by 33 feet of seawater. Every additional 33 ft of water will add another 1 atm (14.7 psi) of pressure. At 66 ft we have 2 atm’s of water pressure (gauge pressure) plus the 1 atm of atmospheric pressure, for an absolute pressure of 3 atm. At 99 feet, the pressure is 4 atm’s (58.8 psi), and so on.

As a diver descends the increasing hydro-static pressure compresses the air, which decreases it’s volume. For example, picture a beach ball with a circumference of 16 inches at sea level. At 33 ft the ball would only be half its size at 8 in. At 66 ft, 3 atm’s of pressure would compress it to 1/3. And at 99 ft the ball would be 1/4 of its original size, or 4 inches around. This is why an understanding of Boyle’s Law is important in diving. This law tells us the relationship of depth and breathing gases, and why gas consumption increases dramatically with depth.