Imagine this scenario: a snorkeler or free diver approaches you and asks for a hit of your air — what do you do? Giving strangers compressed air may not actually be doing them a favor if you don’t know enough dive science to be safe. Last week, we discussed the aspects of Charles’ Law and how it applies to SCUBA diving. This week we’ll look at the similar Boyle’s Law and how its principles relate to our favorite activity.
Boyle’s Law states that at constant temperature, pressure and volume are inversely proportional; as one increases, the other decreases. A balloon pushed underwater, for example, shrinks as its volume is compressed. Conversely, a balloon inflated underwater and released expands and may burst as it rises and its volume expands. This principle is the foundation of SCUBA safety training to prevent barotrauma, or injury to the body from changing pressure. Here are some SCUBA diving rules related to Boyle’s Law.
Clear ears and descend slowly
Divers gently force air into their Eustachian tubes so that their eardrums do not burst inward from the vacuum created when the volume of air in the inner ear shrinks. They descend slowly, stopping as needed to adjust. Ascending too rapidly can cause the eardrum to burst outward if the air cannot escape back through the Eustachian tubes as it expands. Diving with a cold or allergy flare that swells the tubes and prevents clearing is ill-advised.
Controlled by the regulator, compressed air fills the lungs according to ambient pressure, allowing more oxygen in a breath as the lungs compress. Its volume expands or contracts with even a few feet of depth change, so Boyle’s Law means it’s important to breathe in and out without trying to save tank air by breath-holding. It’s vital to release compressed air while ascending to prevent lung damage.
Nitrogen enters the bloodstream during descent compression and is usually released unnoticed over time when normal dive precautions are followed. According to Boyle’s Law, a sudden ascension can create gas bubbles large enough to cause several types of injuries, depending on where the bubbles develop. Safe divers return to the surface at a slow pace to minimize the risk of decompression sickness, and they observe the no-fly rule to give their bodies time to release any accumulated nitrogen before undergoing rapid altitude changes.
Images via lobe.ca, grc.nasa.gov