To dive safely, it is important to be familiar with the physical laws that govern the sport. One of the most elementary is Boyle’s Law. In 1660 Robert Boyle conducted experiments with pressure and volume relationships. He determined that doubling the pressure on a gas decreased volume of gas by one-half. Thus was born Boyle’s Law:
At constant temperature, the volume of a gas varies inversely with absolute pressure, while density varies directly with absolute pressure.
Divers And Boyle's Law
This law has particular relevance to divers, as it has bearing on both mechanical effects and physiological effects of pressure.
- Mechanical effects of pressure and volume include variations in buoyancy (e.g. a diver’s use of a buoyancy compensator to descend or ascend).
- Physiological effects of volume and pressure include potential impact on air-filled spaces in the human body (e.g. areas of potential baratrauma injuries).
- In addition, Boyle’s Law interplays with other gas laws such as Dalton's Law—which, though not covered here—combine to dictate additional stresses on the a diver’s respiratory, circulatory, and nervous system.
Boyle's Law As A Mathematical Formula
Mathematically Boyle’s Law is expressed as
P1V1=P2V2, where:
P1 = Starting Pressure (expressed in absolute pressure, i.e. atmospheres)
V1 = Starting Volume (liters)
P2 = Ending Pressure (expressed in absolute pressure, i.e. atmospheres)
V2 = Ending Volume (liters)
What Boyle's Law Means On A Dive
In volumetric terms, this means that for a diver:
- At the surface, or 0 feet seawater (fsw), the air in a diver’s lungs--is under one atmosphere of pressure (1 ata), which equals 14.7 pounds per square inch (psi) pressure. Likewise for the air in their buoyancy compensator (BC).
- At a depth of 33 fsw, the weight of the water column is equal to another atmosphere, so now the pressure on the diver’s body is two atmospheres absolute (2 ata, or 29.4 psi). The diver’s lungs are now compressed to one-half their surface volume. In addition, their BC has lost volume and therefore buoyancy ; they may need to add air to to control their descent.
- At a depth of 66 fsw, the pressure is 3 atmospheres absolute (3 ata, or 44.1 psi), and the diver’s lungs are one-third their surface volume. Again, it may be necessary to to add air to the BC.
- At a depth of 99 fsw, the pressure is 4 atmospheres absolute (4 ata, or 58.8 psi), and the lung volume of the diver is one-quarter their surface volume. At this depth, the amount of air that must be added to a BC to maintain control begins to be significant.
- At a depth of 132 fsw, or two feet past the depth the Navy Dive Tables allow any time for non-decompression diving, the pressure is 5 atmospheres absolute (5 ata, or 73.5 psi), and the diver’s lung volume is one-fifth their surface volume. At these depths it is very easy to become negatively buoyant; their BC may need a major hit of air!
When a diver breathes compressed air underwater, it is delivered by the regulator to their lungs at the ambient pressure of surrounding water. This prevents the lungs from collapsing under the pressure. Divers need to be aware, however, that:
- Additional gas laws governing the partial pressures in a breathing mixture (not discussed here) come into play the moment a diver starts descending. Their body is now subject to additional stresses associated with breathing compressed air under pressure.
- The deeper they go, the more rapidly they will burn air, both for breathing and for buoyancy maintenance. It is critical they monitor their gauges and turn around with enough air to reach the surface.
- If they hold their breath during an ascent (even a few feet or less!), the compressed air in their lungs will expand as the ambient water pressure decreases, potentially causing a lung overpressure injury. Lung over pressure accidents can be fatal, and are possible in only a few feet of water when using compressed air. Burst lung alveoli make a mess; some veteran divers refer to this type of injury as "making pink soup."
- It is vital that a diver who seeks to descend or ascend carefully monitor their speed. If they do not control air to/from from their BC in accordance with ambient water pressure, their BC can easily take them for a ride, either into the depths or to the surface. Uncontrolled descents and ascents bring the potential for a whole new suite of injuries!
Keep Boyle’s Law In Mind
It can be seen that diver should always pay attention to their depth and air gauges, never hold their breath underwater when using compressed air, and always dive according to regulation dive tables. Diving is an incredible skill and safely opens up the world beneath the waves—but only if divers familiarize themselves with pertinent gas laws, starting with Boyle’s Law—so they understand the forces they are playing with.
References
Joiner, James T (editor). 2001. Physics of Diving. In NOAA Diving Manual: Diving for Science and Technology, 4th Edition. National Oceanic and Atmospheric Administration. Best Publishing Company, Flagstaff Arizona, pp. 2.1-2.18.
