Boyle’s Law
The Significance of Relative Changes
At first glance, the reader might well observe that the changes associated with Boyle’s Law occur in a simple straight-line progression. Ambient pressure increases by 1 bar for every 10 meters of depth, or by 1 atm for every 33 feet of depth, in seawater. This information seems to suggest that changes in pressure, volume, and density always are uniform over various depths, regardless of the depths involved. Such is true, if one considers these changes only in terms of actual pressure.
More importantly, the reader should focus upon the relative changes in pressure, volume and density that occur over a range of depths. For example, ambient pressure doubles when the diver descends from the surface to 10 meters or 33 feet (which is an actual change of only one 1 bar / 1 atm), but ambient pressure does not double again until the diver descends from that prior depth of 10 meters or 33 feet to a new depth of 30 meters or 99 feet (which is now an actual change of 2 bar / 2 atm). The prior table is repeated here, for ready reference:
| Depth in Seawater | Ambient Pressure | Volume | Density | |
|---|---|---|---|---|
0 m | 0 ft | 1 bar/atm | x 1 | x 1 |
10 m | 33 ft | 2 bar/atm | x 1/2 | x 2 |
20 m | 66 ft | 3 bar/atm | x 1/3 | x 3 |
30 m | 99 ft | 4 bar/atm | x 1/4 | x 4 |
40 m | 132 ft | 5 bar/atm | x 1/5 | x 5 |
50 m | 165 ft | 6 bar/atm | x 1/6 | x 6 |
As another example, volume increases by only 25% during an ascent from a starting depth 40 meters or 132 feet to a new depth of 30 meters or 99 feet (which is an actual change of 1 bar / 1 atm), yet volume increases by 100% during an ascent from 10 meters or 33 feet to the surface (which also is an actual change of only 1 bar / 1 atm).
As demonstrated, the greatest relative changes occur at the shallower depths. In many cases it is these relative changes that will be most apparent to the diver, because the results can be dramatic. This situation emphasizes the importance of a diver equalizing early during a descent. It helps explain why a new diver may have a bit more difficulty in effectively controlling buoyancy at shallower depths, and why he or she might unexpectedly pop to the surface. It also demonstrates the need for vigilance in maintaining a slow rate of ascent throughout the entire ascent, especially as the diver nears the surface.
The first issue with air inhaled at depth was previously referenced above. Upon inhalation the pressure of the air inside a diver’s lungs will be equal to the ambient pressure. If that diver happens to ascend to a shallower depth while holding his or her breath, the expanding volume of air inside the lungs can result in a lung rupture.
The second issue with air inhaled at depth relates to the diver’s rate of air consumption. Consider the fact that it requires a certain number of molecules of air drawn from the scuba cylinder to fill a diver’s lungs at the surface (where the ambient pressure is 1 bar / 1 atm). When that diver descends to 10 meters or 33 feet (where the ambient pressure is 2 bar / 2 atm), the same number of molecules will now only half fill the lungs. At this depth, the diver will need to inhale twice as many molecules of air with each breath to fill the lungs. The diver’s air supply, provided by the scuba cylinder, will last only one-half as long at this depth as it would at the surface. Similarly, the diver’s air supply will last only one-quarter as long at 30 meters or 99 feet (where ambient pressure is 4 bar / 4 atm), and only one-sixth as long at 50 meters or 165 feet (where ambient pressure is 6 bar / 6 atm), as it would at the surface.
The third issue with air inhaled at depth relates to its density. At 10 meters or 33 feet, the inhaled air will be twice as dense as it is at the surface, and its density further increases at deeper depths. Though it is often unrecognized by the sport diver, denser air requires greater breathing effort, thereby increasing the diver’s overall workload during a deeper dive. The denser air itself generally creates no noticeable problem for a diver within traditional sport diving depths, provided that the diver continues breathing at a normal rate. However, in the event of overexertion the diver’s breathing rate will quicken and, when combined with the increased density of inhaled air, may result in turbulence as the air passes down the diver’s airway towards the lungs. Turbulence impedes the efficient flow of air. When turbulence occurs, the diver likely will sense some degree of difficulty in breathing. It is stressed during training that divers always should move slowly and steadily underwater, avoiding overexertion.
d nasal sinuses. In addition, a third air space is artificially created over the face and eyes by the diver’s mask. These three air spaces are impacted by the pressure-related changes associated with Boyle’s Law. During descent and ascent, each of these air spaces must be equalized to the ambient pressure in order to avoid discomfort and possible injury. In entry-level training, every student diver learns the importance of breathing continuously and never holding one’s breath while underwater to avoid the risk of a lung over-expansion injury. Inevitably a student’s first pool session clearly demonstrates the need to equalize the middle ears and sinuses. The student also quickly learns to recognize and deal with the sensation of a mask squeeze.
Being labeled a professional, in any field, carries certain connotations. It often involves some form of credentials. It clearly suggests that the individual possesses a certain body of knowledge and a particular set of skills. It implies adherence to a prescribed set of standards in performing one’s duties and ethical conduct in all dealings with clients and colleagues. Often, being a professional is most clearly demonstrated by the individual’s overall attitude and approach to the task at hand. Most importantly, the professional’s own behavior and performance should serve as a role model for others in the chosen field. Finally, the professional normally is compensated for his or her work.
In addition to educational opportunities and diving opportunities, it must be pointed out that every Divemaster and Assistant Instructor also plays an essential role in the third element of the dive business, equipment. This occurs even when the dive leader is not directly involved in the retail side of the business. Experience has shown that the single biggest influence in a customer’s equipment purchases is the dive leader. Though it may not be readily apparent, almost every student and certified diver will be taking a close look at the dive leader’s equipment. Often they will conclude that if the equipment is good enough for a knowledgeable professional, then that’s the same equipment they want to purchase. Accordingly, the Divemaster and Assistant Instructor always should be outfitted exclusively in the same brand of equipment that is sold by their own store; it all should be top of the line, and obviously it should be well maintained and in excellent condition. Furthermore, because a customer frequently will turn to the Divemaster or Assistant Instructor regarding equipment-related questions and advice, it is equally important for the dive leader to be fully familiar with all of the equipment carried by his or her store.
Geographic location and other related circumstances will dictate, to some extent, the business focus of an individual dive facility. Some facilities will concentrate more heavily on certain key elements than others. For example, there may well be a significant difference between the focus of a dive operator in a resort setting, compared to a local dive store in a more temperate climate. The local dive store usually will schedule a variety of training programs throughout the year, with a clear emphasis on group classes; conversely, the resort dive operator likely will offer its training programs primarily on an individual basis. The local dive store often will have a well-stocked retail space promoting the sale of equipment and repair; the resort dive operator usually will be more concerned with equipment rentals and the sale of smaller accessories. The local dive store may promote local diving during the warmer months, and offer group travel to exotic locations throughout the year; the resort operator obviously will focus most heavily on local diving right at the resort, with any travel promotions geared towards getting the customers there. The local dive shop will seek to develop a year-round relationship with its customers; the resort operator will interact with its customers perhaps once a year, but similarly will seek to promote long-term relationships that generate repeat visits. The immediate needs of an individual customer might differ with time and place, but eventually customer loyalty will be bestowed on the dive facility that best fulfills all of the diver’s needs.