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.

Boyle’s Law

How this Principle Relates to Diving (cont.)

As previously noted, Boyle’s Law has no effect upon the contents of a sealed rigid container. Accordingly, changes in ambient pressure will have no impact upon the air inside a diver’s scuba cylinder, so long as it remains within that cylinder. However, at the very moment that air is inhaled from the cylinder by the diver, it enters the flexible air space of the respiratory system and immediately becomes subject to the pressure-related changes associated with Boyle’s Law. The issues related to the inhaled air are three-fold.

1The 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.

During training it should be stressed that divers should breathe continuously and never hold their breath.

2The 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.

3The 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.

Boyle’s Law

How this Principle Relates to Diving (cont.)

While making buoyancy adjustments with the BCD, it is should be noted that pressure-related changes also impact the BCD itself. The BCD has an inflatable bladder that typically contains some amount of air. The bladder is a flexible container. Upon descent the increasing ambient pressure causes the air within the BCD’s bladder to compress, less water is displaced, and the diver becomes less buoyant. Upon ascent the decreasing ambient pressure causes the air within the bladder to expand, more water is displaced, and the diver becomes more buoyant. Again, in order to maintain proper buoyancy control the diver needs to add air to the BCD during descent and vent air from the BCD during ascent.

Divers Descend

A similar situation occurs with a dry suit because, by design, it traps a layer of air within the suit around the diver’s body. As is the case with the BCD, this entrapped air will compress upon descent due to increasing ambient pressure. In addition to the diver becoming less buoyant, compression will reduce the suit’s insulating properties and also can create a suit squeeze. Therefore the dry suit is equipped with an inflation valve that allows air to be added to the suit. Of course the entrapped air also expands upon ascent. The dry suit is equipped with an exhaust valve that allows air to be vented.

Boyle’s Law

How this Principle Relates to Diving

Many aspects of diving are affected by the pressure, volume, and density changes associated with Boyle’s Law. Before addressing these, however, it is important to recognize that Boyle’s Law usually has an impact only upon gas confined within a flexible container. The one other situation impacted by Boyle’s Law involves gas confined within a rigid container with an open bottom, such as a drinking glass (or any similarly shaped vessel) held in an inverted position (upside-down) while underwater. Boyle’s Law has no effect upon the contents of a sealed rigid container, such as a cylinder.

The tissues of the human body are primarily liquid, and liquid is non-compressible; thus much of the body is unaffected by pressure. However, within the body there are two significant air spaces: the lungs and the interconnected middle ears anBoyles Law Chartd 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.

The impact of Boyle’s Law may well be most obvious in the three air spaces identified above. It is important to recognize, however, that the effects are more far-reaching. For example, these pressure-related changes also may have a direct impact upon the diver’s buoyancy during a dive.

The most common type of exposure protection worn by sport divers is a neoprene wetsuit. Rather than being a solid mass, neoprene is a closed-cell foam-like material wherein each small cell encapsulates an equally small bubble of air. The closed-cell structure of neoprene contributes to its insulating properties. Neoprene is relatively light in weight compared to the water it displaces, and therefore neoprene is quite buoyant. However, the closed-cell structure also is compressible. As a diver starts to descend, increasing ambient pressure quickly causes the neoprene to compress. As the diver descends deeper, the material compresses further. When the material compresses, less water is displaced. In effect, as the diver descends deeper, he or she becomes more negatively buoyant. Later, when the diver begins to ascend, the neoprene material starts to expand and more water is displaced, and the diver becomes more buoyant at shallower depths. Naturally these changes will be most apparent when the diver is wearing a relatively thick wetsuit simply because there is a greater quantity of neoprene material to compress and expand. To maintain proper buoyancy control, the diver needs to add air to the BCD during descent, and vent air from the BCD during ascent.

Diving Physics: Part 4

Topics Covered in this Chapter:

  • Boyle’s Law
    • Pressure and Volume
    • How this Principle Relates to Diving
    • The Significance of Relative Changes
    • Pressure and Volume Calculations
  • Review Questions
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Chapter 6 Quiz

Professionalism

Attributes of a Professional

professionalBeing 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.

Naturally, in the case of a dive leader, the credentials are the SDI and TDI certification cards issued to the Divemaster and Assistant Instructor. The applicable body of knowledge and skills are detailed throughout this text. The additional issues, outlined above, will be further addressed in this section.

The Business of Recreational Diving

Positive Selling

It must be recognized that at all times and in all activities the dive leader is a representative of his or her dive facility. In effect, the dive leader is the face of that business, because it is through this dive leader that the customer is currently interacting with the business. As a result, the dive leader becomes the front-line salesperson for the business. Importantly, this role as salesperson relates not only to immediate retail purchases by that customer back at the store, but also to the customer’s long range decisions to enroll in the educational programs and to participate in the organized diving activities offered by this dive facility.

In any interaction with a customer, the dive leader should seek to establish and reinforce a positive mindset for the customer. Often this is accomplished through a few well chosen words, where the message is delivered in terms of “when” rather than “if”. For example, in discussing equipment, the message should be “when you buy your regulator” and not “if you buy a regulator”. Similarly, it should be “when you enroll in this course” and “when you dive with us” again instead of “if”. Though subtle, this positive reinforcement can have a significant influence on the customer.

When drawing comparisons, the dive leader always should address relative merits in a positive rather than negative context. This applies to any discussion regarding one agency’s training programs versus those of another, different brands or models of equipment, or one’s own dive store opposed to its competitors.


Nothing turns off a customer more quickly than hearing a purported professional rant about the alleged shortcomings of another person or product; in fact, some find such behavior to be rather offensive.


Instead the dive leader should highlight the benefits and advantages of whatever is being endorsed in an objective manner, and allow the customer to draw his or her own conclusions regarding the alternatives.

Most importantly, rather than any eloquent sales pitch, ultimately the sale most often is based upon the quality of customer service and the fulfillment of a customer’s needs.

The Business of Recreational Diving

Role of the DM and AI in the Business of Diving

Obviously SDI Divemasters and Assistant Instructors play an important role in the business of diving, especially in regards to two key elements: educational opportunities and diving opportunities.

Though it is primarily the responsibility of an Instructor to conduct diver training programs, often other dive leaders also are actively involved in these programs. The Divemaster or Assistant Instructor may take responsibility for many of the behind-the-scenes administrative details, such as making sure that all appropriate equipment is ready for a classroom or pool session. It will be the Divemaster or Assistant Instructor who supervises the rest of a class while the Instructor is dealing with an individual student. It may be the Divemaster or Assistant Instructor who works directly with a particular student, helping him or her overcome a problem and master a skill.


Sometimes a student will view the Divemaster or Assistant Instructor as more approachable than the Instructor, and might initiate a discussion concerning some issue or concern that otherwise wouldn’t be raised with the Instructor.


With the appropriate qualifications, Assistant Instructors may conduct approved specialty diver courses, further expanding the educational opportunities offered by a dive center.

When it comes to diving opportunities, often it is the Divemaster or Assistant Instructor who takes the lead. Divemasters and Assistant Instructors directly supervise the activities of certified divers. Divemasters and Assistant Instructors may escort certified divers on guided underwater tours. Divemasters and Assistant Instructors also might lead group trips, sponsored by the local dive store, to exotic dive destinations.

RetailIn 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.

While the above discussion centers upon the most common roles for the Divemaster and Assistant Instructor, they are by no means the only roles in the overall operation of a dive business. Dive leaders who possess additional qualifications undoubtedly will be an asset to the dive store, and also will be more employable in the industry. These qualifications might include experience in boat handling as a mate or licensed captain, compressor maintenance, gas blending, regulator repair, computer skills, website design and maintenance, travel arrangements, and of course retail sales.

The Business of Recreational Diving

Interrelationship of Key Elements

The three elements previously stated, educational opportunities, diving opportunities, and equipment, always go hand-in-hand. A comprehensive offering of training programs will help generate equipment sales, as well as stimulate participation in local diving and dive travel. Active divers are more likely to enroll in continuing education programs and more quickly will recognize the benefits of owning their own of equipment. Those who already have a full complement of equipment also are more likely to enroll in continuing education programs, and additionally are motivated to get out more often and dive. Effectively each of the three elements, individually, helps to promote the other two.

Pool ClassGeographic 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.