Friday Night Scuba Review


Larry "Harris" Taylor, Ph.D.


This article is based on an open water review session used by the author in teaching basic open water classes at the University of Michigan. It is intended to bridge the elapsed time gap between close of lecture sessions and the open water weekend. It is typically done on the Friday night immediately before the first weekend of open-water diving. Slides from the Power Point presentation are used to preserve the "flavor" of the session. (The graphics for this article are available separately as a  PDF File. )  I have chosen NOT to include the slides dealing specifically with our training site  (so students know what it looks like before they arrive)  and discussions of the snorkel run, surfaces exercises & 5 open water dives they will do. Basically, I have just added text to an existing Power Point presentation. I hope readers will understand that each class lecture is a bit different depending  upon the  individual students and their needs. This review is also an indicator of the level of knowledge I consider appropriate for a basic diver training program.


This material is copyrighted and all rights retained by the author. This article is made available as a service to the diving community by the author and may be distributed for any non-commercial or Not-For-Profit use.

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Potential Problems   Buoyancy   The Dive   Problem Management   Philosophy


Dive Tables   Table Practice Problems   Final Thoughts

Since there is often a substantial period of time (a semester in academic circles) between the end of class and the open-water weekend, I feel that it is important for students to spend a few minutes refreshing dive principles before donning their equipment and diving. This session (typically 2-4 hours, depending on student generated dialogue) allows students to meet and become re-acquainted, an opportunity to insure that everyone is on the same page, and  time to complete administrative matters. I believe that this session sets the tone for the weekend of diving and, in general, provides a much better open water experience.


Many moons ago, while teaching my first 100+ hour Master Diving Class, one of my students  (a 54 year-old nuclear physicist)  pointed out that "It is difficult for the brain to absorb, what the ass cannot endure!"  So, I went out and bought padded chairs. The point is that the lecture tonight is a democracy. YOU control the flow of the class. YOU do this with your questions and your comments. Since this is the last time we meet before diving, please use this time to answer any questions you might have 'cause tomorrow, underwater, with a regulator in your mouth may be too late to ask a question. Anytime you want a break, for whatever reason, let me know. In other words, the flow of the lecture is a democracy under your control.

<<I will use a slide of question marks to indicate where I typically take a short break for questions>>


But, the water work is a dictatorship. Hopefully, a benevolent one, but, make no mistake about it, you dive this weekend under the supervision of my TA's and me and the conditions we establish, or you do NOT dive at all! Any questions before we begin?

Some people tend to refer to the first weekend of diving as a "check-out". I tend to view it more as a transition state between your experience in the pool and functioning as a diver in open water. So, we will start with a bit on how open-water diving is practiced.

Obviously, it is critical that you feel good: physically, mentally and emotionally. It should be a given that ANY condition that might compromise your ability to function in the water should be evaluated by a hyperbaric-knowledgeable physician. When in doubt, do not dive. The decision not to dive as a recreational diver is ALWAYS correct!

Since our perceptions are altered by immersion in water, it is important that we do not scuba dive with these senses compromised by pharmacological agents.

I would particularly like to comment on alcohol. Besides being a diuretic (increases DCS risk), it turns out that absorbed alcohol changes the viscosity of the fluid in the three semi-circular canals of the inner ear. These canals are an integral part of the way we determine our orientation. Head movements affect the fluid in these canals and as the fluid moves, it strikes tiny hair cells that send electrical impulses to the brain. The brain then interprets these signals to evaluate three- dimensional position. We do NOT have a biological mechanism for coping with alterations in fluid viscosity. So, when the inner ear fluid is changed by the presence of alcohol, the signals sent to the brain to determine our orientation are a bit messy and the brain is unable to accurately use the input. The result is the inability to walk upright without feeling ill, as well as nausea and sometimes incapacitating vertigo. Since the inner ear is not well perfused,  it may take several hours for the inner ear to eliminate all of the alcohol and establish a proper communication between the inner ear and brain. Since the inner ear, while diving, tells us which was is up, it is important that we enter the water with this ability fully functional, So, its a good idea to refrain from drinking alcoholic beverages, before (12-24 hours), between and after dives. (Post-dive alcohol, 12-24 hours,  is considered as a risk factor for DCS.)

I'll also briefly mention caffeine. This is a common component in many commonly consumed beverages. Most divers know it is a diuretic. But, it has an additional property that makes it unwise to consume prior to diving. Whenever we dive, as we enter the cooler water under increased ambient pressure, our heart's steady rhythm is altered. This is similar to exercise-induced cardiac arrhythmia. Caffeine increases this arrhythmia. This increased arrthymia is most likely a factor in something called "sudden-death syndrome." In this malady, the diver simply loses consciousness and drowns without apparent explanation. Typically, the victim is over 30 years old, has a history of smoking, is in poor physical condition and often a heavy pre-dive caffeine consumer. The theory is that a number of factors gradually increase the exercise-induced arrhythmia to a point beyond the ability of the heart to recover.  So, avoid caffeine, as much as possible during the pre-dive hours.

Diving can place exceptional demands on the cardio-vascular system, especially when carrying all that gear to the staging area. So, diving is one of those activities where an active life style is a valuable asset.  We do aerobics (like swimming, cycling, rowing, running, walking, or aerobics classes ... anything that requires a sustained increase in cardiovascular effort) to maintain stamina. We do anaerobic exercise (weights) to build strength. Its easy for people to ignore the gradual decrease in physical condition that occurs as a life style becomes more sedentary. One way to check fitness level is called the "jiggle" (or naked mirror) test. It is done as follows: Stand naked in front of a mirror and jump up and down. Various body parts (gender dependent) will naturally jiggle with this running motion. After a few minutes of in-place jogging, stop and look at the image in the mirror. The longer the body parts that should not jiggle continue to jiggle, the more the need for an immediate increase in physical exercise. Remember the "gospel" that the combination of knowledge (from class and experience) coupled with good physical fitness is a pre-requisite to a lifetime of adventure in this sport.


Buddy Diving

Tomorrow, you will take your "primitive brain" (Primitive Brain) cloaked in a wet suit into the waters of Portage Quarry. Your "primitive brain" will remind you that swimming underwater with a metal thingy in your mouth while covered in funny smelling rubber is a new experience. It will take a little time, as in the pool classes, to acclimate your "primitive brain" to this new environment. So, a little apprehension will be normal. The lack of a healthy apprehension to  a new environment is a good way, in this sport, to get into trouble. However, by the end of the weekend, we will convince your "primitive brain" that diving is an awesome adventure that brings sensual pleasure that can not be equaled by any other recreational sport.

As in the pool classes, we will dive the "buddy system." My TA's will lead, you will dive as a buddy team and I will follow slightly behind and above the group. This is so I can see everything that is occurring.  Several years ago, Lee Somers studied Michigan diving fatalities and noted the overwhelming majority of fatalities in Michigan waters occurred to divers who were alone at the time of their death. So, we consider diving as a buddy team to be an important part of safe diving practice. 

Buddy diving ASSUMES that each member is a responsible, capable-for-the environment diver. Dependency (or "trust me") diving is a recipe for disaster 'cause the dependent diver cannot help the  buddy. Coroner's inquests, particularly  as reported by the Ontario Underwater Council also suggest that diving in threesomes in not a good practice (who is watching and who is alone?). Several diving deaths in Canadian waters have diving as a threesome listed as the proximate cause of the death (s).

As part of the dive plan, it is always good to establish a protocol for action if there is a buddy team separation of divers. At the quarry (confined water, no-boat traffic), the procedure is to look around a bit, then surface. Once on the surface, stay there 'cause otherwise there might be a "yo-yo" situation. If separation occurs (and so far, in training there for more than 2 decades, it has not), then we will re-establish the group on the surface before resuming the dive.

A very common buddy separation point is on the initial descent. To avoid this problem, we will descend, facing our buddy, letting the air out of the b.c. and exhaling. By descending (and ascending as I prefer that each dive has the same number of ascents as descents) facing your buddy, you minimize risk of losing your diving partner. I should also point out that a head first dive places the semi-circular canals of the ear (our balance sensing organ) in the worst possible position for determining our orientation in three-space. So, not only does a head first dive make it difficult to track our buddy, but it makes our own orientation more difficult. This inability to perceive orientation can be a problem in low visibility water common to the Great Lakes region, especially if neither the surface nor the bottom is visible.


Before each dive, especially with a stranger, it is important, to establish some guidelines, so every one is diving with the same mind-set. So, the "ritual" we will employ is SEABAG. 

S = Signals ... up, down, wait, I'm cold, share air, cylinder content,  etc Since divers from different areas of the world and with different cultural backgrounds MAY have different signals for common actions, it is important that everyone diving in the team uses the same set of signals.

Two "war stories" (I typically use these incidents in pool sessions and make reference to them at the review session. My TA's have heard the incidents so many times, they have named them "war stories" and have assigned numbers to them.) that emphasize the importance of establishing communication PRIOR to the dive.

War Story #1

On my first Caribbean night dive, I was diving with someone from Canada that I met on the trip. It was our first dive together. About ten minutes into the dive, my buddy, quite agitated, came over to me holding up three fingers in one hand and five fingers in the other. Since I was trained (and train others) to use fingers (one hand only ... each finger represents one-hundred psig) to indicate remaining cylinder pressure, I assumed my buddy was telling me that their cylinder contained only 800 psig. This upset me 'cause we had only been in the water a short time and I was surprised at such an apparent rapid consumption of air.

Later, I learned that the eight fingers was this group's signal for an octopus and my lack of understanding had created a missed opportunity.

The point is: the lack of understanding signals can lead to missed opportunities.

E = Emergency air sharing ... what I will do for you if you come to me in an out-of-air-situation. Every one in the dive team demonstrates location and deployment of secondary air system.

War Story # 2 The on-site chamber attendant told this story to me.

A young couple met on a dive boat while on a dive charter near Catalina Island, California. Because hormones were quite active, and the chemistry between the two was in sync, the couple was thinking about post-dive activities and not about their diving. The girl was a novice and was depending on the "macho" male for her safety. During the dive, the girl ran out of air. So, in a near-panic, air-starved  state, she went to the man she had dreamed about and gave him a very vigorous "out-of-air" signal. Unfortunately, this guy had only been trained to share air with an octopus (single regulator buddy breathing in many circles has been designated an "unnecessary skill") and his secondary regulator was in the shop. So, when the object of surface fantasy approached him, he had no octopus and thus, no available option to assist her with her crisis!

So, to defend himself from this approaching air-starved-dive-parasite,  he kicked the girl in the chest to drive her away from him. She then bolted for the surface and embolized on the ascent. The good news was that Catalina is one of the best places in the world for emergency management of dive accidents. Fortunately, the girl was rapidly transported to the chamber on Catalina island. After regaining consciousness in the chamber, she told her story to the chamber attendant and he told the story to me.

Now, it turns out that this particular chamber attendant is a real 'hard-ass" about dive safety and training procedures. This was, historically, at a time when single regulator air-sharing was first being removed from most recreational training. He wrote the training agency involved and asked them to comment on the male diver's behavior. The response he received was. "Our diver acted appropriately 'cause buddy breathing causes accidents!"

The point of this story is that at 100 feet down, out-of-air is NOT the appropriate time to find out about potential air sharing difficulties. So, it is just good practice to establish signals and emergency procedures before a dive. 

A = Activity of the dive ... fairly obvious, but this is time to make sure every one understands what is going to occur. It is also the time to set termination parameters (time, depth, air supply, etc) for the dive.

B = Buoyancy Control  ...everyone demonstrates weight system and buoyancy control device so buddy knows how to operate the b.c. and weight system should such a need arise.

A - Air On ... the ritual is 

Look at gauge

Breathe 4 times

Look at gauge to insure it has not moved to zero


After 4 breaths, gauge pressure should remain at near full. If psig reading drops to zero, then the cylinder valve has been closed, after the cylinder pressure had been checked. This can occur on a dive boat if someone senses a small leak in your system after you have set it up and are socializing in the galley.





War Story # 3

The "air check ritual" became a part of my diving as a result of the following incident.

Much of my diving is drift diving in the extremely swift water of the St. Clair River at Port Huron. There are 1-2 scuba  deaths every year in this river. The NOAA measured current flow here is in excess of 90 million gallons a minute. It is not a place for novice divers. Since we may drift a mile or more per dive, it is common practice to suit up at the exit point, load all the gear into one of the on-site vehicles, drive upstream to the entry point and go diving. We commonly check cylinder pressure at the assembly point and then turn the air valve off for transport. To prevent overheating on the surface during hot, humid days, we try to minimize our time at the entry point. Typically, we drive to the site, quickly put on gear and jump into the water.

The entry point at the mouth of the St. Clair river involves climbing over a 4 foot rail fence that lines the walkway along the river bank. There is only a very narrow edge on the river side of the barrier. We climb over the rail with one fin on, (height of rail and length of fins makes climbing the fence with both fins on most difficult), secure position on the narrow ledge, put on the second fin and then drop 8-10 feet into water moving in excess of 2 knots. Because we wish to minimize our surface travel distance on entry, we typically deflate our b.c.'s prior to entry, so that we hit the water a bit heavy and immediately start sinking. The buddy team has a rally point directly under the entry location and after entry, meet and start the drift.

On one very sticky day, we moved rapidly to the site, climbed over the rail and entered the water. As usual, the shock of hitting colder water was an instant relief to the over-heating that had occurred on the surface. However, it became readily apparent on my second breath that I had forgotten to turn on my air. I can assure you that descending in cold, swift water without air can be stressful!

After the dive, I decided from then on, I would breathe several times on the regulator before entry to prevent this stupid mistake from occurring again. The idea is that should you (or someone else) turn off your air after initial pressure check  (for transport, or perhaps to stop a leak), the gauge will read full and there will be a few breaths available from the air contained in the hose between the first and second stage. But 4 breaths should be enough to consume this air, so if the spg goes to zero during the ritual, then the air valve is off, even though the gauge reads full. It is much easier to take care of this problem (air valve turned off) on the surface than at-depth.

The point is: we, as humans, are not perfect creatures and we WILL MAKE MISTAKES. The purpose of training is NOT to eliminate all errors; it is to make these errors that will occur because of our imperfect nature nothing more than an inconvenience, instead of a life threat.

G = Let's GO diving!


Your dive equipment should be checked before a dive to avoid traveling to a far away exotic place and then finding  out that something is missing, broken or too small to wear.

When it comes to life support gear, I do NOT care what other people think about me. I believe that I DESERVE THE BEST!  So, when you purchase components for YOUR personal gear, you should accept the fact that you deserve the best and get the top-of-the-line regulator from the manufacturer that you find most comfortable for YOUR NEEDS. Also, remember that if you run across an air-needy-dive-parasite underwater, they make take your regulator out of your mouth before you realize an emergency is beginning. So, secondary, as well as primary, should be top-of-the-line 'cause you might have to use it under conditions of severe stress. If you find yourself in a situation where high air flow is needed, it is too late to wish you had paid a few dollars more for a better system.

Cold is a potent stressor (Not Being Cold) and I consider thermal protection is a critical element of life support. Again buy the best 'cause you are worth it!

When diving inland lakes where boat traffic is a definite threat to divers, it is the accepted (and legally required) practice to tow a float with the diver's flag attached while diving. The regulations about each states requirements can be obtained from that state's DNR. In Michigan, divers are required to tow a float and remain within 100 feet of the flag. Boaters are required to give the diver's flag a 200-foot clearance. It is important to understand this is a courtesy and should be used with a bit of common sense. In other words, stay out of narrow channels and navigational waterways. If the authorities believe your flag is being used inappropriately, they can issue a ticket for being a "hazard to navigation." Besides, a 1000 foot lake freighter may not see a dive flag on an inner tube and even if seen, there is no way, in a narrow channel, for the vessel avoid the flag.

Also, do NOT count on boaters recognizing the diver's flag as a safety signal. I have personally had my float picked up and put into a boat while I was attached to it. The boater thought he had found a cool inner tube with a funny flag attached to it. Even though he encountered resistance, he continued pulling until we surfaced. He was a bit surprised. We were upset 'cause we missed a good drift dive and he was upset 'cause I wanted the inner tube and flag attached to me returned.




Dive locations should be consistent with training, experience and finances. I believe the best experiences in a new location can be had when done under supervision. Regardless of previous training and experience, whenever you dive a new location or environment, you are a novice there. It is also important to remember that each different type of diving may require equipment unique to that style of diving or location. Finally, open water procedures and equipment may NOT be the best in a different environment, so it's always a good idea to get specialty training from an instructor experienced in the locale.

Here in the Great Lakes, there is, in some circles, an irrational pressure to immediately after basic training dive deep, under the ice, or in the swift water of the St Clair. I strongly believe the transition from novice to "experienced" should be done slowly, in the tradition of Bob! (Extend life experiences in "baby steps"  ... in the long term, progress from the accumulation of numerous small steps can be much larger than from giant leaps, especially if one of the leaps falls short of the mark.)

Because of the enormous peer pressure, particularly with young males, there will come a time when you may be asked to participate in an activity far beyond your personal training, experience and comfort zone. One ego-saving way of coping with this scenario is to point out a temporary lack of ability to equalize pressure in the ears. This inability to clear the ears is a situation that occurs to everyone who dives at some time and is, perhaps,  more socially acceptable than a discussion about the insanity of a particular proposed diving activity. Whenever there is some question about a dive or its participants, the safest course of action is to abort. As I have said (and will say again this weekend), the decision NOT to dive is always correct!

Potential Problems

Now, we shift to common problems encountered on the dive site. I'd like to remind you about masks on the forehead and why I consider this unwise. There is the obvious loss of an expensive mask due to head or wave action. (I have seen videotapes of professional rescue units who have lost search effectiveness 'cause masks fell into the water on head movement and the rescue diver was thus unable to dive without a mask.) People under stress (especially high CO2 load) are likely to spit out their snorkel/regulator and move the mask off the face. There are some individuals in scuba that will ASSUME all masks on the forehead are an emergency situation. So, save these folks a swim by keeping the mask off the forehead. Finally, wearing the mask on the forehead allows body and hair oils to accumulate on the mask. These oils form nuclei for condensation and thus, promote mask fogging. So, for a variety of reasons, I prefer that the mask be kept off the forehead. I wear mine around my neck.

The most common problem (some report as much as in 80% of divers) during this first weekend of diving  is related to equalization difficulties. So, let's review what is happening to the ears when we dive.


As we descend in the water column, the weight of the water (and atmosphere) above us pushes against all body cavities (air spaces). These spaces will begin to collapse under this pressure. This collapse is called a "squeeze." As the water pressure pushes against the tympanic membrane, this thin flap of skin stretches inwards, leading to discomfort and pain. If there is too much stretch, the membrane ruptures. It is possible to rupture the membrane of an adult in as shallow as eight feet of water. The mechanism to prevent this discomfort is called "clearing" or "equalization." There are a variety of techniques employed, but they all share a common characteristic in that the goal is to send breathing gas into the middle ear via the Eustachian tube. This increase in middle ear pressure (to the same as the ambient water pressure pushing the tympanic membrane inward) restores the tympanic membrane to its proper equilibrium position and relieves any discomfort.

There are a variety of techniques one can use. The most commonly taught, the Valsalva (pinch nostrils and GENTLY attempt to force air out the closed nostrils), has a number of potential problems. The "pinch and blow" technique raises thoracic pressure, which is transmitted via the cerebral-spinal fluid to the inner ear. A sudden, dramatic increase in pressure can damage inner ear structure. In a worst-case scenario, a membrane on the inner ear called the round window may rupture. Typically, once ruptured, without rapid surgical repair, there is a high probability of permanent deafness. In addition, many people in applying this technique will, through involuntary muscular action, squeeze the muscles around the Eustachian tube and make the clearing process more difficult. This is exacerbated if clearing is difficult 'cause divers will often use more and more force in an effort to clear. Finally, if a PFO is present (~ 25 % of the adult population, perhaps more in adolescents and children), a forceful Valsalva on repeat dives can send air bubbles circulating in the system (present after every dive) through the PFO into the arterial side of circulation. Thus, a forceful Valsalva makes an air embolism on descent theoretically possible. I SUSPECT a too forceful Valsalva is responsible for much of the barotrauma seen in the ears of novice divers.

Another common technique, the Toynbee, involves pinching the nostrils and swallowing. This problem with this technique is that swallowed air at depth, on ascent, will expand and cause gastric distress. It is best while submerged to avoid swallowing air. Stomach discomfort in novice divers (presumably from swallowing air) is quite common. In a worst-case scenario, the expanding air can rupture the stomach.

I believe the least stressful, most effective way to clear the ears is the Frenzel procedure. It is done by placing the tongue on the roof of the mouth, as far forward as possible. While the tongue is held in place, the back of the tongue is gently moved upward. (Imagine driving the tongue through the back of your head. Obviously, you cannot do this, but striving to do so will move the tongue such that a gentle puff of air moves up the Eustachian tube to increase the pressure in the middle ear). Often a "click" sound is heard internally as the tongue is moved.  This action does NOT constrict the Eustachian tubes, does NOT over pressurize the inner ear, and leaves the hands free. It is the safest, most effective way for divers to equalize the pressure in he middle ear.

Other behaviors that can assist in equalizing pressure are beginning the clearing procedure  prior to dive descent and to "clear" continually during descent (If you feel pain, you have already descended too far ... continual repeating of the clearing technique during descent prevents a pressure build-up in the outer ear), moving the jaw back-and-forth, tilting the head away from the affected ear, and a feet first descent. A feet first descent allows the natural pressure gradient (Pressure is greater on lower portion of the body than on the head in a vertical-feet first descent) to assist clearing (head first descent forces the diver to clear against the pressure gradient), facilitates buddy contact and helps maintain orientation.

If you feel pain at any time, stop! Do not descend any more until pain is relieved. Often, a slight ascent will relieve the discomfort.

IT IS YOUR RESPONSIBILITY TO LET THE NEAREST PERSON KNOW YOU HAVE A DIFFICULTY! 'cause no one but you knows of an internal pain! If you can not clear, then we will simply dive another day. I  prefer that you abort (to dive again on another day) rather than force a situation that may cause permanent injury.

The guiding principle in any equalization technique is to emulate driving in Maryland ... Drive ("clear") GENTLY!

On ascent, the situation in the middle ear is reversed: pressure in the middle ear is now greater than ambient. Typically, the expanding gas vents via the Eustachian tube without a problem. However, there may be times when localized swelling of the tissue surrounding the Eustachian tube (repeated stretch and collapse during a long dive), especially on repeat dives, hinders the escape of venting gas. If drugs were used to assist in keeping the Eustachian tubes open, it is possible, particularly with repeated doses, that the drug effectiveness will decrease with time and have little effect on ascent. In  the worst-case scenario, the expanding air has difficulty moving through the restricted channel. This is called a "reverse block."

If pain is felt on ascent, immediately stop the ascent and let your buddy know there is a problem. Often a slight descent will relieve the discomfort. A common error is to repeat the same technique used for descent ("pinch and blow"), but this makes things worse 'cause it drives air into an already over-pressurized volume. So, in the presence of discomfort on ascent, pinching the nose and GENTLY sucking will assist in draining excess air from the middle ear.

After the dive, it is important to do a self-assessment of your physical, mental and emotional state .Any feeling of ear discomfort  (pain, a "fullness", ringing, deafness, etc.) should be evaluated by a diving knowledgeable physician before resuming diving. A feeling of "fullness" may be acutely removed with a very GENTLE pinch and suck (relieve any excess middle ear pressure). However, the use of this technique to temporarily relieve discomfort suggests it is appropriate to visit an ENT for a quick check. (My basic philosophy is to always seek medical confirmation of dive status: A slight delay to receive an exam is far more desirable than diving with a dismissed problem and doing permanent damage. Again, I say, the decision NOT to dive is always correct!)

The key to having a good time underwater is knowing how to control buoyancy. Proper buoyancy control is a great example of a practical application of the Easy Diver's philosophy, "Dive with your brains, not your back," 'cause without an understanding of buoyancy (and lots of in-water practice time), lack of buoyancy control makes submergence on scuba an exercise in frustration and over-exertion. Not controlling buoyancy is just plain being miserable. But, having good buoyancy control allows you to float effortlessly, truly in defiance of gravity, in a warm, wonderful world that is a sensual pleasure unique to our sport. Being weightless, I believe, is the essence of the very real "diver's high!"


When we think about buoyancy, it is easiest to comprehend what is happening if we forget about positive and negative buoyancy 'cause this leads to confusion in solving buoyancy related problems. (See Archimedes, A Gold Thief and Buoyancy  for a discussion on solving buoyancy problems.) The fastest route to understanding this whole buoyancy business is to remember that buoyancy is a force, always directed towards the surface.


Archimedes Principle states that this upward force is equal in magnitude to the weight of the water displaced by the submerged object. Weight is a downward force directed towards the bottom.

This downward force is equal to the sum of the weight of the diver and everything attached to the diver. So, vertical position of the diver in the water column is primarily the result of two forces acting in direct opposition to each other. Buoyancy moves the diver up in the water column and weight moves the diver down.


If the forces of buoyancy and weight are balanced, then the desirable "weightless state" (hovering ... just like a helicopter ... there the forces of lift (buoyancy) and weight are balanced. The difference between a helicopter and a diver in this hovering state is that a diver does no work to remain motionless while a helicopter is expending energy spinning its blades to derive the buoyant (lift) force. However, the end result, being stationary in the vertical direction is the same) If these forces are NOT balanced, then work by the diver must be done to remain at a constant depth. Balancing the forces of buoyancy and weight leads to an enjoyable experience. Diving over-weighted (the most common problem in novice divers, (See B.C.'s Are Not Weight Belt Compensating Devices) leads to fatigue and a "this is NOT fun!" type experience.

Let's look at these forces and how we can use an understanding of them to facilitate our diving. Before our diving, we will establish our proper weight (See B.C.'s Are Not Weight Belt Compensating Devices). Once this is done, then extraneous effort while diving is avoided. 

The dive should not start until proper weighting as been established.

To descend, we simply look at our buddy, deflate the B.C, and exhale. A common problem in novice divers is to take a deep breath ( the "primitive brain" knows it does not breathe water and is about to submerge, so the natural, human reflex is to deeply inhale on the first descent. However, this inhalation increases lung volume, thereby increasing the volume of displaced water, which increases buoyancy and makes descent more difficult. But, by releasing some air in the B.C. and exhaling, the volume occupied by the diver decreases, buoyancy decreases, and the diver descends 'cause the force of weight (downward force) now exceeds the force of buoyancy (upward force). So, on descent, to slow or stop downward movement, we must either decrease weight or increase buoyancy. Rather than discarding weight (as a balloonist would do to move upward), divers find it convenient (since multiple up and down movements are common on a dive) to increase buoyancy by adding air to the B.C.

We face our buddy during descent because this helps maintain contact (especially in limited visibility water). Facing the buddy also allows instant communication about descent problems like improper weighting or ear equalization difficulties.

As we descend, gas spaces in the diver and the diver's gear will, because of the increased pressure, decrease in volume with a corresponding decrease in upward buoyant force. So, as we descend, SMALL amounts of has are added to the B.C. to provide an upward force to compensate for this lost buoyancy.

The goal is to compensate for, not overcome, the decreasing buoyancy caused by gas volume changes. If you start to rise, you have added too much gas.  This will, of course, occur. It takes time to develop a sense of "how much" gas to I add or dump to maintain my position in the water column.  

Many divers who have descent problems have failed to drain all of the gas out of their B.C.'s. Some inexpensive bladder stabilizing vests actually trap large volumes of gas. This is not a problem with higher quality jackets. If you are having problems descending, it might be wise to have a buddy look at your jacket to determine if large air volumes are being trapped between the bladder and the outer covering. This is the type of situation that is easily solved: Throw money at the problem (buy a better quality jacket) and the problem disappears.  Often divers do not actually vent all of the gas from their B.C. Divers should examine their own individual gear and assume a descent position that places the B.C. vent at the highest possible point. (I personally prefer B.C.'s that allow gas to be vented in the vertical position by pulling down on the B.C. hose. This eliminates the need to alter body position to place the vent at the highest point of the jacket.) It is helpful to have a buddy/instructor examine descent procedures to determine the optimum venting position. Descent in this position allows the water pressure to push all of the gas out of the B.C. (I have seen divers remove 10 lbs of lead, or more, when they were shown how to properly vent their B.C.'s.)  

During the first open water dives, there will be moments where you feel out-of-control. This is because you are truly out-of-control. You see, fine-tuning the balance between the forces of buoyancy and weight, especially during your first experience in a full wet suit takes practice. So, as one force or the other becomes larger, you will move accordingly in the water column. With a bit of time (and that's why I want more than 200 in-water on-scuba minutes the first  weekend of diving), your vertical oscillation in the water column will become smaller and smaller. There is no way that you can become comfortable in the water without mastering buoyancy control and that's why we will spend much time in the water. You will be amazed at your comfort level differences between the start of first dive and the end of your fifth dive. (There is no substitute for in-water experience!)


On ascent, gas volume increases, displaced water volume increases, weight is constant (unless dropped) and thus, buoyancy (upward force) increases. To compensate, gas is released from the B.C. This decreases the air volume in the  B.C. which drops the diver + gear displaced volume and so buoyancy decreases. Since the Boyle's law curve is hyperbolic (greater change in volume per unit of ascent is near the surface), left unchecked, the greatest buoyancy increases will be in areas near the surface. So, as divers, we must pay particular attention to our buoyancy as we near the surface. The area near the surface on ascents will be one of the places this weekend where you will need to pay particular attention to your place in the water column.

You will definitely notice the thermocline when we hit it. In Michigan fresh water lakes, the first thermocline has a temperature of about 55 oF. When you descend through the thermocline into the decidedly much colder water, the magnitude of buoyancy change that would be present without the temperature difference is increased. The gas temperature drops, so the volume it occupies drops (See Gas Law Primer). So, the displaced volume of water is less (buoyancy decreases) and you will fell heavy ('cause you are!). With practice, this slight buoyancy change in movement into and out of a thermocline will not be a problem. But, it is likely that the first time you experience this transition, the magnitude of change may surprise you. But, like every other in-water skill, a few excursions and this temperature-induced buoyancy shift will not pose a problem.

Likewise, when you ascend out of the thermocline into warmer water, the temperature of the gas in your b.c. increases. The gas expands and displaces more water volume and so buoyancy increases. So, when this rise is first felt, dumping gas from the b.c. will compensate.

The natural human  ("primitive brain") learning curve will be to not respond to the buoyancy changes until there is significant perceived movement. Then there is a tendency to overcompensate. This can lead to "yo-yo diving" in a cycle of buoyancy change and compensation. This can rapidly cover a lot of vertical distance, especially in shallow water where there is the greatest change in gas volume per unit of depth change. (Remember Boyle!)  With time and experience, the magnitude of this oscillation will decrease. Also, with experience, slight changes (as in beginning ascent, dropping over an outcropping. entering a thermocline, etc) can be anticipated and compensated for with minimum effort.

Get into the practice of adding or releasing air from the B.C. in small ("baby steps") increments as this technique will minimize the magnitude of in-water depth changes as you learn buoyancy control with your equipment.

I hope by now you have accepted a common theme that underwater skills require time to master and that you will be uncomfortable when submerged until you spend enough in-water time to master the art of buoyancy control.

Although we will set our initial training buoyancy without wearing the scuba gear, a good way to fine tune buoyancy is to float vertically at the surface of the water with about 500 psig remaining in the cylinder. You should, breathing with your snorkel,  float at eye level such that exhaling leads to descent and inhaling leads to ascent. This is typically done as a buddy operation with someone holding your B.C.. just in case of over-weighting to minimize descent when venting B.C. and exhaling. This way, with  full cylinder, you will be just slightly heavy with an empty B.C. and first ascent will be facilitated by the "extra" weight of the breathing gas in full cylinder.

The Dive

So, after setting our buoyancy, doing the snorkel run and our SEABAG briefing, we begin the scuba dive with a relaxed, slow feet-first descent initiated by venting the gas from the b.c. and exhaling. 


At depth, we swim slowly, arms at our sides (See Visualization For Improved Swimming Efficiency)  with slow, deep and continuous breathing and a wide, slow flutter kick. Get accustomed to monitoring your breathing ... it is your window into yourself. ANYTIME you sense an increase in breathing, try to stop and consciously breathe deeper and slower (The typical cause of more rapid breathing is high CO2 (from too much exertion or anxiety) which can be eliminated with slower, deeper breathing.)

One way to slow down is to visualize a STOP sign ... it is red, with a white border, octagonal, and has the potential for disaster when ignored.

We plan to end the dive at 500 psig at the entrance point. The is a bit less than desirable for more strenuous open-water diving, but we match our diving to our environment. We will be diving in a closed quarry where most of the instructional staff has been diving for years. So, we end the dive whenever someone reaches 500 psig, is cold or ANYTIME someone in the dive group signals to surface. Anyone, at anytime, has the right (moral obligation) to end the dive if conditions are not acceptable to them!

We will end the dive with a sloooooooooooooow ascent, breathing continuously. We will do a safety stop of three minutes on the first dive and five minutes on any multiple dives during the day. I anticipate bottom times of 50-60 minutes per dive.

It should be a reflex that immediately after surfacing, the b.c. is inflated to insure a stay-on-the-surface scenario. Most dives accidents (and fatalities) occur at the end of a dive when a cold, tired diver has difficulties coping with an otherwise trivial incident. An inflate-the-b.c.-on-surfacing-reflex insures that no mater what happens, you will be in an environment you can breathe. This facilitates problem solving.

Diving Philosophy

Since the risk is NEVER zero, it is important to continually evaluate self, environment and buddy. There is a great movie line about "Son, your ego is writing checks your body cannot possibly cash," This is most true in diving. This is a sport where ego and ignoring or dismissing risks can lead to tragedy or fatality. Coroner inquests are filled with incidents where inappropriate behavior of the now-dead diver was driven by ignorance or ego. So, always consider the risks associated with the dive before making the decision to enter the water. Remember, the most often heard term at a funeral is "If Only ..."

Problem Management

Since we are not perfect creatures, diving with 100% functioning gear that lasts an infinite number of dives in locations where Mother Nature is always totally cooperative, problems will  occur. It is the nature of our sport and our species. The best way to manage a problem is to cut it short by recognizing a potential disaster long before it becomes catastrophic In other words, the best rescue you will ever do is the one that requires no effort at all 'cause the problem was recognized and dealt with long before it escalated into a major event.

Fortunately, truly life-threatening scenarios are rare. The most common rescue does NOT involve swimming a great distance, rescue breathing and helicopter evacuation. The most common first aid procedures in shore diving are treatments for sunburn, cut and scraps, and insect bites. So, we train for and anticipate the worst, so that when anything less than the worst occurs, it is fairly easy to manage.

A major dive magazine has a slogan, "A Good Diver Is Always Training." I absolutely agree. So, as you gather your experience, consider taking specialty classes in the area of your interests. I think every diver should have training in rescue, first aid, DAN Oxygen Provider, and equipment maintenance. I would advise caution (get some experience and be comfortable in open water) before going deeper, under the ice, or tackling the current of the St. Clair River. (Just for perspective, I require 50 logged dives as a pre-requisite for training in river diving) Your best defense in solving a dive problem of any kind is a strong background of knowledge and experience.

Gather your experience slowly.

The way to solve ANY problem is as follows:

1.Stop and define the nature of the problem because the definition of the problem is 90 % of the solution.

2. Evaluate what you know and what is at hand to go from present situation to problem resolution.

3. Act to resolve the problem.

Acting without a plan often just increases the magnitude of the problem.

To assist in solving problems, every dive site should have an Emergency Action Plan (EAP), first aid kit, oxygen and demand delivery device, a tool kit and spare parts. But the best problem solving tool is a knowledgeable, physically fit, well-trained, properly equipped diver.

In summary, the primary problem-solving tool is recognition. So, once you commit to this sport, then you realize that the more you enhance your knowledge and skill, the less hassle your diving will involve.

 Dive Tables

The last topic of this review will be the dive tables. Although the actual entry times for each dive will be determined by me, you will be expected to tell me, according to the particular table you have chosen, what your no-deco obligation will be. This means I will expect you to be able to plan your dives in the absence of dive staff. 

All dive-planning devices are mathematical constructs. Some, like the US Navy tables, have been modified a bit based on decades of use. But, no table can absolutely guarantee freedom from DCS. I have given talks where some instructors in the audience truly believed that a popular recreational dive-planning device had "removed all the risk from diving." The belief that anything could remove all risk, in my opinion, is a dangerous philosophy to adopt.


Consider that most serious recreational dive DCS hits involve the spinal cord. Since the spinal cord allows you to walk, excrete and have sex, it is in your best interests to know the risks involved in this sport so that these risks may be evaluated and minimized. Every time you dive, you are gambling that your planning device will protect your spinal cord to insure a long life filled with hassle-free walking, excreting and sex.

Since there is much at stake, I'd like to take a few moments to compare tables. These results were taken from a Decompression Workshop given by Karl Huggins while he was at the University of Michigan.

Each of the participants was given a different table available at the time to recreational divers and asked to plan a series of dives. The next few slides summarize the results obtained.

The first Dive is to 65 fsw for 20 minutes, followed by a surface interval of 2 hours and 40 minutes. The slide, below right, summarizes the time allowed for a second dive at that time to 65 fsw.


Next, since there were so many different times, the group decides (diving by committee?) to dive to 65 fsw for 20 minutes. Then the question is asked about the time needed before allowing a dive to 54 fsw for 20 minutes. Notice that the tables give CORRECT times that range between 23 minutes and 16 hours!

So, before you decide which device you will use to protect your future sex life, it is a good idea to make this an informed decision by evaluating a few tables under the conditions that you will dive. Then, choose wisely and dive whatever table you choose on the conservative side.

Dive Problems

We will finish with a few practice problems. We will begin tomorrow with a snorkel run. Although our time will only be about an hour, assume that you snorkel for 3 hours with a deepest free dive of 47 feet. What is your rep group?

Under most circumstances, nitrogen absorption while snorkeling is irrelevant to recreational diving. While there have been reported cases of DCS in free divers, the incidents involved more than 8 hours of repetitive, working dives to depths in excess of 60 fsw.

Now, let's go diving. Your first dive is to 50 fsw for 45 minutes. What is the rep group immediately post dive?

The rep groups for three common tables for this dive are listed in the left panel below. Now, following that dive, you wait 30 minutes. At this time, what is your rep group and the no-stop time limit for a dive to 50 fsw?

The answers for this dive are listed in the left slide below. Now, consider a dive to 50 fsw for 45 minutes, a surface interval of 1 hour and 45 minutes, followed by a dive to 50 fsw for 30 minutes. At this time, what is your rep group, your residual nitrogen time (RNT), your adjusted bottom time and your final rep group?

The answers for this problem are displayed on the slide in the lower left. The last problem of the evening of the evening involves a dive to 50 fsw for 45 minutes, a 1 hour and 15 minute surface interval, followed by a 50 fsw dive for 30 minutes. How long must you wait before a no-deco obligation dive to 45 fsw for 50 minutes?

The answers are listed below on the left. Any last questions on anything we have discussed tonight?


Final Thoughts

No mater where or when you dive, if you approach every dive with a sense of awareness of self and environment, and an proper attitude,

equipment appropriate for the environment and recreational task at hand,

then no mater where you dive,


the knowledgeable, physically fit diver, has more fun!

Welcome to Planet Ocean and the lifetime of adventure that begins tomorrow!

May you always



Thanks to:

The SCUBA students of the University of Michigan whose dialogues with me were the basis of this slide set,

My U of Michigan teaching assistants 'cause they always make my life so much easier


Lee Somers, Ph.D  of the University of Michigan for allowing me creative liberties while a member of his staff


Photos Used:


Students & TA's with the author at Portage Quarry by Jeff Rice

:Lightning & Submarine from the Microsoft Design Gallery of Office 2000

Images of earth from NASA

Maryland Traffic Sign was a gift from a student

Astronauts in training courtesy of Johnson Space Flight Center

Military Diver was a gift from the Lee Somer's Slide Collection

All other photos by the author

 the fire hydrant was shot by the author as an exercise in a creative photography class

(the picture shot by another class member of him taking this image was more amusing, but

proper academic decorum prevents revealing that slide here!)


In this document go to:




Jump To: Buddy Diving   War Story 1  War Story 2  War Story 3  Equipment   Location


Potential Problems   Buoyancy   The Dive   Problem Management   Philosophy


Dive Tables   Table Problems   Final Thoughts

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About The Author:

Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the University of Michigan. He has authored more than 100 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information In North America.

  Copyright 2001-2022 by Larry "Harris" Taylor

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Use of these articles for personal or organizational profit is specifically denied.

These articles may be used for not-for-profit diving education