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     Hold a bottle of unopened seltzer to the light. The contents look exactly like water. If it is shaken the bubbles quickly go to the surface. There are no lingering bubbles. There is no effervescence. Open the bottle and after the blast of gas there are bubbles galore. Amazingly the bubbles continue for some time.

     Is it possible to open the bottle of seltzer in such a way no bubbles form? Let's say there is a special cap on the bottle that has a valve. When the valve is turned ever so slightly a minute amount of gas escapes. The valve is immediately shut to keep more gas from escaping. If this were done numerous times over a period of time it would be possible to have the seltzer go flat without bubbles forming.

     Analyze what is happening when seltzer or soda is decompressed. When the bottle is opened rapidly the carbon dioxide gas, at a pressure of approximately 50 psi, is ejected rapidly to the atmosphere. The high pressure flows to low pressure. Meanwhile, the CO2 in the water is still under high pressure. The molecules near the surface of the water start jumping from the water to the air. The molecules below them start moving toward the lower pressure CO2 created when those jumped out of the water, and so on. However, some of the molecules find a better escape route. If there is a nucleus nearby, such as a piece of dust, a scratch on the inside of the container, etc., the molecules may latch onto that. More and more join those and soon a bubble appears. When the bubble grows big enough it takes billions of molecules to the surface all at once. That is more efficient than taking the time to have each molecule migrate to the surface.

     If the container was designed to be opened in small spurts, the minute loss of pressure above the water would not create the tension caused when the soda is opened suddenly. Simply, some of the CO2 escapes, a few molecules migrate through the surface to where the pressure was lowered, those below migrate upward to cause equal pressure throughout the inside of the seltzer bottle. There is not enough reduction in pressure to create a bubble on a nucleus. If the process is repeated over and over the soda can be made to go flat without forming a single bubble.

     What does this have to do with you as a scuba diver? You are a bottle of soda complete with a myriad of nuclei: platelets, blood cells, etc. are part of your blood. Going under pressure and then coming up rapidly is similar to opening the soda bottle rapidly. Poof! You have decompression sickness caused by the formation of bubbles in the blood and elsewhere. Once a bubble forms it is attacked by the body's defenses and a clot may forms that could have untold consequences. Coming up slowly after a deep dive will allow the high-pressure gas to leave the body without forming bubbles.

      The question is: "How deep do you have to dive in order to be subjected to bubbles causing decompression sickness?" J.S. Haldane in the early 1900's discovered if a person halved their pressure they could have bubble formation. This is a 2:1 ratio. Haldane invented the first dive tables for the British Navy. If a diver descends to 33' of seawater, where the pressure is 2 atmospheres, and then ascends to the surface too rapidly they could have bubble formation. That is a 2:1 ratio. (The U.S. Navy amended this to a 1.7 to 1 ratio.) If they confined their diving to 23', where the pressure is 1.7 atmospheres and then went to the surface they would not be cutting the pressure by half. To carry this a bit further, if a diver went to 66' (3 atm.) and ascended to 33' (2 atm.) the chance of DCS is slim because that is a 3:2 ratio. If they went to 99' (4 atm.) and then ascended too rapidly to 33' (2 atm.) it would be a ratio of 4:2 which is 2:1 and DCS becomes a possibility.

     When one analyzes decompression sickness bubbles they are found to contain almost 100% nitrogen at the onset.. This was discovered in 1880 by Paul Bert, a French scientist working on cases of the "Bends." Since oxygen is metabolized in the body and carbon dioxide is in limited quantities and dissolves readily in water, it is only logical to find nitrogen as the main culprit in DCS. However, after a short period of time the bubbles with equilibrate with other gases around them (high pressure to low pressure) so oxygen, water and carbon dioxide may enter and mix with the nitrogen. Nitrogen dissolves in fat 4-5 times better than in water. Not only do nitrogen bubbles form in a watery environment such as the blood, they will readily form in areas where fat is located. Nitrogen targets the fat under the skin, in the spinal column, and in the joints. Obese divers must really be careful because they are at an increased risk for decompression sickness.

    A comparison of DCS and AGE bubbles is in order:

     Decompression tables for the US Navy were first developed in 1915 using the information Haldane discovered. They were revised in 1937, 1957, and finally in 1985. At this point it is important to learn how to use a decompression table such as the NAUI Table, the PADI Recreational Dive Planner, and/or the United States Navy Decompression Table. The complete table should be understood before you read further.

     Go to: Learning to Use a Decompression Table

     After learning to use the United States Navy Decompression Table, please do the following 2 problems:

  1. A diver goes to 40' for a bottom time of 135 minutes. After leaving the water the diver stays on the surface for 1 hour and 30 minutes. The diver reenters the water and dives to 100' for a bottom time of 20 minutes. What is the final stage decompression (if any), and what repetitive group is the diver in after leaving the water?
  2. A diver goes to 100' for a bottom time of 20 minutes. After leaving the water the diver stays on the surface for 1 hour and 30 minutes. The diver reenters the water and dives to 40' for a bottom time of 135 minutes. What is the final stage decompression (if any), and what repetitive group is the diver in after leaving the water?

     After doing the above problems it should be noticed the outcome is vastly different but the dive depths, times, and surface interval are exactly the same. In the first case the diver could get decompression sickness. In the 2nd it is highly unlikely. That points out a very important rule in scuba diving: Do the deeper dive first! That is known as a "normal dive profile."

     Divers using computers are doing reverse profiles commonly. That is, they are not always doing the deep dive first. Properly using the computer, and staying away from stage decompression, has not shown that reverse profiles are in any way more conducive to getting DCS! This cannot be said for table use.

     There are provisions in most log books to chart your dive. Noting the profile of your dives is important for many reasons. If, for example, you were found unconscious 4 hours after a dive, the logged profile could be a determining factor in deciding what life-saving treatment would be applied. If you had a record of diving to 100' for 30 minutes with little or no stage decompression then a call to DAN would be in order. Learning how to record your dives in a log book is an important part of any scuba course.

     Decompression computers are now very popular among divers.  They are neat electronic instruments that simulate the diver's nitrogen levels in various "tissue compartments" in the body. They allow more freedom underwater and have been proven to be safe and reliable. Without a computer the diver is at the mercy of the decompression table and that is oftentimes very limited in the way sport divers dive. For example, if you were to go to 100' for 15 minutes bottom time and then ascend to 90' there would be no way to determine how long you could stay at 90'. The USN tables tell you there is a maximum bottom time at 100' of 25 minutes. At that point you must ascend at 1'/second. There is no provision for stopping at 90' on the way up. You are supposed to exit the water 100 seconds (1'/second) after leaving the bottom. But we do not usually dive that way. A typical dive in the warm ocean is to go down to 100' and stay there for 5-10 minutes and then begin heading toward shallow water. There may be some exploring at 60' for a short time, and then there might be a descent to 70' for a minute or so, and then the diver may go up in the water column that is less than 20' and stay there until the tank is low on air.  With a dive computer the in-gassing and out-gassing of nitrogen is continuously monitored. The diver is kept well-informed as to how much time they have left at any depth before a danger of decompression sickness becomes a real threat. Dive computers also keep track of compartment nitrogen levels while the diver is out of the water. Repetitive dives take into consideration the amount of nitrogen left in the body from previous dives and surface intervals.

     There are about 100 divers that die from DCS each year. In the US, slightly less than 1,000 divers are treated for DCS each year. Over 1/2 of those are divers that did not exceed their NDL limits on the table or computer! There was no evident violations of the rules of diving! They get "undeserved hits" because there can be no shown violation of present decompression models.

     If you get DCS what happens? Generally the symptoms/signs include:

     The Diving Accident Network (DAN) did a study in 1990 of the most frequent symptoms reported by divers affected by DCS. The results are:

     Another set of data on the most frequent symptoms of DCS appeared in the magazine, Immersed. The article, in the Winter 1998 edition, showed the following results:

     Another interesting set of statistics concerns the time of onset of DCS symptoms. According to DAN in 1990, 60% of divers that got DCS had recognizable symptoms within 30 minutes. That leaves 40% that will not get "a hit" until after they have been out of the water for 30 minutes or more. Of those 40%, 1/4 will feel the effects before 2 hours, another quarter may go as long as 6 hours, and the remainder might go as long as a day or more. Continuing, DAN found that 95% of divers experiencing DCS got symptoms prior to 24 hours after surfacing. It is possible for some divers to have a dive in the morning and have their head fall into their soup at dinner!

     Many divers experiencing DCS deny it. In fact, denial is the number one problem with decompression sickness. Not wanting to admit to failure and/or being considered a lousy diver, not wanting to go through the expensive chamber route, the termination of a great vacation, and the similarity of DCS symptoms to ordinary problems are some of the reasons for denial. DAN has suggested that about 20% of the divers that finally received treatment for DCI continued to dive after the initial symptoms appeared. Unfortunately, with DCS the condition will usually worsen if it is not treated promptly. The earlier the treatment, the better the chances for complete recovery. According to DAN, half of the divers treated for DCS were seen by medical personnel after 24 hours had elapsed. Incidentally, after 2 weeks, treating a case of DCS will do little good. The damage will most likely be permanent.

     Incidentally, the outdated term "Bends" is was coined because of the way many of the Cassion workers walked when afflicted. They had a stance that resembled the 1800 womens' fashion called the "Grecian Bend". Check out the following photograph:

     The initial treatment for suspected cases of DCS is the administration of pure oxygen. There should be no dilution with air. This will cause the level of oxygen in the body to soar. Even in areas where there might be a bubble of nitrogen from DCS that impedes blood flow, the oxygen will migrate (high pressure to low pressure) into the surrounding tissue. That will save cells. As more and more pure oxygen is breathed, the levels of nitrogen will drop as the nitrogen moves into the oxygen-rich environment, to the blood stream, and then out of the body through the lungs. At the same time, the nitrogen in the bubbles will be at high pressure compared to the extremely low level of nitrogen in the tissue surrounding the bubble. That will cause the nitrogen in the bubble to leave and become dissolved in the tissue fluid. Breathing pure oxygen shrinks nitrogen bubbles. The same can be said for AGE bubbles. The part of those bubbles that are of concern is the nitrogen gas. The oxygen will be used by cells to sustain life. But, do not stop the flow of oxygen! Returning a diver to breathing air will reverse the process and the nitrogen will return to the bubbles and the tissue. Recompression must start before a change in gas mixture happens. It is noteworthy to mention that oxygen is used less than 1/3 of the time, and when it is administered the concentration is less than required in 90% of the cases! Recompression in the water is risky. But, according to the Navy Diving Manual, if there is no chance of reaching a hyperbaric facility within 12 hours water recompression should be a considered option.

     According to John Paul Longphre, M.D. in DAN's Alert Diver 7-8/2005, the pressure gradients (high pressure to low) are:

     In the meantime, DAN should be consulted and evacuation must be arranged (1-919-684-9111). The diver with suspected DCS should be taken to the nearest hospital emergency room. The doctor(s) on duty should be instructed to contact DAN. Then the DAN and EMR doctors can decide on the best treatment. If conditions warrant, a trip to the nearest operational recompression chamber needs to be made. DAN will be able to assist if that is the case. Oxygen must be continued until the diver is under the care of the recompression chamber personnel.

      This is the US Navy Table 6 used for treating DCI. Depth is on the vertical (Y) axis. Green is oxygen breathing, blue is air breathing. Notice there is a 2.4 minute descent to 60' breathing oxygen. That is followed by a 20 minute session breathing 100% oxygen at 60', followed by a 5 minute session on air. The rest is obvious. The entire treatment time is 4 hours, and 47.4 minutes. For difficult cases the oxygen breathing may be extended at the 60' and/or the 30' depths. For brain bubble cases (AGE), pressure may be increased to a depth equivalent of 165'. At that depth bubbles are reduced to 1/5 of their original volume. The switch from air to oxygen is usually accomplished by breathing air from the chamber and then putting a mask to the face for pure oxygen. In single-person chambers that is not practical so they usually treat cases of DCI with oxygen at a depth of 30'. This avoids oxygen poisoning.

     The above US Navy Treatment Table 9 is used for repetitive treatment. It is inverted from the Table 6 in that the surface is at the top and the 45' maximum depth is at the bottom. In any case, if a diver still experiences symptoms of DCI after the initial treatment using Table 6, then Table 9 is used for further needed hyperbaric treatment(s). Because the pressure is less than that used on Table 6, 45' instead of 60', and because the pressure is applied for about 2 hours, rather than almost 5 hours, it is permissible to give oxygen the entire time with no air breaks.

     A few words about DAN are in order: The Divers Alert Network (DAN) was established in 1980 and is a non-profit organization dedicated to helping divers with medical emergencies. DAN operates a 24-hour emergency medical hotline (1-919-684-9111). When a diver or physician calls DAN a medical doctor specializing in underwater medical problems is available for consultation. They also have at their fingertips at an updated list of hyperbaric chambers that might be used for treating suspected cases of AGE and/or DCS. This list is made available to medical personnel only.

     For years this author dove with the mistaken notion that if we surfaced properly (then 25'/minute, now 60'/minute), and did not exceed the limits allowed on the USN Decompression Table at various depths in order to avoid a ceiling, we would avoid having those horrible nitrogen bubbles from forming. Then came the Doppler studies. Doppler detectors were invented that could monitor from the outside of the chest bubbles that were passing through the right side of the heart. Ultrasound pulses are reflected by the bubbles. In 1969 it was used to check divers' blood after a  series of "safe" dives. What they found was amazing. Bubbles were created during what was thought to be a "safe" dive. Evidently these "few" bubbles did not do any damage. They are usually captured in the lungs and are dissipated into the atmosphere. There are no obvious symptoms, hence they were nicknamed, "silent bubbles." However, it may be that these bubbles do cause the excessive fatigue experienced by some divers.

     Silent bubbles are unwanted. Further Doppler studies were done. Referring to the 3 graphs above, it was found the bubbles could be reduced significantly if an ascending diver stopped at a depth of 10 feet and stayed there for 2 minutes. It was found the bubbles could be reduced even more if the safety stop started at 20' for 1 minute and then 4 minutes at 10'. Since the latter is a bit long, a comprise was made: If you are planning to dive to 33' or more, it is always a good idea to make a safety stop at 15' for 3 minutes. It is NOT a required part of decompression. It is only precautionary. If it is not done it should not send the diver into mental anguish! In fact, in 2009 new recommendations have been published suggesting if you dive below 33' on the ascent you should spend 1 minute at 1/2 the maximum depth, and another 1-3 minutes at 15'.

     Some dive boats have a hang bar. It may simply be a piece of pvc pipe suspended to a depth of 15' by lines at the ends. The bar makes it convenient for a diver (or group) to hang for 3 minutes following a dive. Some of the boats have a scuba attached to the bar, or a long-hose alternate air regulator, so that a diver running low on air could switch to that source. Coming up the anchor line or float line and stopping at 15' is an alternate method. However, too many divers on an anchor line may dislodge the anchor from the bottom so a hang bar is preferred. In most fresh water lakes the diver can simply stand or kneel on the bottom. Incidentally, when depths are measured they should be done at chest level according to the US Navy.

     To keep commercial airplanes from expanding and contracting as they change altitude the interior pressure is reduced to about 3/4 atm., equivalent to what is found at 8000'. At that altitude any bubbles that were present in the body would expand by 1/3. It is possible to get decompression sickness by flying right after diving. Although there is no hard and fast rule here, different suggestions have been made over the years. One older recommendation was to avoid flying until the diver has moved into Group D on the Surface Interval Credit Table. Another is to wait 12 hours following a no-decompression dive, and 24 hours following a decompression dive where a ceiling was created. Some recommend waiting 24 hours following any dive. The latter is the safest but may not be the most practical. Diving computers usually have a "Wait-to-Fly" indicator on them which makes it important to not turn the computer off after the last dive until the OK to do so is indicated. The latest research from DAN indicates that it would be wise to wait no less than 18 hours before flying. If the diver has done very deep dives, lots of multi-day diving it would be wise to wait even longer. If a diver has had hyperbaric treatment they should wait 72 hours prior to flying.

     Diving at high altitude presents further problems for DCS possibilities. Although this would not be true, let's suppose a diver makes a decision to dive in a lake at 18,000'. The atmospheric pressure would be 0.5 atm. If the diver descended to 34' (fresh water) the absolute pressure would be 1.5 atm. (1 from the water and 0.5 from the air). Going to the surface would be a change from 1.5 to 0.5 and that is 3:1. Remember, Haldane said DCS could occur if the pressure was reduced 2:1. So, the likelihood of DCS is increased at high altitude. How deep would that diver have to go to have a pressure change of 2:1? Only 17'! At 17' there would be a pressure of 1.0 atm. (0.5 from the air and 0.5 from the water). Going to the surface from only 17' down would be a change of 1.0 to 0.5 atm. and that is 2:1. Decompression sickness could occur in a rapid ascent from 17'! Lake Tahoe in Nevada is at the high altitude of 6,229'. If you stop at a dive shop in the area you will find for sale special decompression tables for diving there.

     Another consideration is the residual nitrogen time (RNT) that must be added to a dive at altitude for a diver that has just ascended for a dive. Foe example, if a diver ascended from sae level to dive immediately at 8000' they would have excess nitrogen in their tissues just as if they had been diving before. In this case the excess nitrogen would put the diver in Repetitive Group G meaning time would have to be added to the dive before entering the water. It would be better to wair 12 hours after the ascent to flush excess nitrogen from the tissues. Most dive computers take the altitude into consideration if they are turned on at the dive site prior to entering the water.

     Whew! There are other altitude considerations. Let's say you are diving in Saba. After the dive you drive to your hotel. Being a volcanic island the are steep climbs to villages and living quarters. As you progress to your hotel, which is at an altitude of 1000' you must pass through 2 small villages which are about 1500' high. This trek is similar to flying after diving. There is a distinct possibility you could get a decompression hit. So, what should you do? The best course of action would be to stay at sea level (the dive site) for a period of time in order to de-gas somewhat. Have a leisurely lunch perhaps. The following table will give you the wait times following a dive before proceeding to altitude.

Download the above Altitude Chart

     The following items will increase the chances of a diver getting DCS:

       So, if you are diving in cold water and/or working hard use the next depth and the next time on the decompression table, or be conservative with the dive computer. If you are fat do the same, as well as get some exercise. Drink plenty of fluids, and wait several hours after diving before heating the body. Take 50 mg/day of vitamin B6 everyday. Stay away from the maximum times allowed at depth on the tables or the computer, and if you are diving everyday for a week or more, take a day off in the middle to allow some of the "slow" tissues to return to normal atmospheric pressure. Lastly, breathe normally while diving. Do not "skip breathe" to make your air last longer. Remember, it could cost you about $40,000 to treat a case of decompression illness.

     Dan Leigh, in the DAN magazine, Alert Diver (January/February 2006): "The Utila Chamber is the only one on the island....(Jim) Engel said the chamber used to handle more than 20 divers a year, but that number is now reduced from previous years. 'The chamber enabled us to gather research and interview divers about their diving,' he said. 'We found that as many as 90 percent of divers were dehydrated. We met with  the shops and instructors and they all have been pushing a lot of water down their divers and have cut our cases to almost none.'" 

     David Buch, in the DAN magazine, Alert Diver (7/8 2004): "...heavy smokers who manifested DCI were almost twice as likely to have more severe symptoms than mild symptoms.  Approximately 37 percent of injured heavy smokers showed severe symptoms, whereas only about 24 percent of nonsmokers manifested severe symptoms."

     According to Dr. Alfred A. Boyd, about 30% of people have an incomplete closure of the foramen ovule. This opening between the 2 upper chambers of the heart is supposed to close at birth so blood is forced through the lungs and the 4 chambers of the heart. In 30% of us the opening remains. It is called a patent foramen ovule, or PFO. Since some blood leaks directly from the right atrium to the left atrium of the heart if a PFO is present, DCS bubbles could also follow that route. The bubbles would not be filtered out of the bloodstream by the lung capillaries. This could result in an arterial gas embolism by decompression bubbles! Should one dive with a PFO? The risk for neurological bends may be greater unless the diver becomes more conservative with bottom times, depths, and ascent rates. 

     According to DAN, the diver suffering from DCI (both DCS and AGE) is most likely to be male (69%), between 30 and 49 years old (64.6%), diving with a computer (60%), and have a total number of lifetime dives of 101 or more (44.2%).  Also, if a diver does get DCI, the best advice from DAN is to wait 4 weeks after all symptoms disappear before diving again. Consultation with a physician that has experience with DCI would be advisable. As far as divers using computers having a higher incidence of DCS, it is my feeling it is because they are now able to risk extreme exposures underwater. For example, before computers a dive to 300' would be difficult to accomplish because table diving would warrant lengthy decompression stops. With a computer it is possible to dart down to that depth and come back up to shallower depths within minutes. Although it is dangerous to do a dive such as this because of narcosis, blackouts, etc. the computer may very well indicate it would be safe to surface in a short time interval.

     Do not dive if you are pregnant. The fetus will absorb nitrogen from the mother's circulatory system through the umbilical cord. Getting rid of the excess nitrogen from the fetus does not happen as quickly as from the mother because of the extra distance due to the cord. It has been shown that decompressed sheep fetuses get DCS when the mother sheep do not. Also, if bubbles do form in the fetus they may travel through the foramen ovule, by-passing the lungs, directly to the brain creating an embolism. Also, if a pregnant diver is treated for DCI, the high pressure oxygen in the chamber may cause blindness in the fetus.

The Double-Lock Hyperbaric Chamber, Single-Place Chamber at the

Regional Hyperbaric Center, Westchester Medical Center, Valhalla, NY, USA

A Single-Place Hyperbaric Chamber at the

Regional Hyperbaric Center, Westchester Medical Center, Valhalla, NY, USA

The DSUS Shop Decompression Table:


Download PDF of  The Deep-Six Shop Decompression Table, Side 1

Download PDF of  The Deep-Six Shop Decompression Table, Side 2

The NOAA Decompression Table:

Download a .PDF of the NOAA Decompression Table



     "I got caught speeding, "Strangely enough, these four words usually provoke a sympathetic and friendly response such as, "Oh, that's too bad. You have to watch out for those cops on I-55." And, for those able to think ahead about their own speeding, there follows the almost universal, "Where did you get caught?" and "How fast were you going?"

     "I always wondered if our national tolerance of highway speeding extended to diving. So, sharing beautiful and dramatic 60-80 foot deep dive in clear water, with 16 other divers, I decided, slate in hand to time the ascent rates of the different sets of buddies. With about five minutes to spare on a 40 minute dive, the Divemaster signaled for an ascent from 62 feet. No anchor line was present since it was a drift dive. There were just 62 feet of blue water to rise through and then we'd be back on the surface. Since diving's "speed limit" is 60 feet per minute, it should have taken the divers a minimum of 62 seconds to reach the surface.

     "Twenty-eight seconds later, the first buddy team broke the surface. The next five buddy teams all came up in 35-45 seconds. Only two buddy teams took longer than 60 seconds, and both of them stopped for a ten foot safety decompression stop.

     "Of course, there are no "smokeys" underwater. But, there are expensive "speeding tickets." Decompression sickness, air evacuation, recompression treatment, etc. can cost as much as $30,000. Lifetime paralysis is thrown in at no extra charge. That's a big speeding ticket.

     I'd like to report that this was an isolated incident, but I repeated my ascent rate survey with four different groups with similar results. I didn't catch anyone going more than "100" again (nearly doubling the safe ascent rate) but more than 50 percent of the divers exceeded the 60 foot per minute rate. The finding, therefore, is that most divers do not make safe ascents - particularly when not using an anchor line or ascent line!

     "The biggest cause of speeding ascents is lack of buoyancy control. Divers who are religious about maintaining their buoyancy on the bottom seem to get disoriented when they're ascending without a line or other reference point. Many divers are also over-weighted, causing too much air to be pumped into their BC's. All this air has to be let out as it expands on the ascent and most of it has to come out between 30 feet and the surface. And, that's the "danger zone", where you should go the slowest!

"Follow these guidelines for making safe ascents:

     "1. Give yourself enough time to make a safe ascent. Start up at least five minutes before the no-decompression limit.

     "2. When you're a new diver, use the anchor line and slowly pull yourself up to the surface (gloves help). As you become comfortable, time yourself next to the line until you become proficient in all aspects of buoyancy control. That means you can stop and maintain your depth (within a couple of feet) at any point during your ascent. (This is much harder than it sounds!)

     "3. Make a 15' safety stop if you have been diving below 30'." 

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