Here are a few items you can include in your logbook to help you stay organized and honest, track progress, and work on self-improvement as a diver.
Buying your first rebreather is no easy task and as with any large investment you want to make sure you are making the right purchase the first time around so you do not spend that money twice.
In reality, Trimix is a risk management breathing mixture utilized by divers typically seeking to offset the consequences of diving normoxic air or nitrox mixtures at a planned diving depth by replacing much of the nitrogen and some of the oxygen with more benign inert gases like helium.
Fortunately, there is a way you can discover whether CCR diving may be for you without ever having to make that investment. It’s called the TDI Rebreather Discovery experience.
There is no fluff when it comes to CCR diving. Each step has value and should be followed as prescribed. And that’s not just my opinion, it’s the best, safest and most secure way.
by John E. Lewis, Ph.D.1:
This article could have dealt with Boyle’s law, partial pressures, oxygen toxicity, and how rebreathers work. Unfortunately, it would be three times as long and could (probably would) prove to be boring. Therefore, I chose a more direct approach. I have designed a single day of recreational diving and compared the experience of three divers equipped with different scuba devices ranging from a common open circuit aluminum 80 ft3 tank using air, an identical tank but one equipped with 32% nitrox, and finally the recently introduced Hollis Explorer semi-closed rebreather.
The Dive Scenario
The day’s diving takes place from a boat that is positioned in an area with a wide variety of depths ranging from walls with maximum depths over 30 metres / 100 feet to reefs as shallow as 18 metres / 60 feet. The Captain of the boat has decided there are to be three no-decompression (NoD) dives separated by two hour surface intervals and are to include a mandatory safety stop of five minutes at 3 metres / 10 feet. He also insists that the divers surface with no less than 35 BAR / 500PSI in their tanks.
This boat does not have a compressor, and it follows that all of divers must bring the tanks necessary for the three dives. The first dive is a multi-level dive beginning at 30 metres / 100 feet followed by a 15 metres / 50 feet second depth option. The second dive is also a multi-level dive to a maximum depth of 24 metres / 80 feet with a 12 metres / 40 feet second depth option. The third dive is to be at a single depth of 18 metres / 60 feet. In order to be able to visit the three sites that the Captain has chosen in the time allotted, he insists that for the first two dives the divers not exceed a 60 minute bottom time.
The Divers and their Equipment
Bob, by far the oldest of the three, carries on board three aluminum 80 ft3 tanks filled with air (21% oxygen). He has been diving for over 40 years, and he has developed the particularly low value of air consumption (SAC of 0.5 cu ft/min)2. Mike has three new aluminum 80 ft3 tanks that are prefilled with 32% enriched air mix and sport Nitrox labels. Ordinarily he has a considerably higher SAC, but for this exercise we have made his the same as Bob in order that we can see what Nitrox brings to the table. Nick, by far the youngest, has a brand new Hollis Explorer semi-closed rebreather that is equipped with a steel 28 ft3 tank filled with a 40% nitrox mix, and he too has two backup tanks similarly filled with 40% nitrox. Nick has a SAC of 0.75 cu ft/min, which represents a more common value among recreational divers.
All of the divers are equipped with dive computers, and with one exception, all have been programmed to reflect a predetermined value of oxygen content. The exception is Nick’s Explorer that has been designed to optimize the dive time by choosing a value of the oxygen fraction in the breathing gas such that the no-decompression limit (NDL) equals the capacity of the device. This of course is subject to the maximum operating depth (MOD) dictated by an accepted value of PO2 = 1.4 atm. In addition, the dive time based on the canister life of the Explorer is limited to 120 min.
The First Dive
The results of the first dive are shown in Table 1, where TBT refers to the total bottom time. The dive times that were controlled by NoD limits are labeled as ND, by scuba capacity as CAP, and 60 for the Captain maximum specified bottom time.
|Depth (fsw)||Bob (OC air)||Mike (OC Nitrox 32)||Nick (Explorer)|
|100||18 ND||30 ND||37 ND|
|50||22 CAP||12 CAP||18 (60)|
|TBT||45 min||47 min||60 min|
As can be seen, Bob, the traditional open circuit air diver, was seriously disadvantaged at the first depth of 30 metres / 100 feet where the other two divers have significantly greater bottom times. Note that if Nick with a more common SAC had been using the open circuit rig, his TBT would have been less than 30 min.
The Second Dive
After a 120 minute surface interval, the Captain has moved the boat to a new dive site where the maximum depth is 24 metres / 80 feet. Again Bob is at a disadvantage at the first stop. However, it is interesting to note that Mike who gained eight minutes over Bob at the greatest depth, lost five minutes in total bottom time. Nick with the Explorer greatly surpasses both Bob and Mike at the first depth even with the imposed 60 minute TBT.
|Depth (msw)||Depth (fsw)||Bob (OC air)||Mike (OC Nitrox 32)||Nick (Explorer)|
|24||80||30 ND||38 CAP||55 (60)|
|TBT||TBT||47 min||42 min||60 min|
The Third Dive
Finally the boat is anchored above a reef that has a constant 18 metres / 60 feet depth, and the Captain has told the divers that they no longer need to adhere to the maximum 60 min bottom time. All of our divers have switched to their clean tanks, and Nick has renewed the Explorer’s canister absorbent. The result is that with no constraint on bottom time, as can be seen in Table 3, Nick has more than twice the bottom time with the Explorer over the open circuit divers.
|Depth (msw)||Depth (fsw)||Bob (OC air)||Mike (OC Nitrox 32)||Nick (Explorer)|
|18||60||46 CAP||46 CAP||112 (IDEAL)|
|TBT||TBT||51 min||51 min||117 min|
We don’t see dramatic differences during the first two dives largely because of the boats 60 min bottom time limit. However, Mike by using Nitrox has had more time at the greatest depth as well as Nick using the Explorer. It is worth remembering that Nick has an average diver’s SAC of 0.75 cu ft/min whereas Bob, the elder, and Mike by caveat was granted the same low SAC rate of 0.5 cu ft/min. Finally, on the third dive where the Captain removed the 60 min bottom time cap, the Explorer had more than twice the bottom time as the open circuit divers. It is worth noting that while the Hollis Explorer is semi-closed, during the entirety of these dives the exhaled gas never reached 10% of that of the open circuit divers.
Based on this example, the Hollis Explorer rebreather has a significant advantage over open circuit divers even those with exceptional breathing control that is 2/3 the SAC of the average diver such as Nick. The Hollis Explorer is not accurately characterized as “no bubbles.” However, the undeniably aesthetic appeal of quiet that is less than 10% of open circuit divers is of considerable potential value to any diver in addition to the increased ability to interact with wild live.
In 1989, the term “dive computers” was first coined to describe expensive and exotic devices that were known by no less than 27 different names. Less than ten years later, the term was common place and the majority of divers dove with one. I will be surprised if another ten years go by before the same cannot be said of the expensive and exotic devices known as … rebreathers, and for recreational divers, rebreathers similar to the Hollis Explorer have the potential to be the standard for the future.
1This article is an updated version of an original article by the author that appeared in the magazine Discover Diving in February 1997.
2Surface Air Consumption
by Mark Phillips:
War is one of mankind’s greatest failings and perhaps one of the greatest instigators of invention.
In war men fight. Ships sink. When men learned how to extend their time underwater to salvage sunken vessels it was their nature to consider the concept of an underwater warrior. Those soldiers would have to be comfortable in most any condition of water. They would have to be able to swim great distances and still have the strength and stamina to carry out their mission and still escape unharmed.
They would need specialized breathing equipment that would allow them to breathe underwater. In order for the apparatus to allow for long range penetration of enemy held areas, it had to reuse the exhaled breath of the diver while preventing exhaust bubbles from escaping and giving away the position of the diver. It had to offer extended time underwater, be light weight, versatile and dependable. It had to be a rebreather.
On the morning of June 6, 1944, Operation Overlord commenced. It was an event like none other and was divided into many parts, each of which had to work with the others to be successful. It was also the beginning of Operation Neptune and began the invasion of France at Normandy. It was D-Day. It was the largest amphibious invasion in world history.
Those landing from water were to land on one of five beaches code named Utah, Omaha, Gold, Juno and Sword. The hard thing about such an invasion is that it is hard to hide. When those occupying the land do not wish to be invaded, they resist.
Fortifications, hedgehogs, steel and concrete spikes, some steel tetrahedral, mines and other hazards had been placed on the beaches and in the water. Guns were placed on heavily fortified bunkers perched on the hillsides. Fortified machine gun nests had overlapping fields of fire. And the guns of Pointe Du Hoc could rain down hell on both Utah and Omaha beaches and sink vessels at sea 15 miles out.
Before the landing craft could land the fortifications blocking their way had to be destroyed. Naval demolition teams were responsible for those obstacles underwater and the Army engineers above water. But plans never quite work out the way they are supposed to.
The weather changed. Conditions worsened but once started Operation Overlord could not stop. The degrading weather cost them time and high tide was missed. Because the tide was out when the demolition teams made it to their objectives, most of the obstacles were out of the water. The naval group took those seaward while the army teams placed explosives on those closer to land. On D-Day, they were not all referred to as frogmen. Those from the Royal Navy were Landing Craft Obstruction Clearance Units. More commonly called Lockyews.
|Landing Craft Obstruction Clearance Units
The ‘LCOCUs’ were a vital part of the D-Day invasion forces in Normandy. Four Royal Navy and six Royal Marines units comprising 120 divers wore newly developed neoprene suits with ‘blast proof’ kapok jackets underneath, helmets, breathing apparatus and fins.They laid the foundations for the Very Shallow Water (VSW) and beach clearance techniques in use today.Those remaining after the war were eventually incorporated into the Clearance Diving Branch.
A RN demolitions team was working Gold beach. When they arrived, they found that the obstacles they were to clear were underwater. Each of the hedgehogs they were to clear had to have 36 small charges placed at strategic positions so that the steel would blow into pieces with none more than 18 inches above the bottom. Each of these obstacles was covered with pressure sensitive explosives designed to punch holes in water craft.
Lt. Hargreaves described the experience:
“We must have been about four hundred yards from the beach when the firing first started, and they didn’t forget to inform us that they knew we were coming. When we finally got on the beach we discovered that we were being systematically sniped, not only with rifles but also by odd bursts of machine-gun fire – a most unpleasant experience”.
On another beach one of the men described his experience like this:
“We were spotted from a tower ashore and were subjected to pretty heavy mortar fire during which a petty officer was killed and two men were wounded. Later the R.A.F. blotted out the tower and things were more comfortable although shells still kept coming over. One shell destroyed our breathing apparatus, which we had not been using as the tide was low. When the water came up later, Leading Seaman A. Robertson and myself tried staying underwater by holding our breath. We blew about fifteen obstacles in this way, but we couldn’t keep it up. We carried on the next morning, after sleeping in a R.A.F. crater, where incidentally we were subjected to fire from an 88mm gun.”
Dennis Shryock was 21 years old when he landed on Utah Beach. Dennis had been trained as an explosives specialist and one of those elite men who were the forerunners of our modern day Navy Seals.
The machinegun fire was deadly. They did not have the protection of being underwater and had to wade to each of the obstructions to place explosives. He said the water “looked like pure blood.” But they did the job.
According to navy statistics, at Utah, the demolition teams lost six men and had eleven wounded. Omaha beach did not fare as well. They lost thirty-one and another sixty were wounded.
|Pointe Du Hoc
The water was rough from the stormy wind and the unexpected rain soaked equipment that was intended to remain dry. It took the landing craft longer to reach the beaches than expected. The plan was to hit the beaches at high tide; for the troops to be able to take shelter in the bomb craters as they made their way up the beaches. But they missed high tide. Most of the obstacles placed to keep the landing craft away were on dry land. When they were able to land, the troops had to run through wet sand 300 to 500 yards just to get to the bomb craters. Landing craft that had been fortified with bullet proof plating caused the crafts to ride much lower in the water. Too many of them were swamped and sunk before they could reach shore. The majority of the soldiers on board, weighted by 70 pound of equipment, drowned. Those that survived had to face a wall of bullets and artillery shells. The guns at Pointe Du Hoc had to be taken. The 2nd Ranger Battalion had trained hard in preparation for this day. They had practiced climbing cliffs and had brought along firefighting ladders and rocket propelled grappling hooks to help make the 100 foot climb. But it had rained.The ropes were wet and the propellant used was calculated with dry rope. The ladders were hard to foot and difficult to climb. German machinegun fire was held to a minimum by sharpshooters on the ground but they could not stop them from dropping hand grenades in an effort to keep them from climbing. The 2nd Ranger Battalion clawed their way to the top using footholds in the mud and rocks and bayonets driven into the cliff side when necessary. It took them twenty minutes to make the climb and take the Pointe. They held it for two more days before reinforcements reached them. Of the 200 men that started, only 90 were left in fighting condition.
What exactly was the breathing apparatus used by the USN frogmen and the Royal Navy Landing Craft Obstruction Clearance Units? It was able to supply an extended amount of breathing gas underwater by reusing the exhaled breathe of the diver. It did not allow exhaust bubbles to escape and was stealthy. It was lightweight and maneuverable. It was a rebreather.
While only remotely similar to rebreather units today, the ones used in 1944 were many generations of development old. In fact, the Italians started the concept of underwater assault teams using specialized equipment. And they recognized the need as early as World War 1.
In 1918 two members of the Regia Marina (Royal Navy) literally rode a torpedo into a harbor and sank an enemy ship. At the time, they had no breathing equipment and had to guide the torpedo at the surface in order to breath. They sank the ship but were captured when they tried to swim away. These human torpedoes became more like mini submarines and were human guided. Obviously the ability to be under the water, able to breath and stay stealthy was an advantage.
By 1941 the Italian navy had both a surface unit that operated fast, explosive motor boats and a subsurface unit that used manned torpedoes. Within this group they also had assault swimmers. It did not take long before other countries developed their own versions.
The idea of an underwater warrior is older than modern history. The functional ability to use such a warrior has always been limited by the inability to breath underwater. Throughout history, man has found ways to extend his time underwater and by the turn of the 20th century, some of the first closed circuit rebreather systems had been experimented with and used. The rebreather units used on D-Day were rudimentary compared to the modern versions we see today. But at the time, they did the job and those who used them had to be beyond courageous.
On June 6, 1944 those underwater warriors had a mission to do and short of being killed or captured, that is what they did.
Mark Phillips is a retired 33 year career firefighter and Public Safety Diver; A Master Scuba Instructor an ERDI trainer, and Publisher of PSDiver Monthly, an Internet magazine dedicated to advancing the safety and knowledge of the Underwater Investigator.
by Steve Lewis:
As if cave diving isn’t challenging enough, how should we feel about adding a rebreather to the mix?
When asked, which happens from time to time, I’ll explain to anyone who’ll listen that the easiest way to really give your diving skills a workout is to enroll in a cave diving class. The customer feedback from folks, who take this piece of advice, and dive into a technical overhead program, usually makes extensive use of the words “humbling” and “embarrassing”. The phrase: “brought me down a peg” or something similar often makes an appearance too.
Cave diving, and to some extent Advanced Wreck Diving (i.e. wreck penetration), is fundamental to technical diving. Most of the information covered and the majority of skills and techniques taught in any technical diving program have their foundations in basic cave diving. The presence of a rock ceiling, rock walls, and a rock floor (often covered in a deep layer of fine-grained mud) tends to focus the mind and put a special meaning and strong emphasis to the sage advice that bailing out to the surface is not an option. As any technical diver will tell you, it’s very unwise to bolt for the surface on any dive, especially one that’s incurred a decompression obligation, but in a cave several hundred metres or feet from open water, that option is completely off the table. Problems of all shapes and sizes have to be fixed at depth.
One result of not being able to surface at will, is the cave diver’s conservative approach to gas management: specifically, carrying enough gas to get them and a buddy back to safety in the event of the most horrendous equipment malfunction at the back of the cave. The Rule of Thirds, the starting point from which cave divers traditionally begin their gas volume calculations, is the ubiquitous gas management technique adopted by virtually all technical divers.
Also, the techniques developed and refined by cave divers operating in North Florida and the Caribbean for communications, propulsion, equipment selection and configuration have to a great extent become the best-practice defaults for almost every technical diver around the world.
Furthermore, it’s long been accepted that the standards required for cave instructors (and their students) to earn their certifications to teach (or dive) in caves, are among the most stringent. Broadly speaking, the consensus is that cave divers and the men and women who certify them, are among the most meticulous and squared away of any group of divers.
So, what happens when we take the rigors of a cave diving course and apply them to a new program for which the core life-support systems have been changed from open-circuit to closed?
To begin any comparison, it’s fair to say that TDI’s training department and advisory panel thought long and hard about the best ways to evolve its successful cave diving curriculum to include the special needs of closed-circuit rebreather diving. I was not at head-office for the whole of the development process, but I know it was the work of a larger development team than any previous program. Which is hardly surprising given the magnitude of responsibility to “get it right” when combining the complexity of a rebreather with a supremely challenging underwater environment. Hardly surprising and somewhat comforting!
Given that, let’s look at what they came up with!
The basic shape of most cave courses is the same regardless of what type of gear the diver opts to use. The first step is Cavern Diver. Graduates from Cavern can move up to Intro-Cave Diver; and once that level is achieved are able to sign-up for Intro to Cave and Full Cave courses.
In the briefest of terms, cavern divers are severely limited in where they can venture; intro-cave divers have to stay on the permanent main line or gold line and are not allowed to make any jumps to side passages; and full-cave divers have a license to learn in most of the cave’s main and secondary passages.
The progression has stood almost unchanged since the first organised cave diving programs that pre-date the formation of most of today’s mainstream certifying agencies… in other words, it’s a progression that’s stood the test of time and held its value well. It then follows logically that TDI’s CCR Cave program follows this same structural paradigm.
WHAT’S A CAVERN?
I don’t think there’s any real confusion about where open water ends and a cavern begins: if you cannot swim straight up to the surface and fresh-air, you’re in an overhead environment. If the ceiling is wood or metal, chances are that you are inside a wreck, and if the ceiling is rock, you’re in a cavern.
There might be more confusion about the other end of the cavern and where exactly it turns into a cave.
The standard definition is that the primary source of light in a cavern is daylight. If you and I swim into a cavern and lose sight of the entrance and daylight, we have exited the cavern zone and entered the cave proper. And for the record, there are no caverns at night… and some cave systems do not have a cavern zone to speak of at all. (The Eagles Nest system in Florida as an example.)
That definition does not change for rebreather divers, but there is a subtle change that fundamentally sets up one of the challenging limits for overhead training on any CCR.
One absolute limiting factor for all open-circuit divers is the volume of gas they and their buddy or buddies are carrying. That volume (X litres or Y cubic feet) helps to define just how far they can travel into an overhead environment… given that they follow the established guidelines for gas volume management.
In TDI’s open-circuit (OC) cavern course, penetration is limited to one-third of the volume of a single diving cylinder or one-sixth if the divers are using double cylinders. This is somewhat further defined to explain that the available volume for penetration for the whole dive team is set by the team member with the smaller cylinder or who has the smaller(est) starting volume.
The same volume limit is suggested for OC intro-to-cave graduates.
This limit very effectively helps to “police” or control new cave divers’ return access to open-water and safety. Since running out of gas is #1 on the list of things to guarantee a cave diver is going to have a bad day, the one-third in a single / one sixth in twins guideline goes some way in keeping new cave divers from venturing too far into the cave.
But a fully functional CCR does not have the same sort of built-in restriction. Certainly both diluent and oxygen supply is limited but those limits are measured in hours rather than minutes.
Let’s take the oxygen supply as an example. (Forgive the use of SI units but cubic feet are more complicated and unnecessary to get the point across. If you are only used to American Customary Units, just think of litres as quarts.)
We’re taught that the average per minute oxygen consumption rate for a diver is 1.5 litres. This volume is depth independent. And unlike their OC breathing brother and sister divers, for a diver on CCR, it really makes little difference whether the consumption is measured on the surface or at advanced trimix depth. One’s consumption rate will vary a little with workload, but 1.5 litres makes a pretty good average to work with. For now, let’s make life simpler and a tad more conservative, and use a consumption rate of 2.0 L/min. This is really quite high, but two litres a minute makes the arithmetic even easier than it would be at 1.5.
Now the smallest rebreather tank in common use has a wet volume of about two litres. That means every full atmosphere of pressure in that tank equals two litres of gas. In other words, a fill of 200 bar means there are 200 X 2 litres of gas. That’s 400 litres of gas. Quick math… at two litres a minute consumption, this volume of gas will last up to 200 minutes!
Even if we follow a sort of rule of thirds and suggest a CCR diver only use one-third of his or her starting volume of oxygen, one third of 200 minutes is more than an hour.
This means that if a beginning CCR cave diver follows the same gas rules as an OC diver, he or she can swim into the cave for an hour before having to turn the dive on gas volume! An hour of swimming into a cave usually translates into about an hour swimming out. Sometimes the flow helps to make an exit a little shorter, but an hour would be a fair estimate.
I think even those of us who have zero cave experience will begin to see the potential for a huge problem with this scenario.
If we were to line up the special concerns of those who teach CCR cave diving, at the front of the queue would be: a rebreather is essentially a potentially wicked cross between a time machine and a gas extender. What makes it potentially wicked is that compared to the classic North Florida set of twin steel tanks (even the big ones) the most inexperienced diver can wander deeper in to a cave system… much deeper than he or she should. If something bad happens, an hour is a long swim nursing a problem.
The “magic bullet” designed to help avoid this type of event centers on bailout gas.
Bailout gas is what a CCR diver carries for contingencies. Should the rebreather become completely inoperable, then they stop using it and start breathing from a tank of compressed gas using a scuba regulator. In other words, they fall back on good old-fashioned open circuit.
Some time is spent in the foundation dives for cavern and intro-cave CCR programs working out how much bailout gas each diver must carry, and how far from the surface that gas allows them to venture.
The calculations for this distance are based on a consumption rate effected by a carbon-dioxide breakthrough on the rebreather. A breakthrough such as this would probably result in a diver breathing like a racehorse on the final furlong of the Preakness. Therefore, the calculations are conservative and the guidelines they offer for penetration are written in stone: a sensible diver would never dream of compromising his safety by ignoring these guidelines.
Is your head spinning yet?
The truth is that the task loading for a student taking a CCR Cave class is really high. In addition to the gas management “thing” they have to master all the skills expected of an OC cave diver. They have to run line, place line markers, read the cave, overcome current, learn navigation, perform lost line drills, lost buddy drills, show their instructor perfectly executed bottle swapping in zero vis, and prove they can swim without kicking up a curtain of silt. And when that’s finished, they need to come up with strategies for rebreather-specific issues. They have to run their CCR manually, in SCR mode, they have to deal with depleted diluent, low oxygen, stuck solenoids, and a raft of other “fun” challenges!
Is your head spinning now?
The truth is that I dive CCRs in caves by choice. I believe that all things being equal, a rebreather is the right tool for cave exploration eight times out of ten. (Sidemount covers the other 20 percent!) Like so many high-risk activities, the pay-off is high-value. It’s also a class I love to teach because it is such a challenge and students walk away with a justified sense of accomplishment.
Is Cave CCR the ultimate challenge in diving? I know Brian [Carney, president of TDI] and the team in TDI’s training department well, and I am sure they have other cards up their sleeve; but as it stands, I cannot think of another program that tests a diver’s mental and physical stamina more than this course.
Is it fun? Yes it is. Is it useful? Certainly. Is it tough? Sure thing. Should you start planning to challenge yourself? Well, I don’t know if you’re ready but if you think you might be… Go for it!
Achieving “neutral” buoyancy takes a little work and practice but that perfect state when a diver works out how to eliminate the pull of gravity and fly weightlessly through the water is one of the most enjoyable things about diving. And the wonderful thing is, its part of the basic skills learned in the beginning. Really, there is nothing to it… Well, nothing to it after a few tries on open-circuit (OC). Things change and all bets are off when an experienced diver who prides him or herself on their control and finesse in the water column plugs themselves into a rebreather for the first time!
Let’s recap a few things before we look in more detail about the issues that make divers rethink buoyancy control when they switch from OC to a rebreather.
So-called neutral buoyancy is achieved by adjusting the volume of you and your kit so that the whole “unit” displaces a volume of water weighing exactly the same as you and your equipment. If the volume of water you displace weighs less than you and your kit, there’s some weight “left over” and you’ll sink; if it weighs more, there’s buoyancy to spare and you’ll float. Archimedes Principle pure and simple.
Most people float. Our bodies contain a lot of water and are essentially a very similar density to water. Of course some parts are heavier – things like bones, teeth, muscles, and so on – while other parts are lighter – the air in our lungs, sinuses, ears, digestive system for example. Fat is important too. In fact it is the most important factor because usually there is a fair amount of it, it’s lighter than water and it therefore has a great influence on whether we sink or float naturally. Simply put, lean people are sinkers, and the rest of us float with varying degrees of ease. I read somewhere that the average person needs to add one to three kilos (about two to six pounds) of ballast to be neutral in a swimming pool or fresh water.
And as we know, if a fully kitted-out diver perfectly balances their buoyancy in a swimming pool or a fresh water spring, lake or river, they will float in the ocean. They will displace exactly the same volume of water in both situations, but salt water is around three percent heavier than fresh and consequently generates a greater buoyant force. Because of this, we know that we must add extra ballast to help control our buoyancy when we switch from fresh water to salt.
One other factor to consider is that volume of water a diver and kit displaces changes as they change depth due to compression.
A neoprene wetsuit or drysuit (and to a much lesser extent, a compressed neoprene drysuit) compresses as the pressure at depth increases. In effect, the buoyant effect of these things is lessened. The degree of change is relative to the thickness of the neoprene being worn and how much of it there is, but there will be some change and most will occur relatively near the surface.
To compensate for these slight variations, divers add gas to their flotation devices (BCD, Wing, or Floatation Cell). Drysuit divers must also add gas to compensate for increase pressure squeezing the small volume of gas trapped inside their suit. (For the record, all divers must compensate for depth by adding gas to the very small volume of gas trapped in their mask, although this does not have any effect on buoyancy, just comfort.)
The final factor involved with buoyancy control is the gas in a diver’s body. The volume of gas in their ears and sinuses does not change as they dive, but its density does as outside pressure increases with depth; and divers learn how to equalize those regions. If they fail to do so, they will suffer a ‘squeeze’. There is also gas in the stomach and intestines. It too compresses with depth and returns to its original volume when the diver surfaces, but the volume of this gas is usually too small to notice any change in buoyancy.
But this brings us to the gas in a diver’s lungs. This volume is a large enough to make a real difference to buoyancy, and this is the key area of difference between open-circuit and closed-circuit diving and buoyancy control.
When an open-circuit diver achieves a perfect balance between buoyancy and gravity, she will ascend when she breathes in and descend when she breathes out. Also, rather than taking full breaths every time, she can control this effect by taking partial breaths and breathing with her lungs almost empty or with her lungs nearly full.
This simple “trick” and buoyancy fine-tuning is one of the most difficult for newer divers to master without some guidance. New divers are usually a little nervous and tend to swim their whole dive with their lungs more full than normal. This translates into a need to wear more lead to achieve a balance with their buoyant effect. As this diver gains experience and learns to relax, they will “operate” with more normal lung volumes and will be able to drop a few kilos/pounds of ballast.
Experienced divers understand these small subtleties and will adjust their buoyancy by adjusting their breathing. For example, when an experienced diver wants to rise a little during his dive, he will take a deeper breath; or he may breathe out fully to go under an obstruction.
This aspect of finessed buoyancy control is different with a rebreather. A rebreather diver has all the same general “concerns, controls and influences” as her buddy on conventional scuba. However, she also has the gas inside the loop and counterlungs of her rebreather to contend with as well. And that is where things can get confusing for new closed and semi-closed circuit divers regardless of how much open-circuit experience they bring to the table
Just in case you’re one of the couple of dozen divers who has NOT been inundated with information about rebreathers at some point in the past couple of years, here’s a quick primer on their workings.
A rebreather is an underwater life-support system that carries away the exhaled gas breathed out by its user via a mouthpiece, one-way valve and various large-bore hoses (called the loop). Then it removes the carbon-dioxide – a by-product of the diver’s natural metabolism – from that exhaust gas using the compact little chemistry set at the core of the unit (called the scrubber), replaces the tiny amount of oxygen used by the diver to stay awake and active (this quantity, an average of only about one to two litres per minute, is interestingly not influenced by depth), and finally the rebreather serves up clean, re-oxygenated gas back to the diver via more hoses and another one-way valve and the mouthpiece.
The metabolized oxygen can be added to the loop via a computer-controlled valve/solenoid, via a constant flow orifice, via an adjustable-flow orifice, via a simple manual button similar in function to the manual inflate button on a BCD or wing or a combination of all four depending on the make and type of rebreather.
Also part of this system for re-circulating and processing gas are a couple of flexible bags called counterlungs (some types rebreather only have one counterlung, but let’s focus on those with the more conventional pair of counterlungs for now). One counterlung is on the exhalation side of the carbon-dioxide scrubbing chemistry set and is called the exhalation counterlung, while the other is on the opposing side and is called the inhalation lung.
The loop, counterlungs, scrubber, mouthpiece and all the paraphernalia which joins them, is gas and water tight. During normal operation, these flexible bags “flex” as the diver breathes in and out, but the overall effective displacement of the diver and her kit, is unaffected.
So, when a rebreather diver exhales, there are no bubbles because gas is not released into the water but redirected to flow through the various stages and regions of the rebreather. There is essentially no change in the volume of gas being pushed around the apparatus. Since there is no emptying and refilling of the diver’s lungs from an inflexible metal high-pressure cylinder – the walls of which DO NOT flex – there is no change in the buoyant effect of the additional air in the diver’s lungs.
The diver’s lungs are essentially part of the rebreather loop and this maintains a fixed volume of gas buoyancy remains unchanged during normal operation.
Every new rebreather diver spends some time getting used to this concept. They swim towards an object, take a deeper than usual breath expecting to rise gently and bump straight into the object that they were trying to avoid.
In addition to remembering that on a rebreather, depth of breathing does not control buoyancy, there are two related things worth noting.
The first is being correctly weighted and NOT over-weighted. Using conventional scuba, every breath exhaled into the environment makes the diver and her kit slightly less heavy. Quite apart from the buoyant effects of the lungful of gas disappearing as a stream of bubbles on their way to the surface, all gas has some mass. A litre of air weighs a more than a gram and at a depth of 30 metres (around 100 feet) it’s not unusual for a diver to “consume” 50 to 60 litres every minute. Over the course of an hour’s dive, the weight of gas consumed by an open-circuit diver can make a considerable difference to the balance between the forces of buoyancy and gravity. On a technical dive, it is not unusual for a diver to use several kilos worth of gas. Consequently, the ballast they carry has to help compensate for this “Buoyancy Shift.”
Many OC divers, essentially begin their dives over-weighted. A rebreather diver does not have to consider or account for much buoyancy shift and therefore, should begin the dive correctly weighted. This will help with control throughout all phases of the dive.
Secondly, with the potential to have to manage gas volumes in the wing, drysuit and rebreather loop, it’s recommended to maintain just enough gas in the loop for a full breath and no more. This one-breath volume is the simplest to maintain and control. Having more gas than is required for a single breath adds complexity to an already complex management skill.
In simple terms, if a rebreather diver feels the slightest tug of resistance drawing a full breath from the loop, the loop volume is probably optimal!
As with all in-water skills, it helps to understand the principles at play and the factors making each skill necessary. Then once one understands those issues, practice is the key. And on that score, the best way to practice is with a mentor.
For more information on TDI courses offered, visit httpps://www.tdisdi.com/tdi/get-certified/
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With airlines tightening luggage restrictions, packing for a dive trip is hard enough with just recreational gear and traveling with a rebreather adds another level of difficulty. What to bring, how to pack it, will the dive center have everything I need when I get there? These are all questions that need to be addressed, but if you tackle them one at a time, you’ll realize traveling with a rebreather can be very simple.
“What should I bring?” This question goes hand in hand with “will the dive center have what I need when I get there?” The first step in determining what to bring on the plane with you should be finding out what the dive center has available. Most dive destinations around the globe now have at least one or two “rebreather friendly” dive shops. It’s very important, however, to call and verify that they can accommodate you. Do they have the correct cylinders for you? Do they stock sorb? Do they have bailout cylinders available? Do they have high pressure O2, and can they blend the diluent you need? It is very crucial to ask these specific questions, as many dive centers advertise themselves as “rebreather friendly,” but in reality are just “rebreather tolerant.” Once you know for sure what the dive center is able to provide, the next step is figuring out what you need to bring. If you are traveling to a remote destination, you may experience a bit of sticker shock when you see what they will charge for sorb and cylinder rental. It’s important to remember that many remote locations (especially islands) incur huge shipping charges and import taxes, and these costs are often passed on to the end user. It may seem cheaper to bring your own cylinders and sorb, but this typically ends up being more hassle than it’s worth. We recommend traveling light and supporting the local dive center by renting/buying from them.
“How do I make all THIS fit in THERE?” It can seem like a daunting task when all your dive gear is laid out in front of you, and you have only a few small bags to fit it in. However, there are a few tricks to helping you get everything you need to where it needs to go safely. Try to carry on as many of the critical components as possible. Things like the head, canister, loop, counter lungs, mouthpiece/BOV, regulators, and electronics can easily be damaged/lost in checked luggage and leave your unit inoperable, so it is best to carry them on. Things like wings, harnesses, fins, masks and exposure suits are pretty resilient to rough baggage handlers and can usually be rented at your destination if they go missing. If you must bring cylinders and sorb with you, it is typically best to check them. Just be sure to include a Material Data Safety Sheet with the sorb and remove the valves from your cylinders. You are required to leave the cylinder openings unobstructed so they are easily inspected; agents have been known to simply confiscate/dispose of cylinders when this rule is ignored. It is always a good idea to photograph everything as it is being packed, this way you have evidence if something is lost or damaged by the airline. The fee for an extra bag is typically less than for an overweight bag, so it’s not a bad idea to bring along a small mesh dive bag that you can pull out and transfer gear into if you end up overweight at the ticket counter.
So everything is packed up, you’re at the airport, bags checked, and you’re going through the TSA checkpoint. As long as you remembered to remove any tools or knives from your carryon, things should go pretty smoothly. It can be fun to watch the look on the TSA agents face as your bag goes through the scanner, but after a quick inspection there usually is not an issue. Remember, they are just doing their jobs, and a rebreather head and scrubber canister looks pretty suspicious on an x-ray. We have found many TSA agents are now recognizing rebreathers, especially in popular hubs to dive destinations. Just assume that your bag will be inspected and plan a few extra minutes to allow for this.
So you know you’ve brought everything you need to enjoy a great holiday with your rebreather and all your critical rebreather components have made it onto the flight with you. Now it’s time to sit back, relax, enjoy the flight, and have a great trip.
Contact SDI TDI and ERDI
If you would like more information, please contact our World Headquarters or your Regional Office.
Tel: 888.778.9073 | 207.729.4201