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Is Cave CCR the Ultimate Challenge in Diving?

by Steve Lewis:

rebreather in cave

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!

Buoyancy and Rebreathers… Rethinking Your Dance with Archimedes

buoyancyandrebreathers

Photo Credit Pete Nawrocky

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 https://www.tdisdi.com/tdi/get-certified/

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
Email: Worldhq@tdisdi.com
Web: https://www.tdisdi.com
Facebook: https://www.facebook.com/TechnicalDivingInt

Traveling With a Rebreather

rebreaterstuff
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

Email: Worldhq@tdisdi.com

Web: https://www.tdisdi.com

Facebook: www.facebook.com/TechnicalDivingInt