Buoyancy and Rebreathers… Rethinking Your Dance with Archimedes


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.

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