Switching to the Dark Side: Rebreather Diving

Dear friends, it’s been a while.  That’s not because there hasn’t been any diving in the meantime, but simply because there has been so much going on over the past months that I kept struggling to find the time to sit down and type a few words about any of my underwater ventures.

The most exciting development that’s happened during this time is given away by the title of this post.  I have finally made the switch to rebreather diving.  For too long had I been eyeing the world of silent diving.  No bubbles, an optimised gas mix for any given depth, vastly extended bottom times with reduced decompression stop obligations, a totally different way of diving, reduced gas bills for eventual trimix dives, and the prospect of a new challenge, were all key factors in my decision to take the plunge (forgive the pun) into the world of rebreathers.  And therefore, I recently trained with the fantastic Dave Gration from Rebreatherpro-Training.

However, before venturing any further, allow me to spare a few words for the uninitiated.  “What is a rebreather?”, you might be asking.  Well, let’s take this step by step.

First things first: The Air you breathe

As a primer, recall that air contains 21% Oxygen and 79% Nitrogen.  (Actually, if we were going for precision, it’s 20.95% Oxygen and 78.09% Nitrogen, with the remaining ~1% consisting of other inert gases, mostly Argon [0.93%] and Carbon Dioxide [0.04%]  However, for the purpose of this discussion, we can round up the numbers to 21% Oxygen and 79% Nitrogen).

Fig. 1: The composition of Air

With every breath you take at sea level, your body metabolises (i.e. uses up for respiration) about one-fifth of the Oxygen content.  In other words, it metabolises just 20% of the Oxygen.  That effectively means that your body is really making use of just 4% of the total volume of the air around you (0.2 multiplied by 0.21 = 0.04).  That small volume of oxygen is converted to carbon dioxide in the process of respiration which is keeping you alive and kicking.  The remaining 96% of the volume of air that you breathe in (17% unused oxygen and 79% nitrogen) is simply breathed out again into the atmosphere when you exhale.  (Nitrogen and the other inert gases are not metabolised by our bodies.)

Fig. 2: During respiration we use 4% of the total volume of inhaled air.

Now consider what this means for scuba diving.  You take down with you a cylinder of compressed air, and for every breath you take in, you use up just 4% of that volume, with the rest wasted in the form of bubbles when you exhale.  And the waste is larger the deeper you go.  Why is that? The deeper you dive, the larger the pressure on your lungs from the surrounding water.  Therefore, you need to breathe in a larger volume of air from your cylinder, otherwise your lungs would collapse under the mounting pressure.  However, we said that 96% of the air volume goes to waste upon breathing out.  Now, 96% of a large volume is certainly a larger number than than 96% of a small volume.  So there you go, the deeper you go, the larger the waste.


Wouldn’t it be great if we could somehow capture the exhaled gas from our lungs and reuse it?  After all, we only use up a small fraction of the oxygen.  Well, the matter is not quite as simple as breathing out into a sealed container and then breathing back in from said container.  If I gave you a plastic bag and asked you to seal it over your mouth and keep breathing from it, at some point you’d suffocate.  There are two points to consider here:

(1) Some of the oxygen is being used up by our bodies for respiration during each breath cycle, so left unchecked, the oxygen would eventually become depleted.  We need to add oxygen as required to keep up the oxygen supply over time.

(2) As we mentioned above, the oxygen that our body uses up in respiration produces carbon dioxide – and carbon dioxide is toxic.  We need a way to somehow capture/absorb the produced carbon dioxide and not allow it to go back to our lungs upon breathing back in.

Fig. 3: The purpose of closed-circuit rebreathers is to replace used oxygen and remove carbon dioxide.

Performing these two functions is the primary purpose of closed-circuit rebreathers.  An electronic closed-circuit rebreather monitors the partial fraction of Oxygen in the gas mixture breathed by the diver, adds oxygen as required, and absorbs the carbon dioxide gas so that the diver does not breathe it back in.  Let us look at both functions in a bit more detail.

(1) Monitoring the Oxygen supply

The first function, that of monitoring the oxygen in the breathing loop, is achieved via galvanic oxygen cells.  These cells produce a voltage that is proportional to the partial pressure of oxygen (PPO2).  That voltage is in turn read in by a computer that determines whether it has to feed more oxygen into the loop or not in order to maintain a given PPO2 (as selected by the diver).

Fig. 4: The three oxygen cells in a closed-cicuit rebreather, with the solenoid that controls the flow of Oxygen visible on the right.

The diver is also relayed a reading of the PPO2 through a computer on the wrist and a heads-up display that displays/projects readings directly  into the diver’s field of view.  You might have noticed I said “cells” not “cell”.  In fact, it is common for a rebreather to be fitted with three oxygen cells.  Why not just one?  Well, galvanic cells can act up for a number of reasons (moisture and ageing being just two).  So we don’t rely on the reading of just one cell but at least three, so that we can make a well-informed decision should one cell (or more) deviate or misbehave.

Fig. 5: Two displays that allow the diver to monitor their PPO2.  On the left hand side is the Heads Up Display (HUD) which is mounted right in front of the diver’s mask, thus relaying the PPO2 value via direct display in the diver’s eye.  (See also Fig. 10 below.)  On the right hand side is the JJ-CCR Shearwater Petrel 2 computer controller. The all-important middle row shows the three oxygen cell readings.  (At the time the picture was taken, the rebreather was on the surface, so all three cells are displaying a PPO2 of ~0.21.)  The upper row is currently showing the diver’s depth, dive time, and surface interval.  The bottom row is showing the mode (closed-circuit), gas mix (Oxygen / Helium), No-Deco-Limit, and Time To Surface.
(2) Removing the Carbon Dioxide

Carbon dioxide scrubbing, on the other hand, is performed by means of a chemical known as sofnolime.  This chemical comes in the form of small granules and is tightly packed by the diver inside a canister.  Every breath the diver exhales goes through this canister where the carbon dioxide is trapped by the sofnolime.  For those interested in the chemistry behind this, the following reaction takes place:

CO2(g) + Ca(OH)2(s) = CaCO3(s)+H2O(l)

The end products of this exothermic (i.e. heat giving) reaction are Calcium Carbonate and Water.  A scrubber canister will feel warm to the touch after a dive due to this reaction that’s taking place.

Fig. 6: Granules of sofnolime packed inside a rebreather’s canister.


Advantages of Using Closed-Circuit Rebreathers

OK, so we’ve seen how rebreathers help you save gas.  That alone results in a number of advantages, along with a number of others.  Let’s mention a few here:

(1) Since with rebreathers you can be efficient and recycle and reuse the air you breathe out, you don’t have to carry as many bulky cylinders with you underwater for a given dive (we’re talking technical dives here; for simple recreational dives, the picture is a bit different – more on this later).

(2) This much smaller volume of gas nonetheless lets you achieve vastly longer dives.

(3) An advantage related to gas economy is that when doing deep dives and using trimix instead of air (i.e. a mixture of oxygen, nitrogen and helium that mitigates narcosis at depth resulting from breathing Nitrogen at a high partial pressure), gas bills are vastly reduced.  Helium is a very expensive gas, and if you reuse it you end up saving loads of money in the long run.

Fig. 7: The JJ-CCR rebreather.

(4) Since the rebreather is able to monitor the amount of oxygen in the loop, we can ask it to supply us with a fixed partial pressure to our liking.  Let us say we were scuba diving on regular open circuit instead of a rebreather, breathing normal air (21% Oxygen).  Then we’d be breathing oxygen at a partial pressure of 0.21 at the surface (i.e. PPO2 = 0.21).  At a depth of 10 metres, where the ambient pressure is twice that at the surface, the PPO2 would be 0.21 × 2 = 0.42.  At 20 metres (3× surface pressure), 0.21×3=0.63, and so on.  With a rebreather, on the other hand, we can choose to have a fixed setpoint of, say, 1.3, and breathe O2 at this partial pressure throughout our dive.  The rebreather takes care of adding the right amount of oxygen for a given depth to ensure that the desired PPO2 is maintained.  That means that we have an optimised breathing mix for any depth, which in turns means that upon our ascent, we’ll have less residual Nitrogen in our bodies that we have to off-gas (i.e. get rid off) during our decompression stops.  That, in turn, means less time decompressing and more time having fun at the bottom phase of our dive.

Fig. 8

(5) The rate of oxygen consumption by the diver is independent of depth (which is not the case in open-circuit scuba), and is determined solely by the basal metabolic rate of the diver and their work-rate (the higher the work-rate, the higher the consumption).

Fig. 9: The author diving the JJ-CCR and posing with a shoal of fish in the background while completing a safety stop.

(6) At a given depth, buoyancy does not change throughout the diver’s breathing cycle.  In open-circuit scuba, when the diver exhales the lungs contract.  As a result, the diver is now displacing a smaller volume of water than when their lungs were full of air, and therefore they start to sink.  (On breathing in, the opposite happens.)  However, with a rebreather, the exhaled air simply moves into a bag called a counter-lung, which expands to accommodate it.  Therefore, the overall volume remains constant all the time and the diver’s buoyancy does not change.

(7)  If you take a quick look at the equation above describing the reaction that goes on between the exhaled carbon dioxide and sofnolime, you’ll notice that one of the end products is water (H2O).  Moreover, as was mentioned earlier, this reaction gives off heat.  The combined result of these two points is that the recycled air is both warm and moist (as opposed to cold and dry).  Consequently, one’s breathing can be more comfortable (less dry mouth) and the diver is also kept warmer.

Why isn’t everyone using rebreathers?

Given the above advantages, you’d think everyone would be diving rebreathers by now.  How come this is not the case?  Here are a few reasons:

(1) Well, for starters, rebreathers are expensive machines.  The combined cost of a unit and the required training can easily set one back 10,000Eur.  Yes, you read that right – there are no typos there.  Unless you’re really set on diving, and in particular envisage doing the kind of diving that benefits the most from using a rebreather, the cost outlay can be downright prohibitive.

(2) They require significantly more pre- and post-dive care and maintenance than regular open-circuit equipment.  Before a dive, you have to pack the scrubber with sofnolime, assemble the unit, carefully check o-rings for damage and re-grease if necessary, calibrate the oxygen cells, and carry out an extensive pre-dive checklist.  The latter in particular is crucial for one’s safety.  Skip or overlook one step, and the rebreather could easily end up killing you without warning.  After a dive, you need to disassemble the unit, clean and disinfect the hoses, clean and disinfect the canister, throw away the used-up sofnolime, and refill if doing a second dive.  Clearly, rebreathers aren’t for you if rinsing out your regulator after a dive already feels like a chore too many!

Fig. 10: In this photo, the HUD’s 3 LEDs can be seen relaying the PPO2 to the diver (note the three lights shining into the diver’s right eye).

(3) You need to be very disciplined with yourself.  Compared to rebreathers, open-circuit scuba equipment is very straightforward, easy to use, and maintenance-free.  Moreover, a mistake on open-circuit can be much more forgiving than on a rebreather.  On open-circuit, you’re virtually OK as long as you have a gas to breathe.  On a rebreather, if you fail to monitor your PPO2, you could end up breathing a hypoxic (too little oxygen) or hyperoxic (too much oxygen) mix, both of which can kill you without warning.  In addition, an elevated work of breathing or a scrubber breakthrough can make your carbon dioxide level creep up until you get what is known as a CO2 hit – an insidious killer that can incapacitate the diver without much prior warning.  A high level of alertness and attention to detail is a must with rebreathers.  A cowboy attitude will most likely get you injured or killed.

(4) Annual maintenance costs can be higher than regular open-circuit equipment.  In addition to the usual regulator service, the galvanic oxygen cells need to be replaced on a regular basis (they have a finite lifetime), together with o-rings and (occasionally) other components (e.g. the solenoid that controls the flow of oxygen into the unit).  You also have to factor in the cost of sofnolime and oxygen fills.

Fig. 11: The mouthpiece of a JJ-CCR taken apart.

(5) Depending on your perspective, training is long and complex.  Before you can proceed to do 60m-range decompression dives, you have to do between one and two previous courses (rebreather diver and rebreather decompression diver, depending on previous experience) and accumulate a significant amount of hours and dives on a given unit.  It should be said, however, that this in itself is not too dissimilar to open-circuit scuba, in that prior to moving up the ladder you are rightfully expected to have attained a certain amount of experience and comfort at the previous level of training.

(6) While diving to gain experience for subsequent higher levels of training, you’ll be carrying much more equipment (i.e. kilograms on your back!) with you than with an open-circuit setup.  You can do a recreational dive on a single 12L cylinder.  If you’re doing a simple recreational dive on a rebreather, apart from the significant weight of the unit itself (which varies amongst units), you also have to carry a bailout cylinder that’s used in case of emergencies.  You can easily be carrying upwards of 50kg with you for each and every dive you make.  You won’t feel the difference underwater, but you certainly will on the ground!

Fig. 12: A bailout cylinder prepped for a dive.

(7) The diver has to have a sound knowledge of diving physics and physiology.  To a rebreather diver, this, I would argue, is even more important than prior open-circuit experience.  And whilst every diver should strive to have a good knowledge base, it has to be accepted that not everyone is ready to commit to the same level of reading material and classroom work.

(8) Diving a rebreather is akin to learning how to dive from scratch again.  You’re either happy with that or you’re not.  In my limited experience so far, I would say that your past open-circuit experience is useful in terms of your being comfortable in the water but not much else.  Deeply-ingrained open-circuit habits, such as fine-tweaking your buoyancy via control of your breathing, have to be eradicated the moment you dive a rebreather.

(8)  Whilst the inception of a rebreather is even older than that of scuba, dating back to the 19th century, its use has mostly been the province of commercial and military diving.  Development of the technology for the recreational diver market has not been as wide and fast as one would like.  For many years, rebreathers have remained amateur garage projects.  Recently, however, as rebreathers started gaining popularity, their market grew, as did investment in more reliable technology.  Nevertheless, one could argue that they are still far from being truly “commercial devices”, and some would go as far as to say that they’re either still in test-pilot phase, or are just emerging from that.

Fig. 13: Diver taking a quick glance at the dive computer to confirm PPO2 readings.

Choosing a rebreather

The growth of the rebreather market has meant that a number of unit options have now become available.  I will not dare enter the discussion of which rebreather is “the best out there”.  Suffice to say that the usual remark that there is no perfect rebreather is a very valid one.  In my case I proceeded as follows.

Firstly, I set out with a list of personal requirements and expectations, the most important amongst which were:

(1) Robust build.
(2) CAN bus-based communication protocol.
(3) Shearwater electronics due to their dependability.  (I’ve been using their products for a while and apart from being already used to them, I’ve always found them to be outstanding, as is their exceptional customer service.)
(4) Simple minimalist design adhering to the KISS (Keep-It-Simple-Stupid) philosophy. No frivolous and distracting “bells and whistles”, and no more complications than really required.
(5) Solid reputation and “proof-of-use” for a number of years by demanding divers.
(6) CE certification.
(7) Reasonable possibility to carry out field-repairs (rather than having to send the unit back for every problem that crops up).
(8) Good work of breathing (WOB).
(9) Good trim characteristics.  (A lot of this boils down to the diver, of course, but some units do trim out better than others.)
(10) Slim, streamlined design.
(11) Access to efficient repair/service if required.
(12) Access to a good, reputable instructor teaching given unit.

Secondly, I asked lots and lots of questions of a number of great exploration/expedition divers out there.  These are people whom I knew to be active divers – people who dive, explore, push boundaries, and teach on a regular basis.  (You all know who you are – and I thank each and every one of you again!)

The JJ-CCR fit my criteria perfectly, and the answers I sought agreed with my expectations. So after long deliberation, my choice was decisively made.  Some of your criteria and requirements might be different than mine, so your choice can end up being a different one for good reason.

Fig. 14: Two JJ-CCRs set up for a dive, with mine on the left shown from the back, and another one on the right shown from the front.  The back view shows the canister where the CO2 scrubber goes, flanked by two 3L cylinders containing diluent (air) and pure oxygen.  The front view on the right shows the inhale and exhale hoses.

If I may mention a couple of things you should NOT do:

(1) Relying on just a single opinion.  Some things are objectively true, and can be said to be brute facts.  However, many others are personal opinions that carry with them the baggage of bias.  That’s human nature.  Accept it and watch out for it.

(2) Taking opinions on online forums and social media as the gospel truth.  Quite often, they’re quite the opposite of that.  In my own experience, I’ve found so much information relayed on these platforms by so-called “armchair divers” to be downright wrong and misguiding.  In certain instances it was immediately clear that the information was wrong.  In others, it only became apparent upon my asking further questions to the people I alluded to above, most especially my brilliant instructor, Dave.

My Experience So Far

The short answer: It’s been great and there is no going back.  CCR diving has completely won me over.

If you’ve read all of the above, you’ll be seeking more than just the short answer.  So here are a few thoughts.

The training can be intense.  The aim is to be in the water with the unit as much as possible, so expect a minimum of two dives per day.  When we did our course, the weather was as bad as it gets in Malta, and the entire week was graced with near gale-force winds of variable and unpredictable direction.  On the last day, it was so bad that there was just a single diveable site on the entire archipelago that suited our purpose.  This also meant that we spent way more time than one normally would in travelling.  Immediately after each diving day came equipment cleaning and scrubber packing, followed by theory class.  They were long days characterised by early starts, late bedtimes, and a lot of sore muscles – many of which I barely knew I even had!  But it was huge fun and immensely rewarding – and having as brilliant, knowledgeable, patient, dedicated and gentlemanly a teacher and mentor as Dave Gration made a world of difference.  (No, I’m not being paid to say that.  Yes, I paid the full price for the course. No, I haven’t even told Dave I’ve said this about him.  I’m just that pleased with his instruction is all.)

Fig. 15: JJ-CCRs and bailout cylinders prepared for a day’s diving.

Emergency drills are easily forgotten if not practised regularly.  Even more important than memorising the order in which to perform a drill is the actual ingraining of muscle memory such that you instinctively perform that drill without deliberating when presented with an emergency underwater.  One of my hurdles to overcome has been overthinking, i.e. pausing to question (and flog to death the reasoning of) every step of a drill.  With the continued practice of important drills, you can keep them fresh in your mind, the repetition turning them into automatic responses.  To this end, I am making it a point to practise at least one emergency drill at the end of each dive I carry out.  It’s not a matter of whether I will ever need it but when.  At some point, the rebreather WILL probably fail.  The diver’s timely reaction to that failure is all that matters.

Fig. 16: The author preparing to stow back the bailout regulator, after having bailed out to open circuit while carrying out a drill simulating a hypoxia event.

Unit preparation takes time, as does proper cleaning and disinfecting.  I went in already prepared for this, so this did not come as a surprise to me, but it’s worth stressing.

As for the experience itself of diving a CCR, which I have purposefully left for last, all I can say is that it’s been exhilarating.  With the noise of bubbles all but gone, I can listen to the sounds of the reef and enjoy the true muffled silence of the underwater world.  I’ve had fish coming right up to my face, unfazed by my presence now that I am another silent being in the sea.  It’s as if they now consider me to be one of their kin.  The sensation of true hovering is pure bliss.  At the moment, I am still during that phase where I catch myself smiling each time I pin down neutral buoyancy at a given depth and then, irrespective of my breathing cycle, I just find myself staying perfectly fixed in the water column, effortlessly levitating in the deep, calming blue.

Fig. 17: The JJ-CCR rebreather after a technical decompression dive on the Karwela wreck in Xatt l-Ahmar, Gozo.

Quite frankly, I would say that diving a rebreather feels more “natural” to me than diving on open circuit (which is somewhat ironic given the increase in technological contraptions involved!).  My bottom times have, of course, increased significantly – and after a dive I’m warmer than usual.  Recently, I did a 70min dive in Dwejra, starting at the Inland Sea, finning all the way to the remains of the Azure Window and back.  A good amount of the dive (~20mins) was spent around the 30m mark.  I never approached my NDL time, not to mention the vast amount of scrubber time left had I wanted to dive for longer.  Rebreather diving has opened a completely new world.  The title of this post referred to rebreather diving as the “dark side”, but it has actually thrown a new light upon my diving experience, and I am looking forward to each and every dive I have yet to make on the JJ-CCR.  So much more to learn, so much fun to be had!  To many more dives!

I take the occasion to wish you all happy & safe diving, and a wonderful start to the new year!

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