Closed Circuit Rebreather Training
by Dr. Jeffrey Bozanic

The gorgonians waft gently in the water, brilliant yellow crinoids spread their arms, and a multitude of fire engine red and forest green sponges adorn the submerged walls.  The fish swim lazily past, not even noticing our presence.  The silence of the underwater world is deafening. 

Suddenly, that golden serenity is shattered, by what sounds like a locomotive roaring down upon us.  “What is it?” we wonder.  Then, from the edge of the 100-foot visibility, we see the cause… a pair of open circuit scuba divers, swimming with their thunderous bubbles trailing behind them.  The fish scatter and flee.  And we, wearing our completely silent closed circuit rebreathers, feeling more affinity with the fish than the divers, turn to follow.

What is involved in learning to dive rebreathers? While training programs differ from one unit to the next, all programs have many similarities. Training is stringent, but enjoyable.  Even though you might be an open water dive instructors with over a thousand open circuit scuba dives, you must still take this training prior to being qualified to use a rebreather.  A daily log of my original training follows, to show an example of what is entailed in learning to use CCRs.

Day 1:

The first day is spent in lectures.  Class begins with a review of enriched air nitrox (EANx) material.  Because closed circuit rebreathers are, in essence, nitrox mixing machines, a good understanding of EANx theory and use is an important first step in learning to use CCRs.  Because of this, basic EANx certification is a prerequisite for the class. Other early topics include rebreather history, types of rebreathers, and a summary look at the various components that are incorporated into the design of rebreathers. 

The afternoon we all pre-dive the rebreathers we will use during the course.  This is not like open circuit scuba, where you just attach a BC to a cylinder, throw on a regulator, and dive.  Instead, we check the three oxygen sensors, set and verify proper operation of the primary and secondary sensor displays, confirm that the two gas regulators are functional, confirm battery power and life, fill the CO2 absorbent canisters, confirm the mushroom valves in the mouthpiece are working, check pressures on the two gas cylinders, confirm operation of the automatic and manual diluent and oxygen injection controls, and perform negative and positive pressure checks.  This takes all afternoon the first time, but as the week progresses we become more proficient and faster  at this task.

Day 2:

As we had pre-dived our units yesterday, we are ready to enter the pool first thing this morning.  Initially, we all swim around, getting used to being in the water on the systems.  The most remarkable thing to me is the silence.  No bubble noise at all, just a quiet swish as we inhale and exhale. 

Tiring of the abyssal depths of the shallow end of the pool, we descend to the deep end.  Our tasks are two-fold: work on buoyancy, and watch our oxygen partial pressure (PO2).  Buoyancy is odd. Unlike open circuit scuba, the gas volume of the diver and unit is fixed.  In open circuit, when the diver inhales, you rise.  In closed circuit, when the diver inhales, the gas is merely transferred from the counterlung (breathing bag) to the diver’s lungs.  Because the overall volume remains constant, so does the buoyancy.  To change buoyancy, we must either add air to the breathing bag by manually injecting gas from the oxygen or diluent cylinders, or we must inflate the BC.  This is surprisingly difficult to master, as the patterns I learned over many years as open circuit instructors are so ingrained in my subconscious that I do not even think of them anymore!

Oxygen partial pressure monitoring is critical to safety.  The composition of the breathing gas is verified with three different sensors every two minutes.  If the PO2 drops too low, the risk of passing out underwater exists (a bad thing!).   If the PO2 gets too high, convulsions might result (also a bad thing!).  Because the PO2 varies with depth, amount of oxygen consumed, and the amount and type of gas being injected into the breathing loop, it can vary significantly in just a few minutes.  Thus, it replaces the overall amount of gas carried by the diver as the important variable, and the PO2 sensors replace the SPG as the most important piece of instrumentation. 

The final skill we practice is a bailout procedure.  In the event of a catastrophic emergency, we need a way to return to the surface safely. As insurance, we carry a small (20 to 40 cf) pony bottle with its own regulator and gauge.  However, pony bottle use is not as easy as spitting out the CCR mouthpiece and inserting the scuba regulator in your mouth.  If you do not follow proper procedures, you can destroy all of the oxygen sensors and electronics.  This could be a VERY expensive mistake!

The day concludes with post-dive cleanup and maintenance on the CCRs, followed by the pre-dive checks for tomorrow.  It has been a long day, and it is off to bed we go!

Day 3:

Open water, at last!  On our first dive we descend as a group, reaching 80 fsw.  We work on two skills, buoyancy control, and switching to and from our bailout bottles, over and over again.  I can’t say we saw much of the reef, because as soon as we finished the skills, we ascended to the boat.

On the second dive, we sight see. We do not have any planned skills, except maintaining personal safety.  Our instructor leads us through the reef… up one sand chute, over this coral head, down that canyon.  Of course, every time we ascend, we have to add oxygen to the system.  And every time we descend, we have to watch that the PO2 stays below 1.4 ATM.  Up and down, up and down, up and down… No skills? Hah!!… sneaky person, our instructor!

On the third dive of the day, we relax a little.  While we practice some skills, the majority of the dive is spent looking at corals, fish, and invertebrates.  My buddy and I are swimming a few feet apart, when a twenty pound grouper swims nonchalantly between us.  Clinging to its back is a small remora.  It is as though we are invisible… this kind of event never happens when I am on open circuit scuba!

Afterwards we post-dive the units, so they are ready for the morrow.

Day 4:

The morning begins with a lecture on decompression tables. We find that special tables have been developed for constant PO2 exposures, based on the same EAD calculations learned in our basic nitrox class.

The next topic is design considerations of absorbent canisters.  I think, “How complex can this be?”  Two hours later, I know!!  The efficiency and effectiveness of carbon dioxide (CO2) scrubbing can be extremely variable, based on canister design.  Will a canister last one hour, or ten hours?  How much scrubber will be needed for this duration?  How difficult will it be to breathe through a canister (known as the “work of breathing (WOB)”)?  What will be the consequences of a system flood?  All are dependent, at least in part, on canister design.

Testing of CCRs follows.  We find that very few CCRs have had any testing done on them.  Yet, our very lives depend on the proper functioning of the systems.  Testing shows that the WOB and canister duration are sufficient to maintain life for the duration claimed as “safe” by the manufacturers.  They should be tested under high diver workloads, since that is when the WOB will become most critical. 

Our last lecture topic of the day is a comprehensive debriefing of yesterday’s dives.  We discuss problems, our inert gas status, buoyancy control, and gas usage.  Because we “flew” the units on manual, with all of the automatic electronics turned off, a major topic of discussion is our variation in PO2.  Several of us either let them drop below our given minimum (which could cause decompression sickness had the variations been sufficiently severe), or let them get too high (which could lead to acute oxygen toxicity).  Our instructor gives us our goal for the day: maintain a PO2 of 1.1 ATM, plus or minus 0.1 ATM.  It is going to be challenging!

After lunch, it is off to the dive boat for two afternoon dives.  The first is a no-decompression dive of forty minutes at 90 fsw.  The second is 60 fsw for a little over two hours (also no-decompression!). By the time we finish diving for the day, we have logged almost three hours underwater… on only six cubic feet (cf) of breathing gas! Considering that many of my research dives require me to carry up to eight 80 cf cylinders (640 cf of gas), I could already see an enormous potential for this technology in my own activities!

Day 5

Today we participate in the last dives of the course.  We dive on a Russian frigate, penetrating into some of the interior passages.  I notice another benefit of the CCR… no bubbles knocking silt and rust from the overhead ceiling. This could provide a significant safety benefit for other overhead environments as well, like cave diving.

During these dives, I notice how calm the fish are.  A ten-pound hogfish swims a foot to my side, leisurely nuzzling the sand for food.  Even the garden eels are so lax that we catch one with our bare hands!  It is as though we are not even here.

After dinner, we take the written examination.  This is a comprehensive test, covering all aspects of the course.  Three hours later, we finish.  We drop into bed exhausted, satisfied with the week.

Day 6:

While class is completed, I have an extra day to dive before departing for home.  I am glad that to have this last opportunity, as I want to try out my underwater video camera. What a day!  The CCR makes filming much easier, as I can approach marine life without scaring it away. Already I see the multiple benefits of this new tool, even though I have yet to really internalize all that has occurred this week.

As I perform my final post dive on the CCR, I reflect on what I learned… CCR operational design, pre- and post-dive care and maintenance, and in water use.  All this took just a little more time than an entry level scuba class would take.  (In fact, I probably would have learned some aspects even faster, had I not had to unlearn some of my “bad” habits from open circuit training first!)  This was time well spent indeed.  Even though I have gone through literally hundreds, maybe even thousands of hours of scuba training, I could not have started diving rebreathers safely without this continuing education.  And, once again, I have had a new horizon open up for me.