You might have considered getting into diving on closed-circuit rebreathers but been deterred by all those incident reports and the apparent complexities of using the equipment.
CCR diving is not as difficult as it might appear, though it does involve a shift of mindset from open-circuit. But even if you have no desire to buy, train for or use a closed-circuit rebreather yourself, with so many now in use at some point you might well find yourself diving alongside another diver equipped with one.
But how can you buddy someone effectively when you don't understand the equipment he or she is using? What if something should go wrong, or appear to go wrong? That's why it's a good idea to have some idea of what CCR is all about. Here are the 21 Things You Should Know:
ONE The closed-circuit rebreather diver breathes by using his body as part of a closed loop that includes a counter-lung and scrubber unit. This loop is automatically topped up with oxygen to give the most appropriate mix of gas for a particular depth, thereby keeping inert gas content and decompression obligations to a minimum.
TWO By rebreathing his gas, the diver can make maximum use of the oxygen carried with him, unlike a traditional open-circuit diver who exhales most of his usable gas away into the water.
THREE The pressure of the oxygen content (ppO2) of the gas breathed is maintained as a constant, so the gas mix is continuously varied. For example, using air as a diluent and a target oxygen pressure set-point of 1.3 bar, the gas breathed at 40m is equivalent to nitrox 26, at 30m it is nitrox 32, at 15m it is nitrox 52 and so on. The no-stop time at 20m (nitrox 43) is around three hours. The CCR diver will either pre-plan his dive or use a CCR-compatible computer.
FOUR The rebreather unit has three oxygen sensors positioned in the loop after the scrubber unit. The on-board computer averages the closest two readings. A solenoid-switch opens and closes the valve of the oxygen supply to the breathing-circuit as needed.
FIVE The carbon dioxide that is exhaled into the rebreather by the diver is chemically removed in the scrubber unit. The CCR diver needs to know exactly what he is breathing. As there are little or no symptoms of impending hypoxia, the diver must rely on his computer hand-set display for this information, just as a pilot uses an altimeter.
SIX Rather than the percentage, it is the pressure of the oxygen content (ppO2) of the gas breathed that is of consequence when it comes to life support. The diver uses the same volume of oxygen regardless of the depth he is at.
SEVEN Because ambient pressure at the surface is only 1 bar, a rebreather diver must set up his unit at a set-point lower than one bar, and typically uses 0.7 bar, which will achieve 70% oxygen in the counter-lung at that time. He needs to change to the higher set-point (eg. 1.3 bar) by using a control on the unit's handset, once at depth.
EIGHT The mouthpiece has a valve on it which must be shut before the diver removes it from his mouth in the water, or water will enter the unit. The unit has a water trap, so can tolerate a small ingress of water, but if you are asked to handle a unit after a dive, keep it upright to save the CCR diver the job of having to clean the oxygen sensors afterwards.
NINE The CCR diver checks for bubbles at the beginning of the dive. This is because it is important to keep the breathing circuit as a closed loop. Even a leak from the mask can mean diluent wasted, because the gas volume of the breathing-circuit will have to be made up from the diluent gas supply cylinder.
TEN During descent, the rebreather is unlikely to need to add oxygen automatically to the mix, because the ppO2 of the breathing loop increases in relation to increasing ambient pressure. However, because his counterlung will become increasingly compressed, its volume needs to be added to with a diluent gas so that the diver can inhale comfortably. This diluent normally contains some oxygen, which adds to that already in the breathing-circuit and not metabolised. Most divers wait to change to the higher set-point until they are at depth. Some CCRs have an automatic diluent valve fitted.
ELEVEN Before diving, the on-board computer of the rebreather lists a series of pre-dive checks that should be answered by the user. The CCR diver will also have other pre-dive checks to do. Don't distract him at this time. An audible alarm sounds and will sound during the dive to attract the diver to his handset display if the ppO2 in the breathing circuit moves outside acceptable parameters. An alarm that continuously sounds before or during a dive denotes an inattentive and possibly endangered diver. Should the power supply to one handset (the master display) fail for any reason during a dive, the unit switches to the back-up or "slave" display.
TWELVE No bubbles should be seen venting from the unit during a descent or when at a constant depth. However, because of a fall in ambient pressure on the way up, there is a fall in ppO2 which causes the unit to inject more oxygen into the breathing loop. Added to that, the gas already in the loop expands. Consequently gas must be vented away into the water in the form of visible bubbles.
thirteen Assuming that a diver had an adequate ppO2 in his breathing-circuit to start with, at a set depth or during a descent it takes some time to metabolise the O2 down to dangerous levels. This means there is a big safety margin, so if there is any interruption to the oxygen supply the diver has plenty of time to take remedial action. He must keep an eye on his handset display.
FOURTEEN A CCR diver has the option of driving his unit manually, maintaining the ppO2 level within acceptable parameters. High or low levels of ppO2 are identified by the handset display and an audible alarm. Warnings of low levels of ppO2 should prompt manual addition of oxygen to the loop, while high levels will call for the loop to be flushed with diluent gas. If in doubt, the diver can always flush his unit with breathable diluent gas. Bailing out to an open-circuit rig is regarded as the final option when all else has failed.
FIFTEEN The diver's buoyancy is constant, whatever his lung-volume. Buoyancy must be controlled with a BC or drysuit.
SIXTEEN The three oxygen pressure read-outs (from the on-board sensors) visible on the handset continue to vary slightly around the chosen set-point figure as the diver breathes and metabolises oxygen and the unit automatically compensates.
SEVENTEEN The CCR diver must switch back to a lower set-point for the ppO2 (eg 0.7 bar) when nearing the surface on ascent, where a maximum pressure of only 1 bar is physically possible. Gas is vented off during the ascent.
EIGHTEEN The scrubber material in the rebreather is good for about three hours of safe diving but it needs to be replaced if it gets too wet. There is normally a small amount of moisture in the unit after use - this will have condensed from the diver's own breath.
NINETEEN A CCR diver uses only a very small volume of gas during a typical dive. Often he will surface with more than 200 bar still in his diluent cylinder. During an ascent the CCR diver requires no addition of diluent to the loop, which leaves some diluent supply in reserve for emergencies such as supplying gas to an open-circuit diver who is without an air supply.
TWENTY If the O2 supply or the electrical supply of a CCR is not turned on, the diver can still breathe from the unit until the contents of the counter-lung become hypoxic.
TWENTY-ONE The CCR diver must always know his ppO2. This is not a distraction. It is the same as a car driver being aware of his actual speed. After his training he should be capable of doing that in addition to watching where he is going.
This is the Ambient Pressure Inspiration CCR. The diver's exhaled air (red) is directed into the right-hand counter-lung, then over the shoulder into the base of the scrubber unit. Here a scrubber absorbs the CO2 and the gas then enters a mixing chamber in which three separate sensors measure the pressure of the oxygen content (ppO2). An electronic controller instructs oxygen from the cylinder (blue) to be injected into the mix, replacing that which has been metabolised by the diver and maintaing the ppO2 at the chosen setpoint. The breathing mix (blue) then passes into the inhalation counter-lung to repeat the cycle.
These divers are waiting for their rebreather units to calibrate before diving
This is the alarm siren. It sounds if the pressure of the oxygen content in the rebreather becomes too high or too low
The mouthpiece valve must be closed before the diver removes it when submerged, or water can get in
There are a number of pre-dive checks to carry out - it is important that the diver should not be distracted at this time
The inside of the rebreather unit, with the oxygen cylinder on the right, diluent on the left and the scrubber unit between them
The cover of the scrubber unit removed, to reveal the three separate oxygen sensors and the battery compartment
One of the two computers in "master" mode. Should its power fail, the unit should switch to the second, "slave" display
What are the advantages of closed-circuit rebreathers over ordinary open-circuit scuba equipment? CCRs are not just for deep diving - they confer great advantages in the normal 10-50m range:
- Gas supply is no longer a limiting factor on a dive, because the diver uses only the oxygen he actually metabolises. That's 1-2 litres per minute, regardless of depth (a 3 litre tank can hold 600 litres).
- The risk of decompression illness with conventional dive times is reduced, because the CCR diver breathes the most appropriate oxygen-rich mix for the depth he is at and therefore breathes less inert gas.
- There is little noise, and bubbles are given off only during ascent. This allows closer proximity to skittish animals and is ideal for naturalists and photographers.
- Without exhaled bubbles, less damage is done to any overhead environment. This is a concern where, for instance, there is rust deterioration to vintage steel wrecks.
- You always have the right gas for conventional diving, and with extended-range diving there is the convenience of not needing large numbers of tanks.