Closed Circuit Rebreathers
Rebreathers have come a long way since their first invention some 200 years ago (or 400 years if you go back to a design by Drebbel in the early 17th century). Recreational rebreathers have improved significantly making them much safer when used correctly and with appropriate training. One such closed circuit rebreather is the MKVI from Poseidon. The MKVI uses a prepacked CO2 absorbing cartridge which provides a dive time of up to 3 hours (and perhaps more) and an integrated computer for setting and monitoring the PO2 set point.
For the last few weeks, John Mckeon and I have been undertaking a course with Fil Gray so that we can be certified with TDI to use the Poseidon MKVI. Mike Loricco started the course with us but had to drop out due to bad ‘flu.
The course involves a combination of theory, which is conducted online, and intense practical sessions to learn how to assemble/disassemble the unit as well as a bunch of new underwater skills. As the rebreather is a life preserving device, extra attention must be placed on the assembly of the unit and even a small error could result in making the device lethal. Most important is the CO2 scrubber which is responsible for removing carbon dioxide out of the breathing loop. John and I had to create our own written checklists to use each and every time we assemble a unit for our use.
Each unit is made up of a breathing loop and two cylinders, one for O2 and one for diluent. The O2 is used to replace the O2 used as we dive. The diluent (air in our case) is used to maintain the volume of gas in the breathing loop and is mainly required during descent when the pressure increases and decreases the loop volume. It is also for losses from the loop such as mask clearing.
The breathing loop as the circulates the breathing gas through the CO2 absorbent removing the CO2 and keeping the gas breathable. The gas flows clockwise through the loop thanks to 2 one-way valves in the mouthpiece. Exhaled gas flows through the left side of the mouthpiece to a bag over the left shoulder known as a counter lung. The gas then flows from the counter lung over the back to the bottom of the absorbent canister, up through the absorbent and through the head where the sensors and injection valves sit. The gas then flows to the other counter lung over the right shoulder and back through the mouthpiece to be inhaled. The two counter lungs expand on exhalation and collapse on inhalation so that the total volume of gas in the loop and in our lungs stays constant – something that causes us a perhaps unexpected problem.
After we had mastered the unit assembly and passed the theory test we were then able to enter the water donning these amazing units. Despite the complexity of the unit and all its components its weight is much the same as a 11L scuba cylinder and I was able to get by with 2Kg less weight than I normally use with my 11.2L aluminium cylinder.
Our first in water sessions were held on a wet Sunday at Lilli Pilli. We were lucky to be wearing dry suits as we spent nearly 4 hours in water just over 14ºC. The first thing I noticed as we descended was just how quiet it is without the bubbles on every exhale. It wasn’t, however, completely silent as the unit is frequently calibrating sensors, adding diluent (to maintain loop volume) and O2 to maintain the required partial pressure of O2 (PO2). The unit a PO2 of 0.5 bar/atm at the surface and in shallow water and this increases to 1.2 bar/atm at 15 metres and deeper. So, as we were swimming around there’s all sorts of clicks, swooshes and other noises: nothing quite like the sound of bubbles and somewhat reassuring that every is working.
There are two major benefits from maintaining a PO2 of 1.2 bar/atm for deeper dives. First, 1.2 is low enough that oxygen toxicity is not going to be an issue. Secondly, the higher PO2 means a lower PN2 meaning a lower decompression obligation and longer dives without mandatory decompression stops. For example, at 20m the total pressure is 3 atm and a PO2 of 1.2 is equivalent to Nitrox at 40% or an equivalent air depth of 13m. That increases the NDL from 45 minutes to around 100 minutes (NDL for 14m is 98 minutes). Even better, the computer in the MKVI keeps track of your decompression obligation and displays your remaining dive time.
The first skill we learned was a bubble check at around 6 meters. The bubble check is to ensure there isn’t even the slightest leak in the system. Unlike an open circuit system where a minor leak is of minor consequence, the volumes of gases in a close system are so small that a minor leak can result in a drastic loss of bottom time. Any leak will be topped up with diluent and the top up with diluent will cause more O2 to be injected to maintain the PO2. This depth for the bubble check is chosen because it gives time for the unit to do its O2 sensor check so it knows it can accurately read a PO2 of 1.2 bar/atm.
With the bubble check completed we could descend further and truly experience the closed circuit rebreather. I mentioned earlier that the counter lungs in the breathing loop maintain a constant volume of gas. This has an impact on maintaining buoyancy – or I should say means that breathing has no impact on buoyancy – and so as open circuit divers we’re used to making small adjustments in our buoyancy by breathing in or out, on a closed circuit this doesn’t work. This means we had to get our buoyancy exactly right with out BCD and/or our dry suit or we’d be slowly ascending or descending. Even after 4 dives I’m still having troubles with this.
The mouthpiece has a switch on it so we can go from a closed circuit for normal operation and an open circuit which sends the gas from the diluent cylinder (air in our case) directly to the regulator. This is used as the first part of a bailout in case something goes wrong with the loop. Switching to open circuit also seals the loop to prevent water getting in. We had to practice switching to open circuit and removing the mouthpiece completely.
The diluent cylinder is only 3L in size and so while it can be used for a short period for bailout, it isn’t big enough for a full bailout and so we carried independent bailout cylinders with their own regulator. We practiced bailing out to open circuit and then the independent cylinder. We also practiced deploying a surface marker and ascending the line. The deployment wasn’t any different from what it would be on open circuit but the ascent was more challenging because of the change in buoyancy previously mention and the need to dump gs out of the breathing loop. With that we’d completed 3 dives and only had 2 more to go to get certified.
A week later we’d planned a long dive from Yena to The Steps or the Monument to complete the course. Unfortunately, the weather was not favourable and so we instead got in at The Steps and swam towards The Leap for the next dive with a plan to swim back for the last dive. We were practicing some skills along the way and John even spotted a seahorse. Unfortunately, 35 minutes into the first dive my unit’s computer started alerting because it no longer had confidence in the O2 sensors. The unit has 2 oxygen sensors and at regular intervals it checks the readings from each sensor to make sure it still trusts them. At least one of my sensors was not reading what was expected and to be safe the computer requests a bailout, which I did. This put an end to the day’s diving. Analysis of the logs later showed that O2 cell 2 had drifted away from cell one and that caused the confidence problem. Interestingly it had drifted back before the end of the dive. The most likely cause was excess moisture in the loop. Humid air from my lungs combining with cool water can result in quite a lot of moisture in the loop.
So, we still have one dive left to do and then we’ll be qualified to dive with the Poseidon MKVI rebreather. This will be just in time for the Komodo trip where we hope to do a number of the dives on the units. We also have plans for some interesting log dives back in Sydney over the coming months when the shop as one or two units available for hire. A slow circumnavigation of Bare Island would be a lot of fun as would a slow drift from The Leap to The Monument – all made possible from the increased bottom time a rebreather enables.