MX Rebreather - Diluent / Bailout Tanks
A fitting place to start, as we want to stay within the spirit of UTD/DIR, so let’s start by looking at our bailout configuration and the important considerations associated with it. First, the overall principle of bailout is “how much gas will it take to come out in an emergency?” In other words, we need to reserve enough gas volume of the correct gas to exit to the next available gas supply, whether that be the surface, deco bottles or staged safeties in cave diving exploration. We term this volume “Rock-Bottom.”
A second, and equally important consideration, is that the bailout volume should be scaleable to accommodate volumetric needs of a wide variety of environments and situations. In other words, you can take as much as you need based on your dive profile, and you can choose bailout cylinder sizes to meet those needs.
Third, the base configuration must remain consistent from recreational to technical to trimix to overhead to exploration to ensure consistent skills and protocols, especially during emergencies.
Fourth, the open circuit bailout and/or donation during an out of gas (OOG) situation should be consistent within the team, previous training, and experience, whether that team be all rebreather divers or a mixed team, such as OC divers, PSCR or CCR divers.
Fifth, the OC Bailout should be easy and quick to switch to and from, to donate and stow, and so on, essentially being convenient to use rather than inconvenient.
Sixth, the whole system should be configured to be as streamlined as possible with the minimalist approach as set by UTD/DIR covenants.
Seventh, the bailout gases (diluent/deco) should be able to be used for not only running the rebreather in fully closed circuit mode but also in semi closed rebreather mode, in the case of PPO2 monitoring system fail, or full open circuit bailout, in the case the rebreather is not functional or flooded.
And last but not least, the configuration should be balanced, promoting proper buoyancy and prone positioned trim.
With this in mind, the UTD MX series mCCR Rebreather, or MXRB, is encompassed by a standard set of doubles that are scalable in size and volume to match the bailout volumetric needs. This allows us to have a convenient and consistent approach to open circuit bailout from our fun recreational diving through exploration projects, as well as when diving within a mixed UTD/DIR team. The choice of tanks could be as small as AL40’s/6L to the steel 46’s/7L or 72’s/8L or 80’s/11L or 100’s/15L, or for extreme exploration projects divers could use 130’s/18L. All cylinders are manifolded together in a similar fashion to our UTD/DIR OC or pSCR, giving a diver sufficient volume of gas for bailout in a situation in which the MXRB failed or a teammate required you to donate your long hose in an out of gas (OOG) situation. A side note - The “common” names given to the MX series of rebreathes “MX90 or MX160” are essentially based on the cubic foot volume of the bailout cylinders. However the UTD configurations are named either as MX Exploration - a unit with a single isolated manifold, or a MX Expedition - a dual isolated manifold version.
Since our volumetric needs for diluent in “normal” rebreather diving operations are very small, rather than carrying a separate small diluent tank, we allow the MXRB to access the bailout tanks, essentially doubling up on their use, and they now become DIluent/Bailout tanks. We account for this use during gas planning by adding the required diluent gas on top of our bailout requirements. This also becomes very beneficial when doing multi-level dive operations, i.e. multi level cave exploration, in that our diluent needs may substantially increase due to vast depth changes. In this case one simply increases the volume of Diluent/Bailout carried.
The Diluent/Bailout gas of choice is based on the UTD/DIR standardized mixes. This ensures that we have the “right” parameters to not only conduct the closed cIrcuit (CC) dive operation, but to ensure that if we bailout to open circuit or PSCR mode, we are familiar with the gas parameters and consistent in our approach to decompression strategy. Remember, we have been diving standardized gases on OC for years and transfered them to PSCR, 10 years ago and now CCR and we completely understand their use, risk, and decompression strategy, so returning to them in a bailout situation adds no additional stress. By using standardized mixes we do not have to change our approach to the dive or the parameters of the gas, or the decompression strategy, so we can easily integrate or switch between CC, PSCR and OC. This helps support us when RB diving and during OC emergencies, and also when diving within a mixed team.
The parameters and arguments of standardized mixes are well defined in UTD/DIR, so I will only recap them here. First and foremost, a standardized mix is based on the UTD/DIR covenant of “NO DEEP AIR” - in other words we keep the narcotic value of diluent/bailout gas to 100’/30m or shallower. In order to do this we use a helium based mix when diving deeper than 100’/30m. This helium will offset the nitrogen and oxygen, both problematic gases from a narcotic, toxicity and decompression point of view. Adding helium to the mix reduces the narcosis, the risk of O2 toxicity, and ultimately it is easier and quicker to decompress from than nitrogen.
A second consideration is that one should have a mix with an “average” bottom PPO2 of 1.2 for shorter bottom times or 0.7 for longer cave explorations. Keeping in mind an absolute max bottom PPO2 of 1.4.
There are additional reasons for using a diluent that has a PPO2 that is close to the target PPO2 for the dive operations - it will reduce your overall risk and facilitate a smooth consistent operation of the MXRB, it will even out large PPO2 fluctuations, and ultimately the gas can be used efficiently and effectively during an emergency exit.
Consider this – if we need to add a gas to the loop to compensate either for a depth change or for our metabolic use, we will be adding a diluent gas that has a PPO2 that is close to the desired target, therefore requiring less, if any, O2 addition at depth. By adding or flushing with a diluent that is close to the target PPO2, we can reduce the huge potential for PPO2 fluctuations. This not only reduces the risk of an O2 spike, but also reduces the risk for low PPO2, or hypoxia.
In an emergency, this diluent/bailout gas is at “or close to” target PPO2, and therefore if the MXRB is not useable, an open circuit exit is not only consistent in protocol, as discussed above, but easy to conduct and manage. A more advanced strategy would be that if the MXRB is functioning but the PPO2 meters or electronics have failed, one could use the MXRB in a “SCR” mode safely and effectively. This is of course an advanced ”emergency” strategy and is taught and practiced in our higher level mCCR3 exploration class. Ultimately, as discussed above, using standardized mixes for diluent gives us many many advantages in both diving operations and emergencies, with little to no downside.