This isn't restricted to a single unit. I've seen something else that is related, but not exactly the same thing, @grantctobin even mentioned doing it in this thread. I'm talking about using a CCR designed for off-board DIL with only one deep bailout bottle.
Typically, when people dive a unit with on-board DIL they also carry a deep bailout bottle. If they lose the DIL reg they still have bailout gas that they can plumb in (or use) on the way up. If they lose the bailout reg, they still have a way to drive the unit and get fresh DIL in there if they need to (and worse case scenario, they can even go "open loop" and get a few breaths of bailout gas if needed on the way up).
There are currently three "mainstream" units on the market that primarily use off-board DIL, the Fathom, Optima CM, and Sidewinder. Typically, most people using these units drive them with some form of redundant setup (2x sidemount bottles or a set of doubles). However, when a person goes down to just one bottle they're putting all of their eggs in one basket. Lose that single reg and they've now lost access to both DIL and deep bailout - they can't do a DIL flush or bail out. It's effectively diving Alpinist.
@MattK911 - a mutual friend of ours (and former student of mine) used a single deep bailout for his DIL/bailout on his Fathom when he dove the Oh in 2021. I assure you, no one told him this was a good idea and when he told me about it after the dive, I explained to him why it was a bad idea. We also have another mutual friend that recently became an Optima CM diver (he bought my ChopChop), and he was told by an instructor that in order to avoid filling a set of doubles with mix, to just use a single stage bottle with bottom mix/DIL and keep his doubles with nitrox whenever he was diving at Helitrox depths (when he told me that I explained why I thought it was not a great idea).
Personally, I do think everyone needs to accept their own risk tolerance and make their own big boy decisions. I also think it's reasonably acceptable to use a single DIL/bailout source for "monkey diving" (open water, no deco, max depth of around 40'), but any time a person starts putting a ceiling (physical or physiological) over their head they need redundancy. For anyone that thinks the risk of losing that single reg is overblown I have two words for you: Apeks Seat.
Typically, when people dive a unit with on-board DIL they also carry a deep bailout bottle. If they lose the DIL reg they still have bailout gas that they can plumb in (or use) on the way up. If they lose the bailout reg, they still have a way to drive the unit and get fresh DIL in there if they need to (and worse case scenario, they can even go "open loop" and get a few breaths of bailout gas if needed on the way up).
There are currently three "mainstream" units on the market that primarily use off-board DIL, the Fathom, Optima CM, and Sidewinder. Typically, most people using these units drive them with some form of redundant setup (2x sidemount bottles or a set of doubles). However, when a person goes down to just one bottle they're putting all of their eggs in one basket. Lose that single reg and they've now lost access to both DIL and deep bailout - they can't do a DIL flush or bail out. It's effectively diving Alpinist.
@MattK911 - a mutual friend of ours (and former student of mine) used a single deep bailout for his DIL/bailout on his Fathom when he dove the Oh in 2021. I assure you, no one told him this was a good idea and when he told me about it after the dive, I explained to him why it was a bad idea. We also have another mutual friend that recently became an Optima CM diver (he bought my ChopChop), and he was told by an instructor that in order to avoid filling a set of doubles with mix, to just use a single stage bottle with bottom mix/DIL and keep his doubles with nitrox whenever he was diving at Helitrox depths (when he told me that I explained why I thought it was not a great idea).
Personally, I do think everyone needs to accept their own risk tolerance and make their own big boy decisions. I also think it's reasonably acceptable to use a single DIL/bailout source for "monkey diving" (open water, no deco, max depth of around 40'), but any time a person starts putting a ceiling (physical or physiological) over their head they need redundancy. For anyone that thinks the risk of losing that single reg is overblown I have two words for you: Apeks Seat.
