I'm responding to this post to address one's level of comfort while solo diving. Given a maximum depth of 50 feet, the least amount of equipment I'd be comfortable wearing would be a snorkel. Current freediving training makes solo freediving out to be an absolute foolhardy no-no to solo freedive. I used to do 200-footers solo on constant ballast when I lived in the Caribbean. Honestly, that was stupid and foolhardy. Today, I would have safety divers. But I would be comfortable diving to 50 feet alone while breath-hold diving because I subscribe to the one-minute rule proposed by Capt. Dr. Frank Butler, Jr., USN (ret.).
Dr. Butler had been a UDT frogman, then a SEAL in Vietnam, then a general practitioner after medical school, and finally an ophthalmologist. Both while serving in the military. Army and navy, I believe. Anyway, he did research on breath-hold diving and proposed we limit breath-hold times to 1-minute due to blood gas analysis of apnea divers including the Japanese and Korean Ama women.
When I wrote my second freediving book, it was a manual for PSAI. In the manual, I suggested divers remember the AMA: A Minute... Always. No more.
Turning back to the intended topic of SCUBA, I normally just wear doubles or a single configured DIR even when diving alone. That's just the way I assemble stuff and never need to change it. I am comfortable with a single cylinder down to 50 feet. After that, I'd prefer doubles.
Why? Having been bent, I'm no longer comfortable with the idea of making an emergency ascent. Sure, I could do a CESA and a blow-and-go. I once was solo diving in a drysuit with one of those Italian FFMs for ice diving when the mask was no longer sealing to the outside of the hood against an intended layer designed to seat a FFM. It flooded at 70 feet, and I had to rip it off and do a blow-and-go like a submariner escaping from a sunken sub.
After having suffered DCS 4 years ago today, which required 3 years to recover and resulted in permanent nerve damage to my right eye, I want extra gas and the ability to isolate after 50 feet. AND a buddy bottle. I'd be comfortable with a short trip to deeper depths without a buddy bottle, but if the dive were planned to be deeper than 50, I'd want a separate gas source.
I wear AL80s with a single bladder wing.