REDUNDACY OUT THE WAZOO! I carry 19cu ft pony for back up air. Will soon be getting a 40 as I plan on going tech later this year anyway and will need two of them. In addition to my computer I also carry analog depth, back up timer(my dive watch), two compasses, reel and finger spool, lift bag and smb, PADI wheel in my bc pocket, Two cutting devices(knife) easily accessible with either hand, just added emt shears, and spare mask. In addition to this I also leave a detailed dive plan with someone on the surface. I follow that plan conservatively and usually surface a few minutes early to alleviate any anxiety on the part of my surface person. First and foremost is your mindset. I admire you wanting to take a solo diver along as an observer. But this can be a double edged sword. My planning for solo dives is much more detailed and meticulous in prep and execution. When diving with a buddy if we want to stop and look at something for an extra minute or two it's no big deal depending on depth and time of course. When soloing however it's a no-no for me. Although I calculate my gas with a generous safety margin it is just that, a safety margin to be used in event of emergency. It does not give me permission to deviate from my timetable and gas consumption figures. My advice is if you are going to do this you must try to forget that the other person is there. You must realize that should something happen you have only yourself to rely on. I made sure that I could do what I needed in an emergency by myself to my satisfaction to get out of trouble. This involved alot of practice in the pool and in ow with buddies present who were instructed not to interfere with what I was doing for issues such as entanglement, loss of buoyancy control, gear maifunction, etc. Now I do not know how many dives you have but that is really not an issue. I have 80 some but have helped certify over 30 ow divers, 9 jr open water, 5 aow and 1 rescue. I'm nearly done with DM and have over 100 hours in the pool as an assistant. I have deep, uw nav, nitrox, drysuit and equip specialties. I dive mainly low vis, mild current, sometimes cold(15 dives in below 45 degree water) with drift, boat, and shore entry experience. Every dive I make it a point to work on a specific skill or two and usually always practice uw nav to some degree on every dive. I also have the mindset especially when in the water with students that except for the instructor who may be with 2 or 3 other people I'm diving solo. I seem to have alot of trouble with buddies who decide to suddenly go off chasing fish or whatever. I don't have to always lead the dive but if we have a plan stick to it or al least inform me there's a change. If you can put it in your mind to be totally self reliant, with your skills and gear, and be comfortable with that decision go for it. If however there is any major doubt or naging insecurity about your equipment set-up, skills, place you are diving, or anything else you can't seem to shake, step back and reevaluate your choice. I look at solo diving and relate it to driving a car. Most people can drive, not everyone can or should in the indy 500.