When I first moved to Jupiter, FL. I went diving with a local boat that uses a guide system for all their drift diving. All the divers followed the guide with the flag. After a couple of trips with this operator, I noticed that the guides were all using some sort of pony bottle rig. I figured that essentially they were diving by themselves and had it in case they or any other diver in the group needed it. These bottles had the same as their back gas EANx36% and were attached to the side of their single tank by a variety of methods.
After a couple of trips I was asked to come and be a guide on the weekdays. I brought along an AL40 STAGE filled with 50/50 and properly marked. All of our dives were 80 to the sand 60 top of the reef. I would explain to the divers in the group that in case of low on air/out of air emergency, I would donate my Primary regulator and switch to my back up. We would make a slow ascent and the STAGE would be used if we needed it for the ascent/safety stop. I carried the stage clipped off to my left side on chest and hip D-Ring. The bottle was charged(gas in the system) and OFF. I never really felt it in the way of anything being used to Double 104s and AL80s as stages this was quite nice. Single 80 and Al 40 STAGE. While I never had to use it on any of those dives, I was thanked for bringing the extra gas just in case. I only lasted about 6 trips before the solo diving (no real dive buddy) concern kicked in. Even though there was at anytime between 2 and 8 tanks and regulators within feet from me, I would not call that a buddy.
Unfortunately for ALL I stopped going as a guide and only go back when I have a buddy.
My buddy and I did plan to take a trip down to dive the Spiegel Grove. We wanted to do 4 dives that day and decided to take our Al40 Stage (50/50) for the ascents and stops from 70. While these dives would have been considered within recreational limits 4 dives in a day with the Florida sun can wear you out. Unfortunately we could not get them refilled before our second 2 dives and decided to not do the 2nd trip. Instead we had lunch and rented wave runners for the afternoon. How can you go wrong when you are in the Keys on a MONDAY?