So, the dive you are thinking of is somewhat similar to a dive I did last weekend. I was up at the Thousand Islands planning to do some solo shore diving with my Chop and a set of LP46s filled with good old fashioned breathing air (why pay for nitrox when the Chop is mixing it for me on the fly, right. So right when I get there Friday afternoon, I stop by my friend's dive trailer parked outside the local shop to grab my scooter that he had borrowed the previous week for one of his students to use when the shop owner asks me what my plans are for the weekend. I tell him my meager plans and he says they have room on the boat in the morning and they're going to the Jodrey.
So, now I'm in! Oh, wait... I wanna be in but I came up here with just the mini doubles (with air) and an AL40 of O2. That will technically work, but is far from ideal (I could breathe air at 150 to 170 ft, but it's not the best idea. I would be limited on time based having only 100 cf of bailout that I would have to breathe up to 20 ft). So the shop owner and I did a little calculating on the bailout requirements for the dive I wanted to do the way it really should be done (18/45 dil, AL40 of 50%, and my O2).
So he let me borrow a full 70 ft bottle (with the promise that I would pay for any gas I used if I had to bail out), borrowed a stage reg out of my buddy's trailer, and then dumped all my air and filled the mini doubles with 18/45. That last part was emotionally the hardest, knowing I had 2 twinsets and 3 bottom stages at home already half full of 18/45, until it occurred to me that the $200 I just spent to to fill the 46s with mix would last me a good long time as long as I did not need to actually use them to bail out.
Anyway, we calculated that for a 30 minute bottom time at 170 ft I would need about 12cf to 100 ft, 8 cf to 70 ft, and then the same amounts of 50% and O2 as I would normally use for this dive profile on OC (a 40 of each is plenty). So, in this scenario, I could even get away with losing my 50%, as doubling the stops for being on back gas would mean the 80 cf left in the doubles would get me to 20 ft. So, if I had that much excess BO gas, why did I only plan a 30 min bottom time? Well, I was tempted to stay longer, but the rest of the boat was on OC with just double AL80s and their deco gas, so I didn't want to leave them sitting on the boat waiting for me for too long. I did stay about 5 minutes longer than they on the wreck, and I took my time getting up to 100 ft. I thought I'd end up catching them at the 20 ft stop, but they ended up back on the boat about the time I got there, so I was about 15 minutes behind them surfacing.
Diving the RB on this dive was like a dream. I was so much more relaxed knowing I had all the time in the world if I needed it for any reason, not seeing dollar signs floating up with my bubbles every time I exhaled, and still being clear-headed on a nice gas mix. As it turns out, the mini doubles are even enough gas for a 10 min scooter back to the up line even if I have to bail out further back the wreck (though if I plan to go to the stern I'll take the AL80s or maybe even the LP108s - but at least the days of 1 or 2 bottom stages for this are gone).
The cost of the mix was minimal considering I still have most of it in the tanks. I started with 3000 psi and ended with about 2900, so this fill should last me about a dozen more dives before it's too low to be enough bailout for this type of dive. That's pretty nice considering how much these dives would cost on OC. (Yes, I know it still takes a lot more than that to amortize the cost of the RB, cells, scrubbers, etc. but I count it as a win)
