Dude: If you HOG!! air then a 30 is more than enough to get you to the surface. I am a air hog, techie and I use a 40 for about 35 min of deco at 15-20 ft. Thats on my doubles with trimix 18/65 for back gas, and nitrox 36 in an 80 for mid level deco stops. That gas load is for 180` foot dives in the straights of Mackinac. If you are just looking for that what if factor pony, cause your going deeper, I would 1. Get an analog pressure gauge for your main reg set, 2. get a steel 13 cu. from OMS with an OMS workhorse reg. and for a pressure gauge use a button gauge, 3. Get a mount set to mount it on your main tank to get it out of the way. The button gauage is all you need cause you just need to know if you filled it, cause if you ever go to it your on your way up, and a steel 13 has MORE than enough air to get you up from 130` with a 3 min safety stop if you stay calm.