- Messages
- 13,447
- Reaction score
- 10,046
- Location
- Port Orchard, Washington State
- # of dives
- 1000 - 2499
I wouldn't bring a 4th gas. Its a cylinder you only need to use for a brief moment (although I agree with @beester about just dropping) and having it in the rotation on the bottom is one more opportunity to breath it below the MOD. Especially if you are coming from 1 deco gas diving and now you have 3 or 4 stages in you.I would be interest in any references / discussions on the use of a travel gas as the 4th gas.
Carrying an al80 of 21/20 instead of a larger cylinder of your deeper deco gas is especially heavy on gear while light on usefulness from my perspective. As far as I can tell from your description you didn't use the 21/20 as a deco gas on ascent. I question the total volume of gas if either of the al40s of 32% or the 100% were fail. I guess you had enough 21/20 but the contingency plan(s) here are a mystery to me.
50% is really easy to make and covers a big chunk of your deco time, and if the 100% bottle is lost can be used almost effectively at 6 and 3m too. You can also use it as a travel gas if you really need one. I'm a big fan of 50%.