| The best thing to do is talk with your instructor. On our Tech2 course we used 1 AL80 for bottom gas, then a mixture of AL80s, AL7's and AL40s for the 50% or O2.
We then had a 2nd set of all the bottles to allow us to take it all out on the boat at the beginning of the day and have gas for 2 dives.
A lot of it comes down to the logistics of the course and what is available locally.
HTH
John |