Akidiver and others. The al40 is not going to give you 40 cu ft. It may hold 40 but that is going form 3000 psi to 0 psi. So to get to the surface with 500 psi (??? a faulty high reading gage) you will have only about 34 cubes available for use. So now you are almost back to your 30. Peter there is nothing wrong with with a 30. You just have to understand the limits imposed by the gas available in the pony.
Now Aki earlier had a post that basically went Let me get this straight I am down 130 ft.......
You need to remember that the 130 or 120 rule is depth pluss penitration. at that depth you dont enter a cavern or cave with the certification being referenced. so lets change the dive to a 70 ft dive and a 50 ft penitration. That will remove 2 atm from the problem. Your pony gas willl now last longer. A 30 will probably sufice but leaves little margin for problems when trying getting out on the 25 cuft avail in a 30.
This has been a good discusion
Regards to all