Oh no! My 72s are too small for tech classes! :(

[rjack321]

For what is worth, here's the relevant TDI requirements for Intro to Cave.

The following equipment is required for each student:
1. Primary cylinder(s); volume appropriate for planned dive and student gas consumption
2. Dual-orifice (Y) or (H) valve, double cylinders or dual valve manifold

If using a single (for some wacky reason) you are allowed to dive 1/3rds at the Cavern/intro level. In doubles it would be 1/6ths. In either case that is 24cf of gas.

If the average depth is 45ft and at a stressed student consumption rate of 0.75cf/min you're going to get about a 12 to 14min dive before you're turning on those double 72s. After doing an S-drill and a primary and secondary tie off (as a student) you're then looking at a 5-6min swim. So you are severely limiting the in-water class time you're paying ~$300/day.

You need larger cylinders for cave courses to actually get in the training time.

 

[Hoyden]

I have a couple of sets of doubled 65s. They are super sweet little tanks and I generally used them for light deco, no overhead dives. One of my old dive buddies was a gas hog (which ultimately led to both of us diving rebreathers) and any time I dove with him I had to step up to my 95s because I had to carry enough gas for him. I understand tech instructors wanting to avoid this issue and requiring larger cylinders because of it.

 

[broncobowsher]

Actually, for this case, it is hard overhead for two of the people I’ve encountered who were surprised their 72s weren’t permitted for cave class.

Sorry, didn't see any comment about cave. Just AN/DP and it sounded like just starting tech diving.

 

[Marie13]

Sorry about that. Must have missed putting that in the OP.

 

[rx7diver]

80s are minimum size

Hmmm:

1. Old-school steel 72 nominally gives 71.2 cu ft @ 2,475 psig. I understand that these are routinely (albeit unwittingly) filled to 3,000 psig, so, 86.3 cu ft.

2. Al 80 nomially gives 77.4 cu ft @ 3,000 psig. "Never" overfilled (since Al).

3. LP 85 nominally gives 81.1 cu ft at 2,640 psig. I understand that these are routinely (perhaps unwittingly) filled to 3,000 psig, so, 92.2 cu ft.

If the average depth is 45ft and at a stressed student consumption rate of 0.75cf/min you're going to get about a 12 to 14min dive before you're turning on those double 72s. After doing an S-drill and a primary and secondary tie off (as a student) you're then looking at a 5-6min swim. So you are severely limiting the in-water class time you're paying ~$300/day.

You need larger cylinders for cave courses to actually get in the training time.

[My emphasis.] Now, this makes perfect sense (to me)!

rx7diver

 

[tbone1004]

I'm not aware of any agency that restricts use of lp72's. If the instructor does that is their choice but it is strange to allow AL80's and not LP72's. From an instructor standpoint, the small bottles are extremely limiting in cave diving because of the limited dive time which is that much worse in caves like Ginnie, Little River, Jackson Blue etc. where you will hit thirds shortly after running the primary and often times you'll hit sixths before you tie in.

 

[Manatee Diver]

If using a single (for some wacky reason) you are allowed to dive 1/3rds at the Cavern/intro level. In doubles it would be 1/6ths. In either case that is 24cf of gas.

I was told that was meant more to be a leash to keep you relatively close to the entrance.

 

[p_kos]

what if you overfill the 72's?

I'm kidding, of course. That sucks.

Yup, get ‘em up to 2800 psi and you’re good to go. My 72s get a transfill from my 133s to make them a bit fuller than 80s. I wouldn’t use them for a tech course per se but they would meet the bare minimum of 160 cf.

 

[p_kos]

I dive 85s. Anything longer and they’ll be past my knees.

72s filled to 3000psi are basically 85s, too bad the only shops that will consistently do that are in North Florida.

 

[rjack321]

I was told that was meant more to be a leash to keep you relatively close to the entrance.

1/6ths of doubles is a short leash
1/3rds of a single is dumb