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

[Jim Lapenta]

It does depend on the class. 72's might be fine for some tech classes. If a diver has a SAC rate of .26 like my GF had when she was still diving, 72's would have been fine for some AN/DP stuff or even Advanced Wreck depending on the depth and time plus whatever stages you'd be carrying. Can't make a blanket statement that 72's are not enough.

 

[boulderjohn]

The deeper you go, the faster you go through gas. You need to carry enough gas for the planned dive, plus a healthy reserve to deal with any problems that arise--and not just for a buddy. 72s would be fine for shallower deco dives with limited bottom times, but deeper and longer would call for more gas. When it comes to team diving, everyone on the team plans turn pressures based on the most compromised diver, so if you have a limited capacity in terms of total gas, so does everyone else.

 

[Marie13]

The deeper you go, the faster you go through gas. You need to carry enough gas for the planned dive, plus a healthy reserve to deal with any problems that arise--and not just for a buddy. 72s would be fine for shallower deco dives with limited bottom times, but deeper and longer would call for more gas. When it comes to team diving, everyone on the team plans turn pressures based on the most compromised diver, so if you have a limited capacity in terms of total gas, so does everyone else.

Add in the overhead factor…

 

[broncobowsher]

Add in the overhead factor…

That would likely be a virtual overhead (deco obligation), not a hard overhead (cave)

 

[Marie13]

That would likely be a virtual overhead (deco obligation), not a hard overhead (cave)

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.

 

[boulderjohn]

When I was taking my cave classes, on one of the days, my instructor and I were joined by someone interning in the class as part of his training to be an instructor. My instructor and I were using LP 108s. The intern had LP 85s. My instructor was clearly peeved at the intern. Because the intern had shown up with lower capacity tanks, I had to calculate my turn pressure on his tanks rather than mine. That meant we had significantly shorter training dives that day.

Putting it in simple terms for those who have no training in this....

Bill and George are planning a dive. Bill has a total gas capacity of 150 cubic feet, and George, with his much larger tanks, has a total capacity of 250 cubic feet. They plan to dive to thirds, but they must use the lowest thirds, meaning they must turn the dive after one of them consumes 50 cubic feet. That means George has to turn the dive when he has used 1/5 of his total gas rather than 1/3. That's a significant difference in bottom time.

 

[Squirm88]

80s are minimum size

Marie is this a certifying agencies requirement or a specific instructors preference? Never seen specific tank requirements in the cave courses I have done. TDI lets you take intro to cave in a single tank with an H valve. I wouldn't recommend it, but its allowed. You would think doubled up 72's should be enough, to dive to 1/6 and do some skills on the way out.

 

[Marie13]

Marie is this a certifying agencies requirement or a specific instructors preference? Never seen specific tank requirements in the cave courses I have done. TDI lets you take intro to cave in a single tank with an H valve. I wouldn't recommend it, but its allowed. You would think doubled up 72's should be enough, to dive to 1/6 and do some skills on the way out.

Multiple people getting trained different places with different instructors. If it’s instructor preference, it appears to be a common one.

 

[Squirm88]

Multiple people getting trained different places with different instructors. If it’s instructor preference, it appears to be a common one.

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

 

[Divin'Papaw]

Why would you want to dive caves with 72s? Those would lead to some SHORT dives. My guess is that the rationale has to do with the duration of dives the tanks will safely support in training. I'm honestly surprised the instructor doesn't require at least double 100s. It is cave diving after all.