dive plan

[jj1987]

Why not spend $10, rent a set of tanks that match, and eliminate all the confusion?

[tflaris]

Why not spend $10, rent a set of tanks that match, and eliminate all the confusion?

I had fun doing the math. But you definitely have an easy solution.


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[JahJahwarrior]

Given the 4 year old posts I suspect this dive happened a long time ago...

[tflaris]

Given the 4 year old posts I suspect this dive happened a long time ago...

Gas matching maybe a lost science but still a lot of fun.

Thanks for the heads up.


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[Rich]

Diver 1
72's baseline 2.7 cu/ft for every 100 psi x 2 = 5.4 for back gas
psi @ 2475

72 baseline 2.7 cu/ft for every 100 psi = 2.7 for stage
psi @ 2250

Please feel free to double check my math. I was using the NACD Cavern/Cave student workbook as a reference.

Hey Tony, you might want to recheck your baseline calculations, these are the OLD LP72's we're talking about (rated pressure is 2250/2475, not 2400/2640 and their baseline's a bit higher than that. Hope you're having a great Thanksgiving, bud......

Safe diving,

Rich

[Sludge]

He was thrown off when the 72s were referred to as "LP-72." The "LP" designation implies 2400 wp.

A better nomenclature would be "old style 72s."

[tflaris]

Hey Tony, you might want to recheck your baseline calculations, these are the OLD LP72's we're talking about (rated pressure is 2250/2475, not 2400/2640 and their baseline's a bit higher than that. Hope you're having a great Thanksgiving, bud......

Safe diving,

Rich

You are correct the baseline for the 72's would be lower. Thanks for catching that. I have never owned any 72's so I wasn't aware of their rated pressure or plus pressures.

Safe Diving.

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[DA Aquamaster]

Yes, the LP 72 designation is confusing. There are:
1) "Steel 72s" generally regarded as 71.2 cu ft at 2475 psi (64.7 cu ft at the 2250 psi service pressure),
2) MP 72s (PST and Faber 72s) that hold 72 cu ft at 3300 psi, and
3) aluminum 72s that hold 72 cu ft at 3000 psi.
4) IIRC, Faber made a 2400 psi service pressure 75.8 cu ft tank in the 1980s that then held 68.9 cu ft at 2400 psi.

But there is no 2400 service pressure tank that holds an actual 72 cu ft at 2640 psi.

On the Steel 72 the tank factor would be 2.8, and the same is true with the Faber 75.8.
On an MP 72 it would be 2.2, and
On an AL 72 it would be 2.4

In comparison on the mythical 2400psi service pressure LP 72 it would be 2.7 so the difference betwen it and a steel 72 and Faber 75.8 is minor.

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As for using a steel 72 as a stage, it really depends on the steel 72. I have probably owned 20 of them over the last quarter century and I noted that the buoyancy traits varied a lot depending on who made them and when. I am generally happy with those made by norris industries and those were the ones I have kept and they are a pound or so positive with valve when empty. Some of the others I have owned were boat anchors and were still a couple pounds negative with valve when empty and when full were a bit too negative to carry well as a stage.

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The numbers are what they are, but it's easy to get wrapped around the statistical axel and miss the big picture in terms of where that reserve gas is distributed and located. There are realistically only three approaches.

1) You carry the entire stage as a reserve gas, which means you'd be breathing the back gas right down to nothing (although in practice I exit quicker than I entire by design so I never fully use the 2nd "third" so there would probably be 200-500 psi left over. Something to consider though is that many regs don't breathe well when the tank pressure drops below the IP, so there is 8.6 cu ft of gas in the second "third" that is potentially not usable in a set of double steel 72s.

In the event the guy with the 119s loses all his gas at max penetration, this idea still does not completley as it potentially leaves both divers OOA and trying to be on the stage in the last portion of the exit. To keep it real, you'd need to reduce the penetration gas used by the guy with the 72s to reflect the non useable gas in his back gas to help reduce the probability that both divers would need to share the same stage. And obviously you still need to gas match and adjust the tuirn pressure to ensure that the contents of the steel 72 are enough to get the 119 diver out from max penetration.

2) You use the steel 72 as part of the "thirds" gas plan along with "thirds" in the back gas. The problem here is that if it is part of the total plan, you need to carefully consider where it may be needed. Dropping it when you hit thirds will leave it too close to the exit if you really need it in a OOA situation. You'll need to carry it in so that the full 2/3rds left in it is useful (twice as far as you actually were breathing off it). In the event the guy with the 119's loses all his gas at max penetration, the remaining gas in the steel 72s will get you back to a stage left at thirds, but you've only got one second stage on it, so the "extra" third will be problematic to access with two divers needing to buddy breathe the tank at the same time.

3) You can manage the steel 72 stage at 1/2 plus 200 with all of the reserve in the back gas. In this case you are dropping the stage when you get off it, and in that case it is going to theoretically have enough gas to get you out, but again gas matching to turn the dive appropriately is vital.

Personally, none of the plans are appealing and it makes sense to either just incldue the doubles in the plan and /or just get a bigger set of doubles for the guy with the steel 72s.

I think steel 72 are great for shallow, tight caves when diving sidemount, but I don't think they are a great when bakc mounting in a two person team with a team mate with significantly larger tanks. Adding a stage to make up the difference in gas just creates a false sense of security and a temptation to get into situations that won;t really work all that well in the event an OOA emergency really develops.