PADI Deep Diver course- gas management

[jadairiii]

hmm.

I know the area i took the course in can get some pretty gnarly currents even down to the ocean floor. Could be a local thing, could be a shop thing.

Regardless, I got to shoot 4 of them from as far down as 120ft. All in all good skill to learn, and they saved me from having to take the PADI DSMB Course

What size tank were you diving with on the 120' class dive?

 

[MacDuyver]

What size tank were you diving with on the 120' class dive?

We were approaching NDL at 105 and 120 on air with Aluminum 80’s.

120 I surfaced a little under 800psi, 105 I came up with 1000 left, so not a huge loss on time. Also considering our time at those depths was about 15ish and 10ish minutes respectively, it’s not a tremendously huge amount of time we’d be talking about gaining.

I would have liked to have done NITROX, maybe do an intro to NITROX twins and have messed around a bit more. that said, pretty much all courses are just intros. I left with a notebook full of planning notes and considerations, got to see some cool stuff and hang out with some cool folks so I was happy.

 

[jadairiii]

We were approaching NDL at 105 and 120 on air with Aluminum 80’s.

120 I surfaced a little under 800psi, 105 I came up with 1000 left,

So, as you were just about to surface, your buddy has some kind of catastrophic failure/issue and goes on your octo, could you get both of you to the surface, with safety stops, on your remaining 1000 psi?

 

[lowwall]

So, as you were just about to surface, your buddy has some kind of catastrophic failure/issue and goes on your octo, could you get both of you to the surface, with safety stops, on your remaining 1000 psi?

It's a NDL dive. 25 cubic feet of gas is plenty to get 2 people up.

 

[jadairiii]

It's a NDL dive. 25 cubic feet of gas is plenty to get 2 people up.

For you, and me.....but I would wager there are some on this site that would, in an emergency, suck down the 25 cf pretty quick.....

 

[Blackcrusader]

So, as you were just about to surface, your buddy has some kind of catastrophic failure/issue and goes on your octo, could you get both of you to the surface, with safety stops, on your remaining 1000 psi?

If you are on an NDL dive and you have a failure you would skip the safety stop as it is not mandatory.

1000 PSI is more than enough.

 

[jadairiii]

If you are on an NDL dive and you have a failure you would skip the safety stop as it is not mandatory.

1000 PSI is more than enough.

That is the failure of "recreational" dive instruction. Every dive is a "decompression" dive and should be treated as such. Coming up like a Polaris missile from 120 when you are right up against the NDL's is a recipe for disaster. But clearly the 8 pages of this thread has taught me, most agencies do not teach proper gas management for a buddy team.

1000 is not "more than enough".

 

[grantmac]

That is the failure of "recreational" dive instruction. Every dive is a "decompression" dive and should be treated as such. Coming up like a Polaris missile from 120 when you are right up against the NDL's is a recipe for disaster. But clearly the 8 pages of this thread has taught me, most agencies do not teach proper gas management for a buddy team.

1000 is not "more than enough".

What is min gas for 36m? Because I have it at ~77bar/1150psi. Very likely he left bottom with more than that to be on the boat with 1000.

 

[steinbil]

What is min gas for 36m? Because I have it at ~77bar/1150psi. Very likely he left bottom with more than that to be on the boat with 1000.

GUE use an estimate of 20L/min (0.7cuft), and a conservative ascent rate of 3m/min (10ft) - both because it's hard to control a quick ascent while gas sharing and to account for an actual higher stressed SAC rate in an emergency.

So from 36m:
Consumption - 2 diver x 20L/min = 40L/min
ATA - 36m / 2 / 10 + 1 = 2.8ATA
Time - 1 minute for gas sharing + 12 minute ascent = 13 minutes
Mingas = 40 x 2.8 x 13 = 1456L (51.42cuft)
in pressure of AL80 = 1456 / 11.1L = 131.17 bar rounded up to 140 bar (2000 PSI)

EDIT:
I would challenge anyone with a less conservative gas plan to try practicing gas sharing ascents with your equipment configuration from 36m and time it. Let me know how it goes.

 

[tursiops]

GUE use an estimate of 20L/min (0.7cuft), and a conservative ascent rate of 3m/min (10ft) - both because it's hard to control a quick ascent while gas sharing and to account for an actual higher stressed SAC rate in an emergency.

So from 36m:
Consumption - 2 diver x 20L/min = 40L/min
ATA - 36m / 2 / 10 + 1 = 2.8ATA
Time - 1 minute for gas sharing + 12 minute ascent = 13 minutes
Mingas = 40 x 2.8 x 13 = 1456L (51.42cuft)
in pressure of AL80 = 1456 / 11.1L = 131.17 bar rounded up to 140 bar (2000 PSI)

EDIT:
I would challenge anyone with a less conservative gas plan to try practicing gas sharing ascents with your equipment configuration from 36m and time it. Let me know how it goes.

3m/min ascent rate is silly slow. Just stay below 18m/min (10m/min is even better) and it is likely you will be just fine.