Do you double or single..

[Boogie711]

Basic mathematics, lragsac.

If you have a single tank, once you surface, that reserve is gone forever. You paid for it in your last fill, but it's now stuck in your tank, never to be seen or used again. Say an average single tank dive is 30 minute dive to 70 feet, and uses 65 cf of gas... leaving you with 500 psi, or 13 cf in an AL80 (all numbers are just for approximation, and don't take into account rock bottom or other gas management rules.) That 13 cf stays in your tank until it gets either drained on land or refilled.

But do a 30 minute dive at 70 feet in doubles and still use 65 cubic feet of gas. Now, when you go to do dive number 2, that 13 cubic feet that you would have just thrown away in a single tank is waiting to be breathed again. Instead of 80 cf in a single tank, a double tank diver has the equivalent of a full AL80, PLUS the 13 cubic feet 'left over' from the first dive.

Make sense now?

 

[SeaJay]

I was kinda hoping someone would ask that.

Oops... Looks like Boogie already beat me to it. He did a better job of explaining it anyway.

Bottom line: Two identical divers dive together doing two identical dives - but one's wearing a single AL80 and the other's wearing doubled AL80's:

When they get into the water on the first dive, the single-tank diver's got 64.75 cuft of usable gas. (That's based on a 77.7 cuft tank, 3000 psi, and 12.95 cuft, or 500 psi, in reserve). The double-tank diver has 142.45 cuft of usable gas with the same assumptions.

On the second dive, the single-tank diver's again got 64.75 cuft of usable gas, with 12.95 cuft of reserve. The double-tank diver's got 77.7 cuft of usable gas, with the same amount in reserve.

 

[lragsac]

Basic mathematics, lragsac.

If you have a single tank, once you surface, that reserve is gone forever. You paid for it in your last fill, but it's now stuck in your tank, never to be seen or used again. Say an average single tank dive is 30 minute dive to 70 feet, and uses 65 cf of gas... leaving you with 500 psi, or 13 cf in an AL80 (all numbers are just for approximation, and don't take into account rock bottom or other gas management rules.) That 13 cf stays in your tank until it gets either drained on land or refilled.

But do a 30 minute dive at 70 feet in doubles and still use 65 cubic feet of gas. Now, when you go to do dive number 2, that 13 cubic feet that you would have just thrown away in a single tank is waiting to be breathed again. Instead of 80 cf in a single tank, a double tank diver has the equivalent of a full AL80, PLUS the 13 cubic feet 'left over' from the first dive.

Make sense now?

In monetary terms, this makes sense, but I'm so stinkin' rich I don't really sweat it
I get my air fills for free and for everything else there's the k-bottles in the garage filled with helium and O2

 

[mossym]

so by the end of diving the doubles you're going to have the 500psi reserve split between your two tanks, basically 250 in each?

 

[SeaJay]

so by the end of diving the doubles you're going to have the 500psi reserve split between your two tanks, basically 250 in each?

That's right.

Pretty sharp observation if this is your first time hearing this concept.

More on figuring out how much you need in reserve (depending on your capacity as well as other things), in terms of both cubic feet and psi... And why the concept of, "Be back on the boat with 500 psi" not only doesn't work all the time, but may be dangerous... Here: http://www.DeepSouthDivers.org/homerockbottom.html

There ya go... You're one step closer to "technical" diving now.

 

[mossym]

That's right.

Pretty sharp observation if this is the first time hearing this concept.

More on figuring out how much you need in reserve (depending on your capacity as well as other things), in terms of both cubic feet and psi... And why the concept of, "Be back on the boat with 500 psi" not only doesn't work all the time, but may be dangerous... Here: http://www.DeepSouthDivers.org/homerockbottom.html

There ya go... You're one step closer to "technical" diving now.

yeah thats the problem, lot of things i'm doing lately taking me closer to technical diving

 

[Jason B]

Not to hijack the thread but, nicely written article SeaJay!
(we now return you to your previous discussion)

Jason

 

[SeaJay]

yeah thats the problem, lot of things i'm doing lately taking me closer to technical diving

That's not a problem. That's a solution.

If it helps at all, the educators that I've learned from believe that there is no such thing as "technical" diving - that is, there's no difference between "recreational" and "technical" diving... And that there's no such thing as "no-decompression" diving.

...And if you believe in that philosophy, then you're simply getting further into the education - and you're understanding more. That's not a problem - it's a solution.

 

[mossym]

very very true i like the idea of the skills that tech diving brings. Like you siad why you wouldn't apply them to all diving seems strange, i can see a tech course coming in the near future...

nice article by the way, i just gave my regular dive buddy a copy,

 

[mempilot]

Nicely said!