Independent Doubles vs. Isolated Doubles

[Soggy]

Thats why sidemount is better for an independent setup. The only way you loose any meaningful amount of gas is in a neck o-ring or burst disk failure. A bad regulator can be swapped out underwater quite easily in sidemount. This is actually one of the biggest advantages to diving sidemount - everything is right where you can see and manipulate it easily without help.

JimC sidemount

 

[JimC]

Of course it all depends on the dive profile and deco obligations but I dont see any reason not to consume half of one tank then switch to the other and finish the dive on that one.Then you always have at least half a tank in reserve. That way when you swap out the empty one for the next dive you still have 1.5 full tanks and full redunancy.

If you are going to swap regs every 20 bar thats up to 20 reg swaps per dive! I dont know who says you are "supposed to do" that but it sounds like a lot on unnessary work to me. And I dont see any bennifit over using a manifold.

From a gas reserve point of view, there is nothing wrong with it. But I prefer to swap every few hundred psi. This translates into about 6 switches on a given 1/3rds in 1/3rds out dive.

- Balance. Draining one tank down 1/3rd leaves me keeled over quite a bit. This is more pronounced on sidemount than backmount, but its still there.
- I prefer to maintain equal(ish) reserves because I cannot be sure which tank will have the theoretical failure.

 

[JimC]

JimC sidemount

My only regret is that I wish I had switched to sidemount sooner.

 

[Soggy]

My only regret is that I wish I had switched to sidemount sooner.

You were too busy with all that DIR crap...

 

[JimC]

You were too busy with all that DIR crap...

So true, but it worked for what I was doing at the time.

 

[mthirsc]

Keeping the cylinders within 1/3 (AL 80's) I haven't had an issue with balance. This usually equates to two reg switches, maybe a third during return to keep things even. I like to breathe 1/3 the first, 2/3 the second, then back to the first on the return. Always enough in either to safely complete the dive with safety margin.

Independents are a lot easier to fill/transport. They are also great for kayak diving because of this. I'm not saying a manifold doesn't have its advantages, but in my opinion indys are safe and simple.

 

[nice-diver]

talking with a buddy last month, if you dive your tank down to 500 psiand finish our dive then...with independant doubles you come back with 1000psi of safty air;

with manifolded dubs, both tanks come down to 500 psi and you get to be in the water that extra 500psi.

I know a couple of people who dive 30' lakes and jetties with doubles, gotta dive where you live.

 

[JimC]

talking with a buddy last month, if you dive your tank down to 500 psiand finish our dive then...with independant doubles you come back with 1000psi of safty air;

with manifolded dubs, both tanks come down to 500 psi and you get to be in the water that extra 500psi.

I know a couple of people who dive 30' lakes and jetties with doubles, gotta dive where you live.

Uhh, say what?

500psi in one and 500psi in the other is the same on independents and manifolded doubles.

 

[amascuba]

talking with a buddy last month, if you dive your tank down to 500 psiand finish our dive then...with independant doubles you come back with 1000psi of safty air;

with manifolded dubs, both tanks come down to 500 psi and you get to be in the water that extra 500psi.

I know a couple of people who dive 30' lakes and jetties with doubles, gotta dive where you live.

An AL80 is really 77.4cf at 3000psi. That's .0258cf per psi. (77.4 / 3000 = .0258)

Say that you are using independent double AL80's and breath both tanks down to 500psi. That's 12.9cf per tank or 25.8cf total. (.0258 * 500 = 12.9 * 2 = 25.8)

Now say that you are using manifolded double AL80's and breath down the tanks to 500psi. That's 25.8cf total. (500 * .0516 = 25.8)

By manifolding AL80's together you double the gas capacity to 154.8cf (77.4 * 2 = 154.8) at 3000psi. That's .0516cf per psi. (154.8 / 3000 = .0516)

Basically what I'm getting at is independent double AL80's with 500psi each = isolated manifolded double AL80's with 500psi total. It's not the pressure that matters, it's the volume that matters.

 

[String]

talking with a buddy last month, if you dive your tank down to 500 psiand finish our dive then...with independant doubles you come back with 1000psi of safty air;

with manifolded dubs, both tanks come down to 500 psi and you get to be in the water that extra 500psi.

I know a couple of people who dive 30' lakes and jetties with doubles, gotta dive where you live.

The maths doesnt work like that. Yes the pressure is the same but there are two tanks so its TWICE the quantity of gas.

500 psi in twin 12s is the same as 1000psi in a single 12l.

So if you use a "back with 500 psi" rule with a single tank then 250psi is the same quantity as doubles.

Doing it in metric as its simple.

1 x 12l tank = 2400 litres of gas
so twin 12l tanks == 4800 litres of gas

for the same PRESSURE a twin tank setup will have twice the volume (actual amount of gas) than the single.

So there is no wasted capacity.