Independent Doubles vs. Isolated Doubles

[mboot_9]

Any real reason to use independent over Isolated, not looking at the cost for either.?

[String]

Some cave divers prefer independent citing issues like reduced snag hazard etc. In theory as theyre 2 totally separate systems something like a manifold failiure cannot possibly lose all your gas (that said a failiure on independent means you will lose half your gas).

Cost is the main issue i use them - can use 2 normal tanks without the expense of dedicating tanks to twins, buying manifolds, backplates etc. Very easy config to swap between single and twin tank use (just the case of changing a cam strap and takes 2 mins).

[amascuba]

If cost is your main issue then you might want to re-evaluate your priorities.

Independent Doubles:
two normal tanks put together in a band system.
You will have extra task loading of breathing down the tanks evenly.
If you a regulator goes bad or lose an o-ring on your tank then you lose half your gas supply.

Manifolded Doubles:
If you lose a regulator then you turn that post off, but still have access to the gas supply in both tanks.
You don't have the task loading of breathing down the tanks evenly.
Aren't as readily available at dive resorts (for now anyways)

[nadwidny]

Some cave divers prefer independent citing issues like reduced snag hazard etc. In theory as theyre 2 totally separate systems something like a manifold failiure cannot possibly lose all your gas (that said a failiure on independent means you will lose half your gas).

Not quite. On independent doubles if one has a failure they lose half of their remaining gas. Huge difference. As well any failure (1st stage, 2nd stage, post) in an independent system means loss of that half. With an isolator only a post failure means loss of half of the remaining gas.

Also on an isolated system there is virtually no possibility of losing all your gas. That's what the isolator is there for.

[String]

Unless the manifold fails (rare but few incidences of it).

Or the user cant reach his valves to isolate (Shouldn't happen but does. I see lots of people incapable of reaching valves).

[Packhorse]

I like the idea of using independat doubles and being able to swap the the least ful tank after a dive for a full one. This will allow you 2 dives with redundant air and only use 3 tanks where with manifolded doubles you will need 4 tanks.
Having said that my doubles are manifolded.

[String]

Not really, if you breathe say 20 bar at a time from each tank to keep them at roughly the same level (which youre supposed to do) then at the end of the dive each tank should be within 20 bar of the other in pressure so there shouldnt be a "fuller" tank. If you arent doing that you've already lost your redundancy plan.

Manifold 12s should give you enough gas for 2 recreational style dives. Independent 12s give you an identical amount of gas.

[amascuba]

Not really, if you breathe say 20 bar at a time from each tank to keep them at roughly the same level (which youre supposed to do) then at the end of the dive each tank should be within 20 bar of the other in pressure so there shouldnt be a "fuller" tank. If you arent doing that you've already lost your redundancy plan.

Manifold 12s should give you enough gas for 2 recreational style dives. Independent 12s give you an identical amount of gas.

The point is if you lose a regulator or blow an o-ring on one tank then you lose that amount of gas and only limited to the gas left in your single tank. With isolated doubles if you lose a regulator then you turn off that post and you are still able to use the remaining gas from both tanks. Now if you blow an o-ring on a post then you isolate the tanks and at that point you are limited to the gas in one tank. With isolated doubles you have an extra option that will give you more gas. You are more likely to have a regulator failure than blow an o-ring in a tank during a dive.

[Packhorse]

Not really, if you breathe say 20 bar at a time from each tank to keep them at roughly the same level (which youre supposed to do) then at the end of the dive each tank should be within 20 bar of the other in pressure so there shouldnt be a "fuller" tank. If you arent doing that you've already lost your redundancy plan.

Manifold 12s should give you enough gas for 2 recreational style dives. Independent 12s give you an identical amount of gas.

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.

[JimC]

Not quite. On independent doubles if one has a failure they lose half of their remaining gas. Huge difference. As well any failure (1st stage, 2nd stage, post) in an independent system means loss of that half.

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.