Doubles vs Sidemount

[DaleC]

His approach isn't flexible, it's adding unnecessary tasks ( reg switches, valve manipulation ) to an already "high risk" dive.

What has the pony got to do with any of this?

When you have a failure what do you go to first; Isolator or Post?

This debate is really about manifolds or Independent tanks. That person, if he is like me, probably prefers to dive ID's. It's a different gas management philosophy based on when certain actions are performed and what risks are being mitigated.

With the manifold closed, yes I do reg swaps but these are all done in non stressed states (and hey, isn't it good to know both those regs work well). I also always have part of my gas supply "isolated" pre-incident so it cannot be lost. My decision is either left or right post. I have access to all gas, post incident, by shutting down offending posts, opening the manifold etc... I exchange true redundancy for reg swaps - that's it. That's important to me because I solo dive and I have no other source of reserve gas.

With the manifold open others have no reg swaps true, but are diving basically one big tank. Preserving gas is based on a reaction under stress, during an incident. Post incident we both have the same access to gas. They are exchanging no reg swaps for the need to isolate.

Having the manifold closed is also one way to prevent breathing through rock bottom reserves. If one keeps that value in both tanks as a minimum. It is a small point I know but one area I see this as relevant is the habit some divers have of using doubles to do two dives. Dive one is ok but dive two can leave very little in the way of reserve gas to isolate and preserve.

I concede that this approach is not for everyone but it is erroneous to assume those who do use it have given no thought to the subject.

The pony question was a subtle way of suggesting that the same piece of equipment can be used in different, but equally serviceable, ways.

---------- Post added March 21st, 2013 at 08:45 PM ----------

He's going to wack himself someday breathing the wrong mix. Judi Bedard was diving Eagle's Nest in 2005 and ended up breathing pure helium because of an isolator which had been closed during filling. She passed out on the dive due to hypoxia, but her buddy rescued her and she managed to survive. I believe she even dives again but I'm not sure about that.

Isolators should remain open except for a functionality check and when there's an actual manifold failure underwater (which are very rare compared to regulator issues).

rjack, I'm not familiar with this persons habits. Was she intentionally diving independent doubles or was it a poorly executed open manifold dive plan coupled with failed fill procedures?

It seems to me that open manifold use is based, partly, on being proficient and diligent in valve drills. Seems an argument against them that a failure to perform just one such a drill could result in such an event.

 

[TLRRHOBO]

Can you list some advantages of doubles over side mount..

 

[ianr33]

rjack, I'm not familiar with this persons habits. Was she intentionally diving independent doubles or was it a poorly executed open manifold dive plan coupled with failed fill procedures?

I think it was a series of mistakes
http://www.scubaboard.com/forums/accidents-incidents/113785-eagles-nest-accident-report.html

Seems the tanks were filled and dived with the isolator mistakenly closed. That could leave one side with the correct gas and the other side lethal.

Sounds like she only just survived it Diver who almost died there eyes return to Eagle Nest Sink | Tampa Bay Times

 

[PR_SMITH]

I think it was a series of mistakes
http://www.scubaboard.com/forums/accidents-incidents/113785-eagles-nest-accident-report.html

Seems the tanks were filled and dived with the isolator mistakenly closed. That could leave one side with the correct gas and the other side lethal.

Sounds like she only just survived it Diver who almost died there eyes return to Eagle Nest Sink | Tampa Bay Times

I have had tank monkeys try and close the isolator prior to filling. I never take my eyes off the tanks during a fill, unless I am at a shop that I trust.

 

[DaleC]

Just read that, hope she was ok in the end.

As I suspected it was a case of a diver making errors and not a failure of intentionally diving with the iso closed. Though the caution is duly noted.

The Hobo asked what the pros are. Here's how I see it.

Manifolded Tanks with isolation valve

Rationale: Carrying large volumes of gas with ability to isolate and preserve gas based on the actions of the diver when a failure occurs.

Benefits: Maximum usable volumes per tank size used. Access to all remaining gas after isolation by a series of valves that can be opened and closed. Reserve gas volume can be divided between tanks.

Drawback: The need to isolate gas loss by manipulating correct valves. Failure to do so will result in the loss of all gas.

Solution to drawback: Routine practice of valve drills.


Independent Tanks

Rationale: Carrying large volumes of gas with redundancy not based on actions of diver.

Benefits: Reliable reserve volume contained in tank not experiencing loss due to failure. Failure cannot lead to loss of all gas.

Drawback: Less usable gas overall. Reserve volume must be contained in each tank (twice that of manifolded doubles). Requires reg swaps.

Solution to drawback: Become proficient at reg swaps. Develop a gas plan that includes correct volume for dive planned.



Further to the points regarding gas above, there are some pros and cons based on handling the tanks that I have found.

Independent tanks are easier to move over rough terrain (as they can be carried singly) but manifolded doubles are easier to slap on the back and walk into the water with.

Manifolded doubles are heavier to lift but easier to fill.

Sidemount feels more stable UW but also feels more complicated, traffic wise, around the front.

With BM independent tanks, I can swap out one side between dives to increase volume while with manifolded tanks I am stuck with what I have. I can counter that by carrying a stage but that means three tanks total while with ID's I can always just dive two.

Manifolded tanks make a neat, complete, well understood package. Independent doubles are less common (though sidemount is becoming somewhat more mainstream).

Independent tanks can be dived together or singly while manifolded tanks are dedicated to that purpose, unless broken down.

 

[DevonDiver]

Independent Tanks

Drawback: Less usable gas overall. Reserve volume must be contained in each tank (twice that of manifolded doubles).

X bar in two non-joined tanks is different to x bar in two joined tanks? Explain...?

I go on a dive, using rule-of-thirds, with 2x AL80 tanks:

Sidemount - 2x AL80, containing 2200 litres per tank (4400L combined) at 200bar. My turn-point is at 133 bar - when both tanks reach this amount, I turn (4 reg switches). Switching from tank-to-tank every 30 bar, I consume a total of 133bar per tank - a grand total of 2932L of gas. I have left, in reserve, 66 bar per tank, totaling 1466L.

Backmount - 2xAL80, joined by isolation manifold. Total combined gas is 4400L at 200bar. My turn-point is at 133 bar - when the tank reach this amount, I turn. I consume a total of 133 bar - a grand total of 2932L of gas. I have, in reserve, 1466L at 66 bar.

 

[DaleC]

I'm a little lost with bars and litres and you make a point regarding thirds (which I don't use). I was referring to rock bottom calculations.

Let's look at a dive in which I calculate that I need 40cuft to ascend from my max depth/duration. This will be my turn pressure. And let's pretend we are using true 80cuft tanks for ease of discussion.

With a manifold I need to preserve 40 cuft spread across two tanks or 20cuft/tank. This allows 60cuft each for the dive. X2= 120cuft.

With Independent tanks I need to preserve 40 cuft/tank. This allows 40 cuft each for the dive. X2=80cuft.

But why do you do 4 reg swaps? Perhaps tanks perform differently but I can breath one side down to turn pressure and then the other and begin my ascent on that reg = 1 reg swap.

 

[DevonDiver]

I was referring to rock bottom calculations.

So, what happens to rock bottom if/when you have to close your manifold? Blown?

But why do you do 4 reg swaps?

To ensure a more even distribution of air across both cylinders. The most appreciated benefit of this is to retain equal trim in both cylinders (especially when diving ali tanks). A second benefit, no less important - but often less recognized, is that it retains the maximum gas in any given cylinder should the other cylinder become suddenly inaccessible.

I swap regs every 20-30 bar. Reg swaps are initiated so as the pattern will result in my final swap onto my long-hose regulator at the end of the dive. That ensures I have X (reserve) on my short-hose tank and X+30bar in my long hose tank - sufficient to account for air-sharing to a diver with a higher SAC rate.

That doesn't account for the possibility of air-sharing plus one cylinder being inaccessibletowards the very end of the dive (i.e. buddy breathing off the long-hose). That'd require a very unlucky accumulation of failures. It would equate to sharing air in doubles, whilst having had to close the isolation valve. If a dive required that depth of contingency planning, I wouldn't be using rule-of-thirds.

 

[DaleC]

So, what happens to rock bottom if/when you have to close your manifold? Blown?

I suspect the thinking is that the offending post can be shut down while allowing the gas from that tank to be rerouted to the other side.

But I don't want to get too far into a defense because I dive independent tanks anyways. Because I solo I would rather have a known volume in reserve, even if allows less gas for the dive, than an unknown volume based on when I manage to isolate and whether that corrects the problem. With ID's, even if the problem is not corrected, the gas plan should allow for exit.
For my diving a secure second source is more important than usable volumes. I understand that for other divers that may vary - especially if they dive teams and view their buddy as an additional source.

 

[FoxHound]

I suspect the thinking is that the offending post can be shut down while allowing the gas from that tank to be rerouted to the other side.

But I don't want to get too far into a defense because I dive independent tanks anyways. Because I solo I would rather have a known volume in reserve, even if allows less gas for the dive, than an unknown volume based on when I manage to isolate and whether that corrects the problem. With ID's, even if the problem is not corrected, the gas plan should allow for exit.
For my diving a secure second source is more important than usable volumes. I understand that for other divers that may vary - especially if they dive teams and view their buddy as an additional source.

I think the point of the question was in reference to what happens when it is the isolation bar that fails. if you blow an isolation bar o-ring you have to close the isolator as shutting down that tank valve does not stop gas flowing to the isolation bar leaving you only access to gas in one tank....while uncommon it can still happen, but has yet to be mentioned here