Independent back-mounted cylinders
I am curious if anyone dives with non-manifold connected, back-mounted cylinders and, if so, how the rig is configured and used. Obviously, 2 spgs would be required, but are 2 long hoses used? What is the protocol for breathing down the gas; eg, switch regs. every so many psi used or other? Lastly, are non-manifold connceted tanks, regardless of where mounted, safer than manifold connected doubles?
Although I do not dive with independents I would imagine its the same setup as a manifold set with the addition of an extra SPG on the right post. But yes you would need to breath them down evenly, i.e. switching regs every few hundred psi or so... You would need the 7' hose cause you can't just donate a tank as you would diving a sidemount rig...
But thats just a guess cause I've never dove sidemount or independent doubles. So someone correct me if I'm wrong.
Chunky
If you are diving independent tanks, you might as well sidemount them...
Joe
Originally Posted by Richard Pyle
I 'think' Gary does BM independents? Anybody know where Gary's been lately?
Hangin' out with the Fuzz
and yeah, he dives independent back mounts....
Safe diving,
Rich
Gary is one of the few that I know of that dives independent backmounts. If you want to dive independents for redundancy, get 'em off your back and on your sides. With few exceptions, everybody I know that has gone sidemount likes it much more than backmount.
Whether or not you put both regs on a long hose is up to you and your buddies. It's what YOU, and those that dive with you, are comfortable with. I normally dive sidemount with two short hoses. When diving with somebody that's on manifolded doubles, either I put a long hose on ONE cylinder, or one of us carries a buddy bottle.
Gas switching is a personal preference. I, personally, keep the cylinders within 300psi of each other. With a 3600psi start pressure, it would go like.....breathe 300 out of the left, then 600 out of the right, then 600 out of the left, 600 out of the right, 300 out of the left, then turn the dive at thirds. Stay on the left to breathe 300 more, then back to the right for 600, then the left for 600, etc... In reality, I switch more often than that...but I don't want any more than a 300psi differential between cylinders.
Mike
Tried it a few years ago and never wore backmount again.
It is personal and a lot of divers I know do the same thing. I find that for myself, I don't notice much of a difference and I breath it right down close to thirds before switching. On the way out I'll switch to try and keep them close.
Re: Independent back-mounted cylinders
Hi.
As everyone mentioned, I dive independent tanks almost excusively - usually as independent back-mounted doubles, sometimes as sidemount. I did all my training from intro cave - thru full cave and Trimix - in independent back-mounted doubles.
Lamar Hires discussed also his team's use of independent back-mounted doubles when diving in Australia - since they were winching the tanks in and out of caves and didn't want to risk the manifolds reaction to such harsh treatment - but he has reported that most of his exploration is in sidemount.
As Mike has mentioned, sidemount will offer even slightly more redundancy than backmount having the tanks in front of you where you can deal with problems better; however, I like the convenience of the back-mounted configuration in most of my diving.
If you are diving exclusively with a buddy in large open passageways in the US I suggest you consider sticking with manifolded doubles - if only because stepping out of the typical backmount and manifold framework will limit your access to training, experienced mentors and dive buddies.
There are advantages and disavantages to any system. Independent tanks are probably more useful to those who dive solo or who may need greater than average self-rescue capability due to restrictions, silt, tight passages etc that may cause buddy separation. Sidemount offers roughly the same advantages in a wider, lower configuration that I find wobbly and inconvenient but is suited to a wide range of diving. Also, many who have problems lugging tanks on their backs or needing to transport tanks in rough areas find sidemount advantageous.
Basically, independent back-mounted doubles are sidemount tanks mounted on your back. 2 spgs is obvious (I run mine both from behind my left arm and in front to under my left chest strap so they are retained across my belly tucked back under the strap). One long hose is all that is required for air sharing with a buddy. As long as proper air switches are maintained there will be air in each tank to supply 2 divers out in the usual fashion. A system of anchoring 2 regs in an accessible fashion for easy switching will be necessary (I clip mine at my shoulder D-ring). I prefer to neatly wrap and bungee my long hose on the tank but the hogarthian hose routing is a more likely method for those wishing to conform.
It is important to ensure air is properly distributed across the 2 tank system. Air switches are maintained in any system favored by sidemounters. There is a procedure in the NSS/CDS manual for the switches (Equipment Configuration: Side Mounting p59) that I favor as the simplest and most convenient method, which consists of breathing 1/3 off tank A, 1/3 off tank B (and turn dive). Continue on tank B to 1/2 and switch back to tank A. They are 1/6th a tank separate at that point; continue switching each tank after every 1/3 of a tank so as to maintain the 1/6th a tank difference.
In practice, this allows me to start my dive on my short hose, switch to long hose halfway in, and stay on long hose (for typical mouth donation air share) until half way out of the cave again.
If I needed to donate air while on my short hose I'd still mouth donate but then trade the long for short as soon as my buddy is stablized - which is the only additional piece of information my dive buddy "needs" to know - and the fact that in this case we are probably also near the exit. More complex dive plans may also result in a short hose donation under other circumstances but those would be rare and handled in the same fashion.
So, as you can see, my independent backmount configuration is designed to be easily integrated into a team of backmount divers with minimal extra considerations - while maintaining my own self-rescue independence.
Independent backmount feels much more similar to standard backmount then sidemount. The changes to equipment, procedures and methods are minimal - the most important being proper air switches.
Re: Independent back-mounted cylinders
Yes and no.
Independent tanks provide better protection against the more critical uncontroled air loss accidents (valve, manifold, overpressure plug, or unisolated/delayed regulator failures).
Manifolded doubles provide better protection against the more common regulator failures (isolated regulator or regulator failed to as to not provide gas).
The advantage to independent tanks is that a critical gas failure is limited to only one tank and that proper air management will ensure that adeqate air reserves remain in the uneffected tank for self rescue (which is a real advantage - even in a buddy team - over an air sharing exit).
Manifolded tanks perform better in the case of a regulator becoming unusable: the whole gas supply is still availiable through the remaining reg.
The reason I prefer independents has a lot to do with the performance of each in their weak areas (where the other is best):
Independents: if a regulator fails you have adequate air to make a safe, self supplied, exit from max penetration on the unaffected tank. Also, the air in the affected tank is not lost and can still be accessed through various means (even if you have to cycle the valve or breath to breath off a free flowing reg).
Manifolded: a free flowing reg can drain a pair of manifloded doubles in ~3 minutes if not isolated. A manifold failure can be unisolatable, a HP plug unscrewed/blown or a tank neck o-ring that is not isolated via the center isolator bar will drain both tanks.
The defense against the worst case failures for manifolded tanks is to share air with a buddy for an exit from max penetration on minimum safe air supply computed for 2 self supplied divers not having to share air. As we all know this is not realistic as it takes longer for 2 people air sharing to make an exit sharing air together then it does when swimming independently.
The defense against the worst case failures for independent tanks is a self supplied exit for one diver on his own minimum safe air supply - but (if not solo diving) actually still having his buddy's full reserve air supply available as well.
Many people still prefer the manifold option and either method is defensible as IF you are buddy diving with a dependable and always availiable buddy in clear open passages the simple failure of a regulator is more likely than a catastrophic failure affecting both air supplies. And when a double regulator failure is considered both methods face roughly the same list problems and solutions)
Combining additional independent air supplies (such as safety or stage bottles) opens up a whole new list of options. Additional air supplies provides additional margins of safety to air system - vastly improving the self-supply ability of the manifold systems especially. An additional independent source of air or the ability to hand off "extra" air in the form of a stage bottle to a buddy to allow him self-supplied exit untethered is an advantage rarely considered in basic cave diving classes.
Visiting friends in Baltimore. Just got back.
It snowed on me
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