Double Tank Manifolds, Bad Idea!

[Akimbo]

One of the most important system design lessons I learned from the bloated Navy saturation diving systems I trained on is that backup systems can conspire to make you less safe. Nobody trained more than we did but there comes a point where quickly and accurately diagnosing a problem is dangerously compromised. Granted there were hundreds of valves, gauges, regulators, and sensors but the principal is valid for the Scuba diver as well.

Any serious study of accidents, diving or otherwise, reveals that most have multiple failures (system and/or human). Three failures are very common. Added hardware provides options to work around failures but increases the chance for human error. Nothing is free.

It is all about failure mode analysis and probability. In this case, the isolation manifold provides backup options, introduces failure potential, and operator complexity. However the same can be said for adding a BCD to a Scuba rig. I think it is fair to say that regulator leaks and freeflow have a higher probability than static HP O-ring and burst disk (in the North America) failures. Add a second regulator and the probability increases, thus the ability to isolate regulators with post valves.

The isolation valve provides a limited opportunity to prevent loss of half of your remaining gas supply on the high-pressure side of the regulators. An important question system designers must ask themselves is “would you bet your life on it”. Time to react is always an important factor in accident prevention so this study is important to digest:

Life Ending Seconds, 3000 to Zero in 72 Seconds

Arguments supporting IDs (Independent Doubles) and doubles with isolation manifolds both have validity. A pony bottle provides the same positive features for the majority of dive profiles as IDs with reduced physical burden, which is also part of the safety analysis. Just ask this guy:



I use the isolation valve for Progressive Equalization. Basically the isolation valve stays shut unless the online cylinder is being equalized. There are a number of reasons discussed in the link but minimizing loss of remaining gas with a HP-side failure is the purpose for this discussion. However, routine equalizing several times a dive justifies easier access, thus the http://www.scubaboard.com/forums/ba...ion.html#post6562489[Valve-Down configuration discussed in this link.

Small changes in a dive profile will alter the delicate balance between compromises that yield optimum solutions. Understanding all of them and making well-informed and appropriate choices is what I consider doing it right.

 

[tracydr]

As I get close to taking some tech classes, I've been trying to decide between sidemount and doubles. I broke my neck in 2001 and have some herniated disks above/below my fusion. Now, this year, I started to suffer from osteonecrosis in the knees and ankles. Hoping some of the treatments that I'm going through will help but my joints have certainly been damaged, enough that if the bone marrow stem cell operation doesn't work, I'll be facing double knee replacements. I have no idea what will happen with the ankles, crossing my fingers that 12 weeks of lovenox stops the progression.
So, it looks like sidemount is in my future, unless I'm willing to only dive tiny doubles, which I'm sure would be less than ideal, given I hope to do caves someday.
I only wish that GUE would start teaching sidemount, as I really wanted to be trained through GUE.
For the OP, I think the biggest advantage of manifolded doubles is that its highly unlikely that you would ever lose all of your air, or even half your air. There's a huge reason why overhead divers ( virtual or hard overhead) chose manifolded doubles as the preferred gear. ( with the exception of sidemount, which has other advantages).
GUE has spent a great deal of effort deciding on the best configuration for any type of diving. They chose manifolded doubles for a very good reason. Good enough for me! My only reason for considering sidemount is physical. If I didn't have physical issues, I'd do what GUE does and take GUE classes.

 

[flots am]

I am so close to breaking down my double lp85's and selling the bands and manifold. Since getting my sidemount cert with Doppler I am seeing less and less reason to.have anything but my lp72's doubled up.

At least in the "over 45" range, manifolded doubles seem to losing favor.

I gave my BP and manifold to my (younger) buddy, who used them for a year or so, then took them apart and gave them to someone else. Not sure who has it now, but it seems to be equipment for the young and "fast-healing" crowd.

 

[vicp]

...first let me say that I am certified to both side mount and back mount and i really like the maneuverability i get with side-mount but I am in the process of undergoing intense overhead environment training and would not even think of diving side-mount at this time..

Why?

I know in you post you said that overhead environments are of no interest to you, but that is where manifolded doubles becomes a requirement. Under IANTD manifolded doubles are required for overhead environment training if not training in a CCR.

COMPLETE NONSENSE!! - IANTD DOES NOT require manifolded doubles for any overhead training. I just got off the phone with Tom Mount, one of the original members and the CEO of IANTD, and he confirmed this. As a matter of fact, IANTD encourages the use of sidemounted doubles over backmounted/manifolded (BM) doubles and Tom does not even own any BM doubles anymore.

the big difference between side mount doubles and manifolded doubles is gas management. on the side mount configuration I have to gas match my tanks within 500PSI of each other to account for a complete failure of one of my tanks so that I can safely make it out of a cave. At depth dealing with narcosis and the heavy task loading ( running a reel or navigation) this becomes more difficult to remember to do and it is possible to find your self over 1000PSI off on your tank balance. A tank failure can then quickly become a serious issue and you might have become a statistic.

I know many people are switching to side mount in over head environments because its easier to get in and out of the water,but until they can handle a high level of task loading I think they are asking for complications on their dive.

If you can not monitor your gas supply to balance out your tanks, you will also probably miss your turn pressure and maybe should not be cave diving at all. The same also goes if you get too "narced," in which case you may consider trimix/helium - but then again, more task loading.

AS for the OP's original question,

You have just installed a complicated life support system behind your back! These valves are difficult to reach (at best) and impossible to see, sure, it can be done (valve drills and all) but that doesn’t mean it’s a good idea. Fumbling with valves behind my head at depth in a stressful, life threatening situation is not inconvenient it’s dangerous and unnecessary.

...Manifold doubles are very heavy and awkward to deal with out of the water...

...I dive independent twins...

Not sure what the OP's point is here (troll?). He complains about the valves behind his back and the weight of BM doubles, but still prefers to BM doubles, even if independent (non-manifolded)??

Why not just sidemount the tanks? You see the valves (to troubleshoot if necessary), no need to carry all that weight on your back (the manifold crossbar weighs a pound or 2 at most), have a truly redundant air supply (separate tanks and regs - no need to guess what is going on behind your back), and best of all -- MUCH better trim, balance and freedom once it all is adjusted. I know that I will NEVER backmount doubles again, and I knew that the first minute I dove sidemount!!

 

[mathauck0814]

I switched to independent back mounted doubles (~21ft^3 steels) from manifolded back mounted doubles (~120ft^3 steels) and never looked back

 

[Akimbo]

Dr. Tracy

Other options include donning gear in the water and mounting gas supplies on a DPV. Treat your onboard gas as bailout when DPV mounted gas is used.

On a related note, I am a volunteer with the Monterey Bay Veterans' working on their Lift to Freedom Project. The objective was to accommodate the full range of disabilities from ambulatory weight limitations like yours to severe non-ambulatory disabilities like David Riley’s (see link).

The Lift to Freedom is in the final stages of installation and we hope to start sea trials in the fall. We are working on the hydraulics and Nitrox systems now. It is a pretty ambitious project for a small non-profit to undertake but a lot of dedicated people have put it an unbelievable amount of work. It is one of the most gratifying projects I have ever worked on. Hope you can come aboard and dive her soon. Here is an image by Dida Kutz the day she was put back in the water after installing the Lift.

 

[vicp]

I switched to independent back mounted doubles (~21ft^3 steels) from manifolded back mounted doubles (~120ft^3 steels) and never looked back

No fair - those are your RB dil and O2 bottles!

Where do you put your bailouts?

 

[mathauck0814]

No fair - those are your RB dil and O2 bottles!

Where do you put your bailouts?

Depends on the dive and how many tanks I have to bring (and whether or not I've got the scooter)

 

[Instructor_Tom]

vicp;

why?

because I'm training in a 3 man dive team plus the instructor doing staged penetrations into Ginnie and then doing a full length exit with a blackout mask on, because we are in the catacombs and doing no light exits while sharing air going through restrictions.

and I stand corrected page 83 of the IANTD training guide does allows for side mount, but it also allows instructors to modify the training and to use their judgement in how to teach to course in a safe manner. my instructor wants everyone in manifolded doubles.

as a DiveMaster at a tech shop in this class the instructor is relying on me to assist with monitoring the other students in the class whom have less experience than I do. training and certification are very different than actually diving for recreation. in a training scenario every possible failure is thrown at you in one dive to complicate and task load heck out of the dive, something that would probably not happen on any real dive, such as 9 light failure across 3 divers all at the same exact time - never going to happen anywhere but in training.

would I take up side mount in a cave after my certification, maybe. more likely I will continue on to CCR and get off the whole Side-mount vs back mount issue.

hope the OP finds value in this thread, and either way safe diving...




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[TSandM]

Jeff Bozanic agrees with the OP . . . “Ideal” Manifolds… Not So Ideal?

I think it's really difficult to do anything but argue opinions on this topic. The failures that require isolation are so rare to occur DURING a dive (and failures of the isolator or manifold even rarer) that coming up with any statistics is difficult. There is no question that diagnosing a failure is easier in sidemount -- and no easier in backmounted independent doubles, nor are the valves easier to reach that way. The failures that lose you half your gas in sidemount are as rare as those requiring isolation in backmount. The difference is that, to use the gas in the cylinder with a regulator malfunction in sidemount, you have to feather the valve, which can be done (I learned to do it in my sidemount class) but I wouldn't want to have to go very far that way.

There may be some increment in safety in one setup or the other, but it would take a long time of gathering careful data to find it, I think. What I do believe is that independent, backmounted doubles is the worst of all possible worlds, because you can't see the valves, you still have trouble reaching them, and you don't offset that with the manifold's ability to access both tanks if a regulator malfunctions. If you want independent tanks, set them up where you can see and reach them (at least that's my take on it).