Double Tank Manifolds, Bad Idea!

[pocky21]

It's my understanding that by adding another HP hose and SPG, you're not only adding another couple of failure points but the task loading of monitoring additional equipment. Since I have seen several hose/reg failures even in my relatively short diving career, I'm inclined to believe that statistically speaking, the addition of a hose/gauge is more likely to introduce failure than introducing an isolator.

So the argument goes - higher chance of losing 50% of your gas vs lower chance of losing 100% of your gas. This assumes you can reach the valves (but seriously, if you can't reach the valves, what the hell are you doing with a manifold?), and can shut them down in a modest amount of time (again, if you can't do this, why the hell are you using a manifold?). In this situation, in order to lose 100% of your gas, you would need a failure of the isolator that would vent to the outside at a rapid rate regardless of position (virtually impossible), or a second failure that would compound a stuck-open isolator.

Personally, even diving open water, there are a hundred thousand things that are statistically and realistically more likely to kill me than the failure of an isolator. The same can be said about losing 50% of your gas with independent doubles. I like knowing that if I do have a failure, I can quickly and safely shut down the offending post and still have the rest of my gas to fix other problems, get out, ponder the meaning of life, or whatever else I need to do. That is a preference, and a risk I assume given the incredibly low odds of the failures needed to compromise the system. That's my argument for using them, and there are other very well stated arguments for the other system as well. If your dive plan does not necessitate access to 100% of your gas throughout the length of your dive, then use whatever system your heat desires, just know HOW to use it! I'm starting to get the feeling you're looking for validation that independent doubles are in some way superior - and I really don't think you're going to find it here.

 

[DevonDiver]

Do you breathe down one tank and then switch, or do you swap regs during the dive to keep the tanks balanced?

The procedure I've seen used most often is to balance the tanks by 1/3rds: 1/3rd from the right tank, 2/3rds from the left tank, 1/3rd from the right tank. This preserves more gas in the long hose for sharing throughout the dive.

See diagram:

How do you calculate thirds? I am guessing you would have to do a little bit of math adding the pressure used from both tanks to determine 1/3 gas used.

See above diagram.

Are pressure gauges clipped on d-rings on both the left and right hips?

That seems quite logical.

 

[gcarter]

It's my understanding that by adding another HP hose and SPG, you're not only adding another couple of failure points but the task loading of monitoring additional equipment. Since I have seen several hose/reg failures even in my relatively short diving career, I'm inclined to believe that statistically speaking, the addition of a hose/gauge is more likely to introduce failure than introducing an isolator.

So the argument goes - higher chance of losing 50% of your gas vs lower chance of losing 100% of your gas. This assumes you can reach the valves (but seriously, if you can't reach the valves, what the hell are you doing with a manifold?), and can shut them down in a modest amount of time (again, if you can't do this, why the hell are you using a manifold?). In this situation, in order to lose 100% of your gas, you would need a failure of the isolator that would vent to the outside at a rapid rate regardless of position (virtually impossible), or a second failure that would compound a stuck-open isolator.

Personally, even diving open water, there are a hundred thousand things that are statistically and realistically more likely to kill me than the failure of an isolator. The same can be said about losing 50% of your gas with independent doubles. I like knowing that if I do have a failure, I can quickly and safely shut down the offending post and still have the rest of my gas to fix other problems, get out, ponder the meaning of life, or whatever else I need to do. That is a preference, and a risk I assume given the incredibly low odds of the failures needed to compromise the system. That's my argument for using them, and there are other very well stated arguments for the other system as well. If your dive plan does not necessitate access to 100% of your gas throughout the length of your dive, then use whatever system your heat desires, just know HOW to use it! I'm starting to get the feeling you're looking for validation that independent doubles are in some way superior - and I really don't think you're going to find it here.

A high pressure hose will empty an 80 is what, 20 minutes? Free flow or a blown o-ring can empty an 80 in more like 2 minutes. There was a test done, I don't recall the exact numbers and can't be bothered to look for it, but that is the order of magnitude.

Point being, not all failure points are created equal. Just last weekend, did a dive on a shallow (30') wreck. My buddy - my son - evidenced a bubbling from his HP SPG hose from one of the crimp connections as we entered the water. We considered the risk acceptable. We surfaced 65 minutes later and he had 1300 psi remaining in his AL80. Had the hose failed completely, he would not only not have had to surface immediately, he would have had sufficient gas and tiime to do a sub-surface return to shore. Had the bubbling been from a LP hose we would have called the dive before it started.

So losing a HP hose on ID does not lose you 50% of your gas -- unless you don't shut it down and stay at depth. Shut down the valve, and it can still be called upon in a pinch.

FTR, I do not dive any kind of doubles - which does not invalidate my point, but I thought I should make it clear how little a stake I have in this discussion. I just think it is disingenuous to suggest all failures are equal.

 

[DevonDiver]

A high pressure hose will empty an 80 is what, 20 minutes? Free flow or a blown o-ring can empty an 80 in more like 2 minutes. There was a test done, I don't recall the exact numbers and can't be bothered to look for it, but that is the order of magnitude.

Here: Life Ending Seconds • ADVANCED DIVER MAGAZINE • By Curt Bowen

 

[gcarter]

Here: Life Ending Seconds • ADVANCED DIVER MAGAZINE • By Curt Bowen

That's the one.

---------- Post added September 20th, 2013 at 12:13 AM ----------

As an aside, this is the second HP hose failure at a crimp he has had in the last year, roughly 50 dives. This time we replaced the miflex with a tried and true rubber hose.

 

[Tortuga68]

I'd still like to hear about all these failure modes. I'll start you off:

1. Catastrophic manifold failure
2. ???
3. ???

Still waiting...

 

[pocky21]

A high pressure hose will empty an 80 is what, 20 minutes? Free flow or a blown o-ring can empty an 80 in more like 2 minutes. There was a test done, I don't recall the exact numbers and can't be bothered to look for it, but that is the order of magnitude.

Point being, not all failure points are created equal.

Thank you for bringing that study up, it was a really interesting read. I knew the HP side was sent through a restricted orifice, but having hard numbers does shed a lot of light. It was not my intention to imply that all failures were created equal, in fact I really don't see how you got that out of my statement to begin with. While some carry greater impact, a failure is a failure, and I will be turning the dive if something fails. While a HP hose failure certainly isn't a death sentence in itself, it does create additional stress and divert attention that could otherwise be used elsewhere. I have yet to see or hear of a catastrophic manifold failure, yet hose ruptures are just a part of the scuba life. The point being, when every failure is a check mark against you (some of greater impact than others), prudent risk mitigation involves minimizing both the probability and impact of failure. I see ID vs. MD as a trade off, and barring necessity for a particular dive, there is very little reason to say that one is objectively superior to the other.

 

[DaleC]

One is not superior to the other, in absolute terms. The benefits vary, dependent upon the situation.

Quite frankly, I don't worry about mechanical failures - they are quite rare. I worry about user error - they are quite common.

 

[Slamfire]

Here's a more likely scenario that I usually face which illustrates why I choose what I choose (and why I now am leaning more towards SM). As you can see, max usable gas is not the issue as much as dependable bailout is:

I am solo diving a remote setting at a site where there is no information regarding conditions. However, if history dictates, I anticipate the water is cold, low vis and there is an unknown entanglement potential. I plan a max depth cap/RB volume and want redundancy with more gas than a single/pony combo will afford. I am also probably shooting video. My choices come down to two small steel cylinders because they won't kill me getting in and out of the water (I'm alone if I fall and can't get up) and I need to wear less lead yet can achieve neutral buoyancy at the beginning of the dive. I can either use a manifold or dive them independently. The manifold offers me nothing, because I don't need to use all available gas, and the manifold requires a step, post incident, to achieve true redundancy. Independent tanks offer me all the gas I need and two completely separate gas systems.

Up to here you still have not presented a compelling reason why independents are better. If I'm reading this correctly you're saying they are no better, but neither are IDs any better... so far.

My potential to survive human error is also better with ID's. I can miss my turn pressure and the worst that will happen is I will breath down one tank. I will still have a backup to switch to, surface and slap myself silly with. I can also fail (for whatever reason) to shut down the offending valve and still have enough gas to exit (only losing half gas). With MD's I could, potentially, breath right through my turn pressure to the point I do not have enough gas for exit or fail to isolate quickly or properly. I have thought about this a lot and can say that, for myself, human error is a far more likely failure I will have to contend with than equipment failure. My choice highly anticipates and attempts to address human error while solo diving.

We are still talking under your more likely scenario, right? Just to add detail to your scenario, we are also talking 130ft or less and staying within the ndl or with a very light mandatory decompression.

I've seen you do the upside down funky chicken dance doffing a vintage double hose rig underwater. And yet you expect me to believe that for some strange reason you might be uncomfortable reaching back and shutting down an isolator or a post. I just don't buy it. Neither do I buy that even with your independent doubles you will allow one of them to drain out completely through a free flow for whatever reason. If that "what ever reason" has incapacitated you to the point of preventing you from closing a valve underwater, you have much more serious problems to worry about. Definitely more serious than whether you have a manifold or not. IDs are not going to do much life saving if you happen to have a paralysing cerebral stroke while soloing in a remote interior lake.

You missed your turn around pressure... Well, for you in your likely scenarios it most probably means that you will have to do a blue water ascent and end up doing a long surface swim back to shore. But again, why would you miss your turn around pressure. This situational awareness is so basic that there should be no excuse. Complying with turn around pressures is a function of situational awareness--don't blame equipment. This excuse just sounds too close to "trying to solve a skills problem with equipment", or lack of it [manifold] in this case.

Because I know you I'm having a difficult time buying your explanations. They just don't hold much weight either for or against manifolds. If you tell me you just like it better without manifolds and you feel more comfortable for some subjective personal reason, I can buy that. You like kiwis, but don't like oranges, that's fine. I buy that. I respect your right to have personal tastes and choices. Just don't try to convince me that kiwis are superior to oranges using reasons that don't carry that much weight in real life.

Gabe, I can't help you there (but I'd like to see that boat) as that diving is beyond my training.

The point that I was trying to illustrate is that back mounted manifolded doubles are very versatile. IMHO they are better suited for doing a dive to the Gunilda than sidemount or back mounted IDs. In the sidemount case are you going to leave valuable real estate unused when you need to carry that many cylinders? In the case of back mounted doubles one less spg, not having to manually balance two bottom gas tanks, and the avoiding of several regulator switches on a dive that already has plenty regulator switches is a welcome reduction in task loading for a dive like this.

I realize this is not a typical dive for a lot of divers. Yet manifolded doubles are perfectly capable for less complicated dives. I can understand and sympathize with those who have vertebrae problems. I also know that there are certain restrictions that require sidemount or even no mount for someone to squeeze through. But I do believe that MDs are generally more versatile.

If you would like to see the Gunilda or the Bunker Hill one day, wouldn't you think it's valuable to build in some familiarity with the configuration/rig that is better suited for such dives.

But I would assume (you can tell me if I'm wrong) that on the bottom you can either switch to one of your stages if you had to OR you have a teammates BG you could switch to.

Teammates yes. Stages, yes but no. These are deco gases, not bottom gases. Breathing the wrong gas at the wrong depth is very dangerous. That said, having something to breath is better than having nothing and I would rather go out toxed out first and then unconsciously drowned than fully consciously drowned.

In that case, a state of true redundancy does exist and MD's would seem to be the best option.Would you do that dive solo though with all your breathable bottom gas in MD's without recourse to any other bailout gas?

Clear this up for me: The "state of TRUE redundancy" in MDs is only negated by an inability to close a manifold behind your back or by the highly unlikely event that my triple oring barrel manifold fails, isn't it? If said inability is not present would you concede I have redundancy.

No, I would not dive the Gunilda solo with only a set of MDs and no other tanks. Unless you make it a sharp quick bounce dive, you do not have enough gas with just one set of doubles. Not with my SAC rate. And I'm not about to travel to the Great Lakes and charter a boat just to go do a bounce dive with 15 seconds of bottom time. If I'm going to go to the trouble to get there, I want a decent and memorable bottom time.

To the OP: I am not a cave diver and have never been inside an underwater cave.

 

[DevonDiver]

Teammates yes. Stages, yes but no. These are deco gases, not bottom gases.

Small point, but 'stages' are typically bottom gas. Stages and deco tanks being separate things, by definition.

To the OP: I am not a cave diver and have never been inside an underwater cave.

Which explains the above.