Doubles: manifolded vs other

[Bismark]

Hi Dave:D

Okay, so in the interests of furthering my education, please give me your rationale for why you chose to dive independent doubles? Look forward to hearing from anyone else. I laid on extra koolaid for this one Dave!

[JeffG]

Hi Dave:D

Okay, so in the interests of furthering my education, please give me your rationale for why you chose to dive independent doubles? Look forward to hearing from anyone else. I laid on extra koolaid for this one Dave!
I guess that would be when he isn't diving his Meg.

[wedivebc]

Hi Dave:D

Okay, so in the interests of furthering my education, please give me your rationale for why you chose to dive independent doubles? Look forward to hearing from anyone else. I laid on extra koolaid for this one Dave!

I have a few reasons but one is convenience when travelling. I can always set up a set of independants using rental gear. From a safety standpoint I don't buy the failed manifold argument against manifolds and in fact I am not against them. I don't believe IDs are any less safe than MDs since when operated correctly will never render the diver with less than enough gas to complete the dive assuming they consume the same amount on exit. Those that argue higher stress, diving a siphon etc are free to augment their turn pressure, as MD divers also do to add their margin of safety. The task loading iargument is a crock since I can train an open water diver to monitor one SPG and even safely remove and replace a reg. A tech diver surely can monitor 2 SPGs and change regs twice during a dive.
The big advantage I see to IDs is should a major failure occur with MDs a valve shutdown drill is immediately required to safe life giving gas. With IDs the diver can ignore the failure for long enough to locate guideline, pull himself out of a restriction or anything else that might increase chances of survival then shut off those annoying bubbles. This system works equally well either solo or when with another independantly configured diver. I can think of no single failure that will render a ID diver OOA.
And yes Jeff I try to avoid OC diving these days unless absolutely neccesary (like bailout):D

[Bismark]

Okay, what about the view that the failure is most likely to occur on the right post (the working post) and shutting that post down actually saves the entire remainder of my gas supply vs a post failure which means you are automatically restricted to 50% of your supply? As far as losing gas from a failure, it takes me about 5 seconds max to shut down my isolator. (I do understand about the problem of being in a restriction, etc and not being able to reach my manifold) Then I am in the same boat you describe. I guess the trade off here is balancing the statistical chances of what is likely to fail. Have I missed anything here?

As for the extra SPG, one more thing to go wrong, but again, just a question of statistics.

Is this an accurate summary of the MD versus SD argument? It can't be that simple can it? What did I miss?

[wedivebc]

Okay, what about the view that the failure is most likely to occur on the right post (the working post) and shutting that post down actually saves the entire remainder of my gas supply vs a post failure which means you are automatically restricted to 50% of your supply?

I am not sure where that statistic came from. I have seen several LP hoses fail and they didn't care which post they were connected to. A hose could easily get damaged while passing though a restriction on a wreck. Not a good time to be performing valve shutdowns.
Again I don't see a need to save more than 50% of the gas. And that is only at the far reaches of the dive. A failure any time before the turn point will result in more that 50% even for the ID diver.
If I am in a cave or wreck and the bubbles fly and vis goes to zero I want to have my hand on the guideline not on the valves. I will not be stressed knowing I have enough gas to make the exit.

[wedivebc]

As for the extra SPG, one more thing to go wrong, but again, just a question of statistics.



Take your favorite reg, connect to a full al80 and record tank pressure. Remove SPG and open tank for 30seconds. Re-attach and record pressure. Now remove hose from LP side and perform same test and let us know what pressure changes occured.

[Bismark]

I am not sure where that statistic came from. I have seen several LP hoses fail and they didn't care which post they were connected to. A hose could easily get damaged while passing though a restriction on a wreck. Not a good time to be performing valve shutdowns.
Again I don't see a need to save more than 50% of the gas. And that is only at the far reaches of the dive. A failure any time before the turn point will result in more that 50% even for the ID diver.
If I am in a cave or wreck and the bubbles fly and vis goes to zero I want to have my hand on the guideline not on the valves. I will not be stressed knowing I have enough gas to make the exit.

Would be correct here then to say this scenario is applicable to a LP hose failure only and not a failure with the 1st or 2nd stages where the moving parts actually are? I don't have an actual statistic for you here but I think the thinking here may be that the working bits of the right post are working, thus more likely to fail than the bits that are not doing anything.

[J.A.S.]

just a question. when you have IDs do you have a full set on each tank. 2 second stages with spg?

assuming you manage air consumption heavily to balance the weight of tanks properly?

thanks

[TSandM]

My friends who dive sidemount (a form of independent doubles) have a long hose on one tank and a short one on the other -- one second stage per tank. They do shift their breathing from one tank to the other during a dive, to maintain balance.

My guess is that nobody has any statistics that would prove beyond a shadow of a doubt that one approach is better than the other. Catastrophic equipment failures just aren't common enough to collect big series of them.

[Bismark]

My guess is that nobody has any statistics that would prove beyond a shadow of a doubt that one approach is better than the other. Catastrophic equipment failures just aren't common enough to collect big series of them.


Happy for that but I am starting to see cracks in my world view.........might have to call in some heavier hitters......:crafty:

[wedivebc]

Would be correct here then to say this scenario is applicable to a LP hose failure only and not a failure with the 1st or 2nd stages where the moving parts actually are? I don't have an actual statistic for you here but I think the thinking here may be that the working bits of the right post are working, thus more likely to fail than the bits that are not doing anything.
I am not sure I could say the working second stage is more likely to fail because I have seen both fail, although sometimes the cause can be a poorly stowed secondary that drags in the dirt.
Please try my experiment though and try purging a reg for 30 seconds as well just to put these 3 failure modes in perspective.

Happy for that but I am starting to see cracks in my world view.........might have to call in some heavier hitters......:crafty:

That is too funny, calling in the borg mothership, send some nanoprobes:D

[Bismark]

Nanoprobes? Isn't that what happened to you last time you were abducted by alien megdalons?

[Bismark]

Here is a quote from a post several years ago re: the same question we are discussing:

All evolve from the fact that you can't access all the air from either regulator.
(1) Gas management: To safely manage the gas you have to switch regs at least once, and you really ought to switch twice. And that's only on the way to your first third. You still need one more switch coming out. Are you going to use two long hoses so you can stick with the procedure of donating the reg you're breathing, or are you going to have to remember which one you're breathing, which one to donate? Just making sure you're always associating the correct SPG with the correct reg is a pain in the tail.
(2) Fewer options - a simple free-flow, the most common regulator failure mode, costs you all the air in its associated tank. If you're diving thirds (maybe you shouldn't go all the way to thirds on independents - I don't - I should say I "wouldn't" - so far I've restricted my independent doubles use to open water) and lose a tank just as you hit thirds on tank 2, you have just enough gas to get out with no delays, and nothing left for the excitement... this is not good. If for some reason your buddy needs any of your air, you're both dead.
(3) More failure points - two HP hoses vice one.

[Thalassamania]

Nanoprobes? Isn't that what happened to you last time you were abducted by alien megdalons?The answer to Life, The Universe, and Everything is, as we know, 42. What's the question?

For many of the same reasons I favor IDs. Here's Jeff Bozanic's take:

“Ideal” Manifolds… Not So Ideal?
By Jeffrey Bozanic, NSS-CDS 181, NSS 22353 Fellow
October 2005


The Benjamin Conversion manifold, or as it is more popularly called, the dual valve manifold, was utilized for cave diving and other environments in which regulator redundancy was deemed beneficial throughout the late 1970s to the mid-1990s. This manifold was a vast improvement over the pre-existing manifolds of the day, as they allowed two independent regulator systems to be used on the same set of doubles. Thus, if a regulator failure occurred (either first or second stage), the diver still had a viable option for self-rescue from the cave. During this time, cave divers worried about a few possible failures that could still result in catastrophic gas loss from the primary gas supply. These included:
Burst disk failure
Sudden, massive failure of one of the cylinder neck o-rings which seals the manifold
Loss of integrity of the manifold itselfThese concerns lead to the development of the “Ideal” or isolation manifold, which allowed the two cylinders to be isolated from each other, maintaining at least part of the gas in the event of one of the failures listed above. It was considered a vast improvement, and very quickly replaced the use of the “unsafe” dual valve manifold. It is the primary manifold used today for all forms of technical open circuit diving. Yet, my opinion is this valve does not add safety, rather it significantly reduces it.

During the twenty or so years in which the dual valve manifold design was in use, there was only one recorded failure of the type listed above that occurred while diving. This event occurred during a cave dive while using a Sherwood manifold incorporating a metal-to-metal seal. Immediately prior to the dive, the double cylinders were accidentally knocked off the preparation platform. They fell about three feet to the ground, landing on the manifold. The manifold was closely examined prior to diving, but was not leaking, and the divers elected to dive. After the cylinder pressure had been reduced to about 1500 psi, the manifold catastrophically failed, and both divers exited successfully sharing gas from the remaining rig. It was suggested at the time that the fall caused a displacement cylinders relative to each other, which did not manifest itself until the pressure reduction allowed the metal-to-metal seal to shift and lose integrity.

In addition, in 30 years of accident data collection, there are two instances of in-water burst disk failure recorded. Both events occurred with cylinders that had been pressurized beyond the working pressure of the cylinders (in one case almost to the hydrostatic test pressure!), and occurred within minutes of the cylinders being placed in the water (prior to cave penetration, while in a safe environment). Also, in both instances, the burst disks had not been replaced in many years. It can be hypothesized that the old disks had metal fatigue from small flexing associated with repetitive filling and emptying over the years, and failed due to thermal shock when placed into relatively cold water after being sun warmed on the surface. Cave divers used to alleviate this risk by double disking or soldering the disks shut, but these are not recommended procedures. A far better practice is to replace all burst disks annually. As both of these incidents occurred at the surface, prior to beginning the dive, an isolation manifold would not have benefited the divers, since they would have called the dive anyway.

In contrast, since the isolation manifold was introduced in the early 1990’s, there have been many, many incidents related to misuse of the manifold. Most of these have been rectified without harm to the divers involved, but all of them had the potential for very serious consequences. The types of problems associated with this design of manifold along with representative case histories include:
The isolation valve being closed prior to the dive.Case #1: This involved a cave diver who began the dive with 3,000 psi (200 bar) in his doubles. He and his buddy did a S-drill prior to descending, indicating that both regulators were working fine. About 15 minutes into the dive, he noted that the pressure on his SPG was not dropping as expected. He reached up, opened the isolation manifold, and watched as his pressure dropped from 2,700 psi (180 bar) to 1,700 psi (110 bar). He called the dive, and exited the cave with no further incidents.

What happened was that the diver was in the practice of always leaving his isolation manifold open. However, when he had it filled, apparently the fill station operator closed it. Thus, only one cylinder was being utilized during the dive. The diver using the cylinders did not check the isolation valve, since it was “always” open. The pressure drop seen was due to the S-drill usage, BC and drysuit inflation, and cylinder cooling after being placed into the water.
The isolation valve being closed during filling of the cylinders.Case #2: A cave diver planned a nitrox dive to a depth of 110 ffw. Prior to the dive he analyzed his cylinders and found that he had EAN32, as expected. He proceeded to a depth of 50 ffw, whereupon he began to experience symptoms of CNS oxygen toxicity. He immediately began sharing gas from his buddy, and aborted the dive.

After examining the cylinders on the surface, the team found EAN32 in one cylinder, and 100% oxygen in the second. Apparently, at some time during the blending process, the isolation valve was shut, resulting in only one cylinder being properly prepared. This was the cylinder that was analyzed, and so everything appeared normal prior to the dive. At no time prior to the dive did the diver check the isolation valve.
Roll off of the left manifold valve.Case #3: A cave diver swimming through a tight cave passage experienced a sudden failure of his gas supply. He switched regulators, and aborted the dive. After surfacing, he found that left manifold valve was closed. It had been open prior to the dive, as evidenced by his utilization of that regulator for the entire period up to the sudden supply failure. His forward movement through the overhead environment resulted in the “auto-shutdown” of the valve, as the hand wheel turned shut off as it scraped across the ceiling.

These failures are only representative of those in the files, and related to me anecdotally from other sources. Cases like this are very numerous, and any of them could have resulted in a fatality. In my opinion, it is only a matter of time until one does.
One might argue that these incidents did not need to occur, and that it was the divers’ fault for not checking the isolation valve prior to their dives. I do not disagree with this. However, when a piece of equipment opens itself up to a multitude of cases of “pilot error,” while not providing any concrete improvement in other areas of safety, then the net result is one of additional risk with a commensurate reduction in safety. For this reason, and the history of misuse of the manifolds in the field, my belief is that we should go back to using the standard dual valve manifold of the 1980’s or adopt another type of technology.

NOTE: This is one of a series of articles planned for Underwater Speleology, NACD News, and other journals of interest to the technical diving community which will discuss findings from the combined accident analysis files collected by the cave diving community.

About the Author:
Jeffrey Bozanic
P.O. Box 3448
Huntington Beach, CA 92605-3448
(714) 775-4462

E-mail: JBozanic@HQonline.net

Jeff was certified as a NAUI Instructor in 1978, and for the NSS-CDS in 1983. He is certified to teach diving for the NSS-CDS, IANTD, TDI, and NAUI. Jeff is active in teaching cave, rebreather, nitrox, technical nitrox, and trimix diving courses. Together with his wife, Rebekah, he has maintained the combined accident files for the cave diving community (a joint project of the NSS-CDS, NACD, and IUCRR). He has published extensively on diving education topics, with heavy emphasis on cave diving safety techniques. He has edited/reviewed many diving textbooks, and is the author of Mastering Rebreathers. He has served on several Boards of Directors in the diving community, including as Chairman of the NSS-CDS and as Vice Chairman of NAUI, and as Treasurer on the AAUS Board. Jeff has received the NAUI Outstanding and Continuing Service Awards; the Silver Wakulla, Abe Davis, Henry Nicholson, and International Safe Cave Diving Awards; the SSI Platinum Pro 5000 Award, and is a NAUI Hall of Honor inductee.

[wedivebc]

Here is a quote from a post several years ago re: the same question we are discussing:

All evolve from the fact that you can't access all the air from either regulator.
(1) Gas management: To safely manage the gas you have to switch regs at least once, and you really ought to switch twice. And that's only on the way to your first third. You still need one more switch coming out. Are you going to use two long hoses so you can stick with the procedure of donating the reg you're breathing, or are you going to have to remember which one you're breathing, which one to donate? Just making sure you're always associating the correct SPG with the correct reg is a pain in the tail.
(2) Fewer options - a simple free-flow, the most common regulator failure mode, costs you all the air in its associated tank. If you're diving thirds (maybe you shouldn't go all the way to thirds on independents - I don't - I should say I "wouldn't" - so far I've restricted my independent doubles use to open water) and lose a tank just as you hit thirds on tank 2, you have just enough gas to get out with no delays, and nothing left for the excitement... this is not good. If for some reason your buddy needs any of your air, you're both dead.
(3) More failure points - two HP hoses vice one.

The person who posted this clearly didn't understand the best method or protocols for diving IDs. I think I have addressed most of these so called issues with IDs.
One comment I would make concerning keeping the spg aligned with which reg you are breathing off of I use a left hand reg for left tank. An Aqualung titan, Scubapro R190, a posiedon extreme, Oceanic omega to name just a few regs that can be converted for left hand operation make keeping track of which side you're breathing from a cinch.

[Bismark]

Interesting reading Thalassamania. What would you say to the argument about gas donation in the case of two independent 1st stages? In other words, you now have to have two separate skills to donate gas depending on which 2nd stage you are breathing. Further, what type of lp 2nd stage hose arrangement would you have in the case of ID's?

[wedivebc]

Interesting reading Thalassamania. What would you say to the argument about gas donation in the case of two independent 1st stages? In other words, you now have to have two separate skills to donate gas depending on which 2nd stage you are breathing. Further, what type of lp 2nd stage hose arrangement would you have in the case of ID's?
If there is no single failure that could require an ID diver to need to share gas why would they require a protocol for 2 ID divers to share gas? A truly redundant breathing system means no reliance on a buddy, no need for a long hose. This is why IDs are so popular with solo divers.

[Thalassamania]

Interesting reading Thalassamania. What would you say to the argument about gas donation in the case of two independent 1st stages? In other words, you now have to have two separate skills to donate gas depending on which 2nd stage you are breathing. Further, what type of lp 2nd stage hose arrangement would you have in the case of ID's?wedivebc has it right, there is not single failure that puts an ID-Diver in an air sharing position. Even so, since I (similarly) tend to use sidebreathers or a lefty/righty combo it's really a non-issue. It's not that I have an issue with with an "Ideal Manifold," I'm happy to use one, its just that the "Ideal Manifold" is (as so many of its users like to call lots of other things) a solution in search of a problem.

BTW: There was a time that a well traveled diver was quite used to using air coming from either side ... the Europeans had their IP hose over their left shoulder.

[Bismark]

So if you are diving with a buddy? Not so much for you but your buddy? Or do you just adopt the mindset that you are solo diving all the time? If not, what about my question?

[Thalassamania]

So if you are diving with a buddy? Not so much for you but your buddy? Or do you just adopt the mindset that you are solo diving all the time? If not, what about my question?If I have to use doubles the I am diving with with a trained and drilled team member (much more than a buddy in the standard concept) so its really not an issue for me, or for him or her.

[wedivebc]

So if you are diving with a buddy? Not so much for you but your buddy? Or do you just adopt the mindset that you are solo diving all the time? If not, what about my question?
You of all people should understand the value of like-equipped and trained team members. In a ID team there is no need for gas sharing. In a mixed team there is still the availablity of sharing gas. For that and most cases I keep a 7ft hose on my right post. An OOA diver may still take the reg from my mouth but should it turn out to be the left tanks I am breathing from at the time it is easy to change over to the long once the OOA diver gets it together. The same rules for gas matching apply for IDs as for MDs.

[TSandM]

I asked about donation when I was diving with my sidemount friends, as I was backmount. They have a single long hose, and the other hose is short (but deco reg short, not 24"). They can donate the reg they are breathing, no matter which it is, or they can unclip and donate the long hose as neither tank is ever turned off. Yes, there would be a one or two second delay in donating the clipped off reg. I was okay with that, as I was diving doubles, so it would be an unusual circumstance indeed that would put me in instant OOA status.