Twin tanks: manifold or redundant

[Sas]

Yes. I would stress that I dive this system primarily because I solo. I also think Wookies system of closing the isolator and occasionally opening it to equalize would work just as well as IT's. I also recognise that there is a transition point at which having access to all your gas becomes more important that true redundancy (long penetrations or deco obligations). At that point the benefits of IT's diminish IMO.

With the isolator being open or only slightly open, well I am interested to hear more about this. I was taught to have it fully open. Some of the reasons for this include:
-minimising confusion over which way is open or closed - not likely to be a problem on the surface, but at depth, when stressed, might be confusing to people
-easier for it to roll off to closed during a dive

But I'm new to doubles diving so would be interested to hear what other people think of this.

The best advice is to know the pros and cons of each system and plan accordingly either way. I always enjoy this topic in particular as a good (healthy) debate can bring out little things I might not have considered before.

Yes me too, especially as I just decided to go manifolded doubles over independents

 

[TSandM]

I think the point that mattboy was trying to make is that the most common sources of leaks are in regulators, either where they attach to the tank, or in the first or second stage. A manifold permits you to shut off the offending regulator and still access all the gas that remains in both tanks. In the very unlikely event that there is a leak in the tank o-ring, valve or manifold itself, isolation will save half of your gas.

How useful a manifold is DEFINITELY depends on your facility with valve shutdowns. And a manifold DOES make it possible to lose all the gas in both tanks, although you'd have to be awfully bad at valve drills and have lost your buddy to have that happen.

I believe that few people dive independent, back-mounted doubles. You lose the positive things about a manifold, and gain none of the advantages of a sidemount setup (the ability to SEE both tanks, valves and first stages).

 

[DaleC]

I think the point that mattboy was trying to make is that the most common sources of leaks are in regulators, either where they attach to the tank, or in the first or second stage. A manifold permits you to shut off the offending regulator and still access all the gas that remains in both tanks. In the very unlikely event that there is a leak in the tank o-ring, valve or manifold itself, isolation will save half of your gas.

Yes, I saw that too but there is always that debate as to whether one goes to the post or isolator first which shows there can still be some confusion over the issue.

How useful a manifold is DEFINITELY depends on your facility with valve shutdowns. And a manifold DOES make it possible to lose all the gas in both tanks, although you'd have to be awfully bad at valve drills and have lost your buddy to have that happen.

Or have no buddy. I am curious Lynne, as someone who has gone through GUE training, do you discuss/practice working the isolator with both hands? I am just imagining a scenario when the usual isolator hand may be trapped or incapacitated in some way and can't recall reading about it. It would be a skill all manifold users should have considered IMO.

I believe that few people dive independent, back-mounted doubles. You lose the positive things about a manifold, and gain none of the advantages of a sidemount setup (the ability to SEE both tanks, valves and first stages).

It all depends on what one considers advantages and negatives (I think they vary depending on the dive).
I think that most people follow trends and the trend now is for manifolds (which is fine). The trend until recently was not sidemount for OW but look what is happening there. People say SM's gain no benefit over BM's (as a Doc I hope you see the humor) in that scenario. I sometimes worry that the benefits of manifolds are touted over IT's without looking at the possible downsides. A well educated diver will face and plan for these of course but some "followers" may not.

Perhaps the biggest downside I actually see (in reality) is that manifolded double divers sometimes push the limits by doing a second dive when perhaps they shouldn't because they don't want to bring a second set of doubles with them. This of course, is an operator error not a design error, and happily I did see someone from your agency demonstrate restraint recently and sit out a second dive for this very reason.

 

[Nemrod]

Yes it is possible to lose all of your gas in a manifolded system if you can't isolate, which is why I think it suits buddy diving more than solo diving... Your buddy should be diving with enough gas to get you out in the (unlikely) event that you lose all of your gas.

I think both systems have their place.

Yes, which is why the independent doubles are still used by solo divers.

N

 

[halocline]

I don't argue with anyones decision one way or the other but I think this is not correct. With independant twins any gas leak (whatever the source) will only result in the loss of one tank. If you follow your gas plan you should always have enough gas to exit/surface. With manifolded doubles a gas leak (whatever the source) could result in the loss both tanks. That is just an objective fact. Whether someone feels they have the skill to say they will "always" be able to isolate is subjective.

Not arguing against manifolds, just that one detail.

You have to think about probability. The probability of a leak that would empty both tanks in a manifolded set is infinitesimal compared to the probability of a leak that would empty one tank in a set of independent doubles. In that far more likely scenario, you have all the gas, not half. And by far more likely I would venture a guess that we're talking about hundreds of times more likely, or more. Certainly a manifold failure (six consecutive o-rings on mine) is no more probable than two regulator leaks which would empty both tanks in independent doubles (as well as in manifold). So I just do not agree that using a manifold increases in any way the likelihood that you will lose all the gas in both cylinders. I understand the concept; no connection to the tanks, therefore there is no possibility of losing both at once. But in actuality, by FAR the most likely equipment failure that would empty two tanks is two regulator failures. If you think it's impossible to lose all the gas in both cylinders by using independent doubles, you're not correct.

Certainly there is the small increase in skill needed to manipulate the isolator, and you are correct that if the diver cannot close the isolation valve, he/she can lose all the gas in both cylinders via a regulator/valve leak in one tank. But as I mentioned before, if this is even the slightest challenge for a tech diver, that's an indication that there are probably much more serious training flaws. You reach right behind your head and twist a valve shut. Actually the posts are much more difficult to find, reach, and shut down IMO. And of course, with back mounted independent doubles, the posts are no different than with manifold.

I'm not trying to be difficult and I appreciate your attitude towards this discussion. We all have to dive our own way, and it is always interesting to learn from people with different ideas than our own.

 

[jvevea]

Is it okay to dive twins with two redundant rigs or is it better with a manifold? Part of the reason why I like the idea of twin tanks is that you can potentially set them up independently. However, now that I think about it, it seems that I might run into trim problems when there is significantly more air in one tank versus the other. Also, I don't recall seeing anyone dive twins w/o a manifold.

This is one of my six-times-a-year or so posts to make the complaint when I log on after not posting for a while go away.

I have three sets of doubles: manifolded 100s, independent steel 72s, and an antique pair of doubles that I think are 60s, with a non-isolation manifold (AKA Benjamin rig). My original reason for going with doubles is that I most often dive solo and need the redundancy. I dove strictly with independents for about the first year using doubles (my 100s were originally set up independent). Now, except for shore dives where I appreciate the lighter weight of my baby sets, I almost exclusively dive the 100s. The redundancy is still there: I regularly practice closing the isolator, and find it quite easy to do so. (I'm not quite so good at reaching that left post, though.)

If you read archived posts on this subject, you'll see lots of people talking about the task loading of switching regs with the independent rig. My own $.02 is that if that constitutes serious task loading for you, you probably aren't ready for doubles anyway. So if you are interested in a low-cost way of exploring doubles, I see independents as a useful step; but in the long run, I think most people would be happier with the manifold.

 

[shoredivr]

Yes, I saw that too but there is always that debate as to whether one goes to the post or isolator first which shows there can still be some confusion over the issue.

I was one of those taught to isolate first in case of gas loss.

 

[PfcAJ]

I was taught to shut down whatever is broken.

If my primary starts freeflowing, why would I waste time with the isolator when the right post solves the problem? Same with the backup reg. When in doubt, go for the right post since its used more, and if that doesn't work, isolate.

Manifolds fail once in a blue moon, but regs go **** up much more regularly.

 

[Sas]

I was one of those taught to isolate first in case of gas loss.

I was the opposite Taught to identify which post has the problem, and then shut down that post first. If that doesn't work, isolate.

 

[Sas]

Yes, I saw that too but there is always that debate as to whether one goes to the post or isolator first which shows there can still be some confusion over the issue.

Just to respond to this in more detail as I had to do a course recently where doubles were required but doubles experience beforehand was not necessary (i.e. how to use them was covered on the course) so interested to hear what I was taught compared to what others have been. My instructor was critical of people who think that isolating first is the right solution when there is an issue and went through the procedures for various types of failures. As far as I could see it was easy to tell which post was having troubles when we had to deal with this, but I'm interested to know what it is like in real world diving situations and other people's experiences. We were taught if you couldn't identify which post it was, to shut down right post first.

Or have no buddy. I am curious Lynne, as someone who has gone through GUE training, do you discuss/practice working the isolator with both hands? I am just imagining a scenario when the usual isolator hand may be trapped or incapacitated in some way and can't recall reading about it. It would be a skill all manifold users should have considered IMO.

Yes I agree, this is part of training to use a manifold in my experience. Actually the isolator is the easiest valve for me to reach though, either hand When we were shown valve drills though we closed it with right hand.

It all depends on what one considers advantages and negatives (I think they vary depending on the dive).
I think that most people follow trends and the trend now is for manifolds (which is fine). The trend until recently was not sidemount for OW but look what is happening there. People say SM's gain no benefit over BM's (as a Doc I hope you see the humor) in that scenario. I sometimes worry that the benefits of manifolds are touted over IT's without looking at the possible downsides. A well educated diver will face and plan for these of course but some "followers" may not.

Actually before I was shown how a manifold works, I was leaning heavily towards an independent setup but now I can see the advantages of this system over backmount independents, mattboy does a good summing up of this. A lot of the issues that I think could be a problem with manifold twins seem to be more user error than equipment error. For example, having the isolator off when tank is being filled - this happened to one guy on the course and he didn't find out until we were at the site as he didn't check it after it was filled. If you have nitrox fills this could be dangerous as you end up with the wrong mix.

Also as far as I have heard, isolation valve failures are rare too.

Another thing I see is people who cannot manipulate their valves properly. I know people who cannot reach their valves and have no idea how to a manifold works and the impact of turning off various valves. If this is the case you're effectively diving a single tank. Also I think even people who dive independents, should know how to use a manifold if they dive with people with manifolded tanks (which is likely given they are the majority).

Perhaps the biggest downside I actually see (in reality) is that manifolded double divers sometimes push the limits by doing a second dive when perhaps they shouldn't because they don't want to bring a second set of doubles with them. This of course, is an operator error not a design error, and happily I did see someone from your agency demonstrate restraint recently and sit out a second dive for this very reason.

This is an issue too for independent doubles divers too isn't it? If they don't swap tanks before another dive. There is no reason why people who decant their independent tanks equally would not have this thought process also and start a second dive with the same gas as a manifold diver, (other things being equal). Except if they have a failure they are more likely to have even less air than a manifold tank diver (as isolator problems are rare and problems where you can still access all your gas, more common).

Anyway, I am very new at diving with twin tanks, so interested to hear if other people have better ideas about things.