[sausagedog]

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What is best?

a) completely independent twins

or

b) manifolded twins with isolation valve

the reason i ask is that i have a good friend who is a BSAC instructor who claims that completely independent cylinders are better in his opinion.

anyone got any views?

 

[Dominic]

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Depends what you're doing with them

 

[And]

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<font color='#0000FF'>Hi

b. In all cases (Well, except sidemount, which would look a little strange

)

Andy

 

[MATTBIN]

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I recently read a report of how long a shut down would take and how much gas you might lose during this drill/emergency. The conclusion was that for certain size tanks it might be better to be independant.

Andy, why would you say under all circumstances that isolated manifold would be better, given that on small tanks the time taken to shutdown etc would leave you with less gas than you would have had for a single tank.

Matt

 

[Dominic]

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If that was that laughable one in Diver, bear in mind that they were doing isolates and shutdown times in the 30-second range.

I can isolate in about a second and shutdown in only a few seconds more. The article was very evidently written by someody who didn't use twins (Mr Liddard, IIRC?) - there ARE reasons for using indys, but the risk of loosing gas in a freeflow isn't one of hem.

 

[MATTBIN]

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It was, you think it was twaddle Dom?
As you usually put a good case to your arguments I'd like to know where you think indys are better than manifolded twins,
apart from stages. Would cost come into the discussion as indys are a lot cheaper to set up.

Matt

 

[Dominic]

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It wasn't that it was "twaddle" as such - they put a lot of time and research into the article, I'll give them that.

They just didn't use realistic timings for their tests. They talked about 30-60 second times to turn off a valve before independants were better than manifolds, when most twin divers would be embarrassed to take over 10 seconds.

Where indys are better? Well, you've got a few cases:

People diving in a cave/wreck where there's too little room to maneuver their arms into place

People who are diving sidemounts

People with physical problems that can't cope with a normal twinset

People who are diving a long way from a compressor - being able to swap around multiple single cylinders can give you more options than a couple of twinsets

People who travel a lot and will be typically using rented cylinders

People who are too stingy to pay for a manifold

Those are the ones that come to mind straight off..

 

[Finless]

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Manifolded for me (barring the exceptions mentioned by Dom) - you can dive them as independants if you want to + if you have a problem then any gas saved in the problem cylinder (after you have closed the cylinder valve) is still available for use by opening the manifold again.

For the diving I do I don't see that there is any arguement NOR am I prepared to listen to any. Fingers are now stuck in my ears - LA LA LA LA LA.

As I progress into trimix this might change (I don't know yet) - esp when I receive my QR hose pics from A the C 2.

 

[jasondrake]

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Think this has been covered before, but can't find it in the archives and I don't think anything useful came out of it. Needless to say both have their pro's and con's but probably most divers now would say manifolds are the best way as long as you can effectively use them in  a gas emegency.

 

[Twinset & Wing]

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Manifold and INVERT


LOL

DD

 

[Bob Cooper]

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Quote[/b] (sausagedog @ Oct. 19 2003,16:22)]

Quote[/b] ]What is best?

a) completely independent twins

or

b) manifolded twins with isolation valve

As ever, you need to make an objective appraisal of the advantages and disadvantages of each system and then make a decision based on that information.  There is a "risk based" decision making process which means that you end up with the system which presents the least "risk".

Now, assuming we are talking about UK ocean diving or clear water cave diving within a "team" environment (by that, I mean you have a similarly equipped buddy or team) you need to assess the additional risk of using a manifold against not using one.........

A manifold is a potential failure point. There is no doubt about that.  There are additional o-rings in there.  "Failure" can manifest itself in other ways too.  For example, the isolator could be closed when filled with Nitrox or trimix, resulting in high O2 on one side of the isolator.  Even if the isolator is open during filling, it could get inadvertantly closed after filling, resulting in the diver "running out of gas" when he still has 50% of his available gas remaining.  A simple freeflow can empty both tanks if not correctly managed when a manifold/isolator is used.  All of these failure modes are potentially life threatening and all of these failure modes are eliminated by using independents. That's the argument "for" independents.

The above risks can be mitigated as follows:

1. Closed isolator - keep the isolator open at all times and continually check, check, check even when in the water.
2. Freeflow - learn and practice how to shut the isolator. This needs to be a reflex action.

It's really a simple matter of learning how to properly manage a manifold/isolator.

The disadvantages of independents are as follows:

1. Gas management - you need to swap regs with independents.  With a manifold/isolator you have access to all of your gas at all times, even if a regulator fails.  With independents, you loose gas even in the event of a simple free flow.
2. Planning - we tend to plan dives so that, even in the event of a catastrophic loss of backgas, our buddy can donate enough gas to get us out of the cave or to our first deco gas. You can't do that with independents.
3. Failures - we have a system which will tolerate at least two independent failures.  For example, if two regs fail, our buddy can get us out of the cave or to our first gas switch.  You can't do that with independents.
4. With independents, you will need two pressure gauges.  This adds an unwanted failure point. With a manifold/isolator, you only need one pressure gauge.
5. With independents, which cylinder do you use for BC inflation?  With a manifold/isolator, you can use both.

I've gone on far too long and I think you will see which system I'm in favour of.

Hope this helps.

Bob

 

[Dominic]

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Quote[/b] ]With independents, you loose gas even in the event of a simple free flow.

That's not an absolute - if it's a freeflow due to icing-up, you can do the same as with a manifolded twinset and shut off the valves until they defrost..

Quote[/b] ]in the event of a catastrophic loss of backgas, our buddy can donate enough gas to get us out of the cave or to our first deco gas. You can't do that with independents

Devil's advocate and all.. but surely the point of indys is that you're less likely to GET that catastrophic failure as it calls for TWO systems to go pop..?
Why can't your buddy donate enough gas tho? I don't follow. A buddy with twin 12s is a buddy with twin 12s whetehr they're indys of manifolded, surely..?

 

[Finless]

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Quote[/b] (Dominic @ Oct. 21 2003,08:39)]Devil's advocate and all.. but surely the point of indys is that you're less likely to GET that catastrophic failure as it calls for TWO systems to go pop..?

OK you devil, you and previous posts have forced me into doing another poll.

I understand the danger of underwater failures BUT I think this potential failure point thing is being taken too far (Dom - NOT aimed at you even though I have used your post as a quote). By all means be aware of where the potential failure points are with your kit and what to do if ...

See you all at the top of the forum (I hope).

 

[Bob Cooper]

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Quote[/b] ]Why can't your buddy donate enough gas tho? I don't follow. A buddy with twin 12s is a buddy with twin 12s whetehr they're indys of manifolded, surely..?

No, the independent diver has 24L of gas, true but he can't share that gas like the manifolded diver can.  The only way to do that with independents would be to have a long hose and  backup on each independent cylinder - 4 regs.

 

[Mark Chase]

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Hi Bob

What about side mounts in cave diving surley they arnt manifolded. Are side mounts not DIR? Do they have two long hoses on side mounts?


Apart from that:

I dive manifold 12 in the UK and indipendent 10 or 12's when I am in the warm stuff. (see avtar indipendent 12s red sea) Apart for the dive guides being convinced that I must be the worst gas guzler ever I cant see a problem with indi 12s. They work well, reg swaping is no big deel and they are
safe if your working to reserves based on one tank ascent. That said I would rather dive a manifolded set and at least have the chance of additional air in a cluster F situation.

ATB

Mark Chase

 

[Bob Cooper]

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Mark Chase[/quote]

Quote[/b] ]What about side mounts in cave diving surley they arnt manifolded. Are side mounts not DIR? Do they have two long hoses on side mounts?

I really don't know about sidemount diving.  Of course the tanks aren't manifolded. As far as I know, they have one reg per tank and the gas management is a bit of a nightmare!

The point about independents is that you can't access all of your gas all of the time like you can with a manifold/isolator. The point about gas reserves is OK for solo diving but it doesn't work in a team environment.

Bob

 

[Spike Jackson]

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I'm not sure I understand why gas management with independent 12's is a problem.
Surely it works like this: I breathe 1/3 of my first cylinder on the way in, I then swap to my second cylinder and breathe 1/3 of that and then turn around with 2/3 remaining in each cylinder.
If I lost either reg or gas supply at this point I still have the 2/3 in the other cylinder to get me out.
If I lost BOTH regs, then my buddy has 2/3 in each cylinder, one for each of us. If just one hose was 2m long I would use that and we would both get out.
It would only be a prob if we lost 3 regs between us, as problematic on a manifold as on indies!
Or am I missing something?

 

[Phil Ennis]

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<font color='#810541'>But with manifold you have 2 thirds in both cylinders available, not just the one.

 

[Spike Jackson]

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Yes you do and I agree manifolded twinsets offer more advantages over indies. However I think you can still manage your gas effectively with indies eg when travelling abroad.