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"hardly ever here"
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Hi all.  I have a question about this whole twinsetting malarkey, which I hope that someone can answer to my satisfaction......

Right - as I understand it, the whole point of diving with a manifolded twinset is that if you get a free-flow you firstly isolate, then work out which side has failed and shut down that cylinder. Most of the stuff I've read then talks about slowly opening the side that has failed, so it is available again.......correct me if I'm wrong?

So, assuming the above to be the case, I'm wondering what is the point of a manifold? (apart from not having to swap regs during the dive). With a manifold, once you get a free-flow you are (briefly) losing gas from both cylinders, then you isolate, then you shut down the tank that has failed.  With independents, you skip the first part altogether, and go straight to identifying and shutting down the one that has failed. So you must end up with less gas lost.

That much seems obvious to my little brain, so my question is about the next stage of the process...... How likely is it that you'll be able to open again the cylinder that has failed? I don't have any experience of this process, so I'd be grateful to hear from people who have. If it is generally possible to open and use a tank that has free-flowed (I'm thinking mainly from cold rather than equipment failure I suppose), then surely independents save more gas in the long run? On the other hand, if it isn't generally possible to access the gas in the failed cylinder, obviously you'd need a manifold to get to it.

The background to this question is that I'm moving to twin sevens from a twelve and a pony (for comfort and trim reasons, not to do deeper diving). I don't particularly want to shell out for a manifold, and I want to know that I won't be completely crazy to dive with independent sevens.

I therefore invite everyone to come and pick holes in my arguments, as I'd rather be corrected now than when I'm 30m down. Mary, I think this'll probably be of interest to you too, as you're also thinking of independent sevens. (When did you disappear on Saturday night by the way? Did you miss the gold lipstick episode?!)

Cheers!
 

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Well in the case of a freeflowing regulator then you would have an idea which side to shutdown and you can just shutdown the valve on the relevant cylinder.  You would then still have access to the air in both cylinders via the manifold although you have stopped the flow to one firststage.

If the regulator has freeflowed due to cold then hopefully by stopping the flow of cold air it will warm up a bit and you maybe able to reopen the valve and it might be ok.  

Taking it even further if both regs freeflow, you could shut both sides down and by turning one valve on and off you could still use it to breathe from.

The isolator is only used generally in the event of you not knowing which side you are loosing air or a catastrophic failure between the cylinder and the actual valve.

From the reports I have seen, most failures have been easily isolated using the cylinder valves, the isolator was not used.  

So basically the isolator/manifold gives you access to the air in both cylinders in the event of a failure downstream from and including a firststage.

I think that is reasonably correct but someone will correct me if I am wrong.  


HTH,
Daz
 

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Finless: You couldn't invent him...
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[b said:
Quote[/b] (daz @ Nov. 05 2003,12:05)]I think that is reasonably correct but someone will correct me if I am wrong.
I was taught that you shut the isolator down first in all cases - it should only be open a max of one turn and therefore can be closed much faster than the cylinder valves.

I did argue (on the course) that if I knew which cylinder to close off it would be better to go straight to the cylinder valve in question and close it - the answer was "definitely not".

It is not the kind of thing you can really test so I believed the man.
 

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[b said:
Quote[/b] (Finless @ Nov. 05 2003,12:17)]
[b said:
Quote[/b] (daz @ Nov. 05 2003,12:05)]I think that is reasonably correct but someone will correct me if I am wrong.
I was taught that you shut the isolator down first in all cases - it should only be open a max of one turn and therefore can be closed much faster than the cylinder valves.

I did argue (on the course) that if I knew which cylinder to close off it would be better to go straight to the cylinder valve in question and close it - the answer was "definitely not".

It is not the kind of thing you can really test so I believed the man.
Well there we go,  a classic case of opinions differ  


Personally if it was a freeflowing second stage or basically something I could positively identify I would always shut down the relevant valve on the cylinder.  

I would not be confident in identifying which side a 1st stage leak was coming from so in that instance I would go for the isolator.

There is a valid argument for a standard shutdown drill that is exactly the same in all circumstances though.

YMMV
Daz
 

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[b said:
Quote[/b] (Kate R @ Nov. 05 2003,11:54)]
<font color='#0000FF'>Hi Kate

[b said:
Quote[/b] ]So, assuming the above to be the case, I'm wondering what is the point of a manifold? (apart from not having to swap regs during the dive). With a manifold, once you get a free-flow you are (briefly) losing gas from both cylinders, then you isolate, then you shut down the tank that has failed.  With independents, you skip the first part altogether, and go straight to identifying and shutting down the one that has failed. So you must end up with less gas lost.
You don't shut off the gas when you close one post, only the valve on that post. The remaining gas in the cylinder is available to the valve on the other post. That is why you have an isolator, so if the valve is still leaking you can 'isolate' on cylinder from the other.

In the situation you talk about the left post fails, say an extruded o ring. You close the isolator, then close down the offending post. You can then open the isolator and use ALL of your remaining gas to get you safely home


Incidentally I was taught to close the right post first if I couldn't identify the offending post, as that is the one most likely to fail given my config as my main reg and main buoyancy are on that post, then, if the bubbles continue, isolate and let my buddy identify the problem.

Using independents is not so easy to manage your gas, you need two gauges, you need to look carefully at how you donate gas in an emergency and has no real significant benefit over a manifold with isolator.

Regarding failures if its a simple second stage freeflow then you may most likely be able to open again. If it is a first stage which may be loose, you should be able to tighten it and reopen the valve. Anything else you are less likely to be able to fix whilst underwater.You can swop first stages between your twinset and your deco bottles, but this is usually if your deco reg fails, not the other way round.

Hope that helps

Andy
 

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Daz,
If you got a first stage freeflow on the one you are breathing from (the most likely one?) then you know which one it was as presumably the second stage wouldnt be delivering anything like the gas you needed, swap to other dv and shutdown the original 1st stage and carry on - yes?
If you can breath then presumably its the other 1st stage.

Do you abort the dive?

Matt
 

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[b said:
Quote[/b] (MATTBIN @ Nov. 05 2003,12:35)]Daz,
If you got a first stage freeflow on the one you are breathing from (the most likely one?) then you know which one it was as presumably the second stage wouldnt be delivering anything like the gas you needed, swap to other dv and shutdown the original 1st stage and carry on - yes?
If you can breath then presumably its the other 1st stage.

Do you abort the dive?

Matt
Not really sure I understand the first paragraph completely (First stage freeflow?, do you mean a leak at the 1st stage?) but I guess to answer your question, any failure at the first stage that also presented symptoms at the second stage, it should be identifiable and yes I would shutdown the offending post.

Would I abort the dive?  If I could not rectify the problem then yes.  Rectifying may involve shutting down the post and reopening to see if it is a problem with it freezing up.  

If it was not delivering the air I required, I suspect I would abort immediately after shutting down.  I can think of solutions for over delivery of air but under delivering even on one side removes the redundancy for me.

Daz
 

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Just not enough dive time.
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Daz
the situation I thought of was to over exert a first stage e.g. purging the DV to the extent that ice formed and the 1st stage then froze open.

Matt
 

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OK gotcha,  I think my answer still stands as is but if I can clarify let me know.

Daz
 

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[b said:
Quote[/b] ]With a manifold, once you get a free-flow you are (briefly) losing gas from both cylinders, then you isolate, then you shut down the tank that has failed.  With independents, you skip the first part altogether, and go straight to identifying and shutting down the one that has failed. So you must end up with less gas lost
Why??

The volume of gas that escapes before you manage a shutdown will be the same whether you're on manifolded or independants. Indys just mean all the gas is lost from one cylinder instead of half from both.
 

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[b said:
Quote[/b] (MATTBIN @ Nov. 05 2003,12:35)]Do you abort the dive?
<font color='#0000FF'>Hi

That depends on what resources you have left. Diving in a pair means you have two gas supplies and 4 first stages. One diver loses a first stage and so is no longer able to donate. Common sense says abort.

However, diving in a three means three gas supplies and 6 first stages. Losing one first stage means you still have 5 first stages between 3, but the position of the 'disabled' diver should be considered as he can no longer donate. He probably moves to the front. I probably wouldn't abort in this case, depending on how much progress has been made so far,  how much we still had to do to complete the dive, and the environment being dived. ie overhead, open water, wreck penetration etc.

Andy
 

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[b said:
Quote[/b] (Dominic @ Nov. 05 2003,13:10)]
[b said:
Quote[/b] ]With a manifold, once you get a free-flow you are (briefly) losing gas from both cylinders, then you isolate, then you shut down the tank that has failed.  With independents, you skip the first part altogether, and go straight to identifying and shutting down the one that has failed. So you must end up with less gas lost
Why??

The volume of gas that escapes before you manage a shutdown will be the same whether you're on manifolded or independants. Indys just mean all the gas is lost from one cylinder instead of half from both.
because you go straight to stage 2 instead of doing stage 1 (taking time, even if only a tiny bit) then stage 2

my logic anyway  
 

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Depends on the situation.. but yeah, fair enough.

You just have to decide if that potential fractional increased loss from an iso'd manifold outweighs the increased chance of being able to use the air in both cylinders when a reg goes pop
 

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[b said:
Quote[/b] (Dominic @ Nov. 05 2003,15:21)]Depends on the situation.. but yeah, fair enough.

You just have to decide if that potential fractional increased loss from an iso'd manifold outweighs the increased chance of being able to use the air in both cylinders when a reg goes pop
do take your point, but i'm afraid the main calculation at the moment is:

how much does it cost to buy a manifold?

and the answer is: more than i can afford

 

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You also have to consider if you're buying a manifold, you may well need to either get the right bands for it, or adjust it to fit whatever bands you have. Don't know if they're standard bands, but they'll need to be right for the tanks, or you'll stress the manifold and that can cause a whole lot of problems.
 

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[b said:
Quote[/b] (Digger @ Nov. 07 2003,12:44)]You also have to consider if you're buying a manifold, you may well need to either get the right bands for it, or adjust it to fit whatever bands you have. Don't know if they're standard bands, but they'll need to be right for the tanks, or you'll stress the manifold and that can cause a whole lot of problems.
hi jack. just going to use buddy twinning bands for now - i might move on to manifold/steel bands etc one day though!
 

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I read/listen to all diving related topics with interest and an open mind. However, this is one of the few topics where I say there is only one option. In the event of an uncontrolled loss of gas on a manifolded twinset the isolator valve must be closed first. Don't waste time thinking, make it an instant reaction to close the isolator as soon as you notice you have a problem.

A max of one turn and it is closed and you have one protected cylinder of gas whilst you try to figure out what has gone wrong.

If you can't see the problem then you have to close the isolator anyway. If you can see one problem - it may not be the only one. If you are super quick at closing down the cylinder valve then it will make f/all difference to your overall shut down time BUT you have immediately guaranteed yourself (as best you can)  one protected source of gas.

Sorry, it is the only way to guarantee you have gas left - barring failures on both cyliners or to the isolator in which case you are in need of a buddy or a quick ascent.
 

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Kate, do you want to borrow my bands? I haven't used them for ages, but can't sell as I may well need them for blue water twinning. Text me.
 

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I have dived a twinset with an isolator since 1978. As I see it, the only time I would need to shut the isolator would be if the O-ring between a tank and its individual valve failed, and in 25 years that has never happened. If a regulator failed, whether because of a blown O-ring at the junction of !st-stage and tank valve, or for a blown hp. hose for example, I would always sut down the valve to the offending reg. and breathe the rest through the other side via the manifold. When away abroad, I often twin up independents. So, with the risk of upsetting everyone, I would say..... no, no, I dare not!
 
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