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Just not enough dive time.
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If I shut down the isolator in the manifold and the left hand post valve to the 1st stage, which has the spg on it, how does the spg know whats in the right hand tank?

Matt
 

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Matt
It doesn't!  The assumption is that if you have to shut down one side and can't re-open, you then open your manifold again, and you then have access to gas from both cylinders (through the manifold).  Anyway, that's how I understand it!
Regards and dive safe
Martin
 

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Either:

You open the manifold again, and the pressure is the same in both sides

OR:

You can't open the manifold again. This signifies the end of the dive. You  ascend immediately. And because you've done your gas planning properly, you KNOW you've got enough gas.

And if you HAVEN'T got enough gas, having a working SPG is NOT going to help you on little bit..

A touch fatalistic, but true nonetheless.
 

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Just not enough dive time.
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Discussion Starter · #4 ·
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Hmm!!
so a bit like no spg on a pony then  


Cheers
Matt
 

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Not really - an SPG on a pony is good as it'll tell you if the pony got emptied on the way out to a dive site
 

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Just not enough dive time.
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Discussion Starter · #6 ·
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I always check mine as I mount it on the wing/bcd but there isnt one on it when I dive.
Matt
 

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Finless: You couldn't invent him...
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Personally, I have an SPG on each cylinder. The one on the left cylinder is velcroed out of the way around the back where I could reach it if I wanted it and I dive with the manifold open if using a 3rd cylinder for a deco mix. If just diving on the twins I close the manifold and use them as independants.

Yes, I do know I have introduced, in most peoples opinion, another potential failure point but iy makes me happy. In the event of having to close the manifold and losing all the gas in one cylinder at least I have the assurance of knowing what gas is left in my remaining cylinder and how long I have left to complete deco etc rather than hanging there wondering how long I can last on this cylinder before having to surface.
 

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<font color='#000080'>Hi

SPG is on left post. This is the post most likely to roll off down the shot line. Your SPG lets you know whether it has (as the gauge will stay the same) and also whether your isolator is closed (same effect).

You are breathing off your right post and so you know that works. This is also the post most likely to fail (as it powers your wing too) and so in the event of closing it your gauge is still operational. Thats the logic anyway.

Having a second SPG provides little benefit but will introduce another failure point and complicate hose routing and you then have two instruments to check rather than just the one.

Hope that helps

Andy
 

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Matt

Dominic is right about doing your gas planning properly. However, I use a remote air-integrated Alladin and put the sender on the opposite side to the SPG. That way I can see see the both tank pressures regardless of whether the manifold is shut or open.

I just like to know now much gas I have.

The breathing rate thing on the Alladan is also at it lowest setting and so the computer does not really compensate for the breathing rate. This means that it doesn't 'fudge' any deco obligations.

ta

Andy
 

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A short fat well off crap cave diver. Likes wrecks
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I have one SPG on the left post. If any thing hapens during a dive that puts me in a position where that SPG is not giving me the available presure in both tanks then the only way I am going is up.

As Dom said at that point you either have enough gas or you dont.

As for most of the other arguments the answer is simple. If your SPG is giving strange readings then you instantly check your isolator is open and both tanks a full on. If they are then you abort the dive. If they were not then this action will bring the dive back on plan and you carry on slightly embaresed about having forgoten to open your isolator.


ATB

Mark Chase
 

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Small, yet perfectly formed...
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we just dive with one SPG and manifold isolator open...
 

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Just not enough dive time.
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Discussion Starter · #13 ·
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That the Royal 'we' Julia  


Glad I asked this question, seems opinion is split, so what's new, but I suppose the bottom line is do want you want but be aware of the consequences of your choice.

Me - I'm going for the cheap option - 1 spg and fewer failure points but aware that if the left post goes its time to surface.

Good weekend folks

Matt
 

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<font color='#000080'>I've got the same system as Bloss - SPG on left post, air integrated transmitter on right.

I remember we had this discussion a couple of months ago;

Right post - main reg and wing
Left post - octopus, suit inflator and SPG

All logical and all based on the premise that the right post was the one you were most likely to need to shut down.

The air integrated transmitter is of course a luxury (and a different argument altogether) so I'd otherwise agree with the arguments above - if you've had a shut down and are unable to open the isolator then you're coming up anyway, and as quick as you can. But having said that, to be without your SPG the only occurance giving rise to this would be an O ring failure on the valve/cylinder connection, which is more or less unheard of, and on the wrong side of a 50/50 chance. In my opinion not a risk worth worrying about.
 

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<font color='#000080'>Fletch,

By the phrase on surfacing "as quick as you can" I sort of took it for granted that all the readers here (being sensible divers) had the nouse to understand that it meant a safe ascent with necessary safety/decompression stops, not a bolt to the surface (especially those diving on twin tanks!). I'll obviously have to reassess that view of YD readership.

The point about the valve/cylinder O ring failure is specific to the point raised about not being able to re-open the isolator after a shut down. I thought I'd made that clear as well. It is the only circumstance where the isolator cannot be re-opened - for any other failure it can be isolated by shutting down the relevant first stage.

Yes, you lose the SPG by shutting down the left post. That's why it's on the left post. That was the point!
 

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That's Dude with an E
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Matt, as your just starting using a twinset, l would sugest that you start off with two pressure gauges, with the isolator shut and use them as indies.
Then on each dive practise reaching all the valves, when you can comfortably reach all them, dive with the isolator open.
At this point you may wish to remove the other gauge.

Or you could leave the manifold open, use one gauge and spend lots of time in some crappy inland site practising.


The first way above means that you can go out diving, practising at the same time when its not critical if you cannot shutdown.
 

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<font color='#000F22'>or just forget the isolator manifold as it's another source of failure, and you only need one SPG, fir the reasons outlined above.
 

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Just not enough dive time.
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Discussion Starter · #20 ·
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Howard
Good call. Just got em in the garage and tried to mount them on the wing and found a few minor glitches, which I think I will post and get some answers to on here under a different thread. But problem 1 of course is that I cant reach the right hand valve, it would be that one wouldnt it? So manifold closed and 2 spg's seems like a good plan at the moment.

Matt
 
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