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
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
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..
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.
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.
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.
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.
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.
<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.
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!
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.
<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.
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.
<font color='#F52887'>People get so confused about where the valve shuts down from, someone asked me this at the weekend.
with the isolator open and the pillar valves closed the air route is open between the 2 cylinders. closing the pillar valves do not affect the manifold, only the isolator valve affects the manifold. If one DV freeflows and you just shut down that pillar valve, then you still have access to the gas in BOTH cylinders from your other DV.
I use a Cobra AI on one 1st stage and on the other a small SPG. at the weekend on my second dive I notice I was using more air than I should have been. when I checked my guages against each other they show different pressures. After the first dive the isolator had gotten closed (dont know why or who) and that meant I was using only air from ONE cylinder and because the other pillar valve was open the feed to my wing was OK and working. but I was only using the gas from one cylinder! If I had been using only 1 guage, depending on what pillar it was on, then I would have thought that I was either using no air OR I was almost out!
simple, I just reached back and opened my isolator and I had air again from both cylinders.
I own 3 SPGs, I own 3 cylinders, I use 3 SPGs. Well, to be more precise, I take 3 SPGs on every 3 cylinder dive but only use two - the lazy SPG is tucked safely away.
If there is a problem and things have to be shut down then I know what gas I have everywhere whatever the circumstance. In the right conditions this could even mean continuing the dive - although I must admit I find single cylinder diving a bit disconcerting now.
Failure points? Poo! Unless your personal bits are ensconced behind a button fly then you are not serious about the subject of failure points. Don't even start me on entrapment dangers if you happen to have an extra hose! .
AND, another thing ..... what happens if the pointy thing on your SPG falls off during the dive. At least I can refer to my other SPG.
Pointy things can fail too you know!
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