[Dominic]

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Quote[/b] ]One thing that I dont understand, if I isolate the LH cylinder or isolate at the manifold, how will I know what is left in the RH cylinder apart from adding another SPG.
I`m assuming that I cant reuse the LH cylinder due to SPG blow out

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The basic thinking is: If you have to isolate and you're unable to re-open the isolator for whatever reason, the dive is over and you're ascending.

At this stage, you either have enough gas to get you back, in which case you don't need a gauge; or you don't have enough gas to get you back, in which case you don't need a gauge.

There might be some psychological comfort in knowing you have enough gas, but practically, the gauge won't add anything: You either have the gas or you don't.

And don't forget, a lot of the time, people on the twin setup will be carrying a stage or two as well, so you only need the gas to get you up to the first stage depth...

However: Why are you assuming that a blown SPG means permanently closing the isolator? The high pressure hose doesn't leak terribly fast, it's very small bore. If you turn off the cylinder valve, no air will be lost, and you can re-open at your leisure. You still won't have a working gauge, but you'll at least have access to all your air..