Double tank SPG question

[Cheizz]

In a back mount double tank setup with a manifold, the SPG usually is on the left-hand post, I believe. And you breathe the tanks down simultaneously. Should the right-hand valve fail, you close that valve and the manifold and you can breathe the left-hand tank down while you end the dive and ascend. You can keep monitoring that tank pressure, since the SPG is on that first stage.

But what if the left-hand tank, first stage or valve fails? You close the manifold and the failed left-hand side and breathe from the right-hand tank while ascending. So you cannot monitor your pressure and have to rely on the last known pressure and an immediate end of the dive. Is that it? No redundancy on the SPG then? Or am I missing something?

 

[SpeedyJ]

The logic is that you've planned your dive, know your gas requirements, and once a post is lost your dive is over - it's time to leave. As you would not have hit your minimum gas requirement, you know you have enough gas to safely end the dive.

 

[Angelo Farina]

My double tank has a not-separation manifold WITH RESERVE.
So if the failure affects the reg with the SPG, I can use the other relying on the reserve for avoiding a complete OAA situation.
In any case it is time to thumb the dive, as when a failure occurs you should not continue a planned deep dive with deco, or the like.

 

[Cheizz]

In any case it is time to thumb the dive, as when a failure occurs you should not continue a planned deep dive with deco, or the like.

Yes, that's what seems obvious to me. Having to shut down one of the two tanks (and with that half of the remaining gas) would be a dive-ending event. For me at least.
I was just wandering about the redundancy principle, which apparently does not extend to the SPG.

 

[SpeedyJ]

Yes, that's what seems obvious to me. Having to shut down one of the two tanks (and with that half of the remaining gas) would be a dive-ending event. For me at least.
I was just wandering about the redundancy principle, which apparently does not extend to the SPG.

Think about what you're adding to the rig in order to get information that's not needed - the hose with spool, o-rings, swages, and the spg itself are all failure points.

 

[grantctobin]

In a back mount double tank setup with a manifold, the SPG usually is on the left-hand post, I believe. And you breathe the tanks down simultaneously. Should the right-hand valve fail, you close that valve and the manifold and you can breathe the left-hand tank down while you end the dive and ascend. You can keep monitoring that tank pressure, since the SPG is on that first stage.

But what if the left-hand tank, first stage or valve fails? You close the manifold and the failed left-hand side and breathe from the right-hand tank while ascending. So you cannot monitor your pressure and have to rely on the last known pressure and an immediate end of the dive. Is that it? No redundancy on the SPG then? Or am I missing something?

The SPG is already your backup for proper gas planning. You’re exiting, and know that your gas plan will cover you. If anything, seeing the SPG will add to your (most people’s) stress. All three of those failures are major and dive ending.

 

[PfcAJ]

It’s kind of binary. You either have enough gas or you don’t. “Knowing” doesn’t change the outcome.

 

[Compressor]

In a back mount double tank setup with a manifold, the SPG usually is on the left-hand post, I believe. And you breathe the tanks down simultaneously. Should the right-hand valve fail, you close that valve and the manifold and you can breathe the left-hand tank down while you end the dive and ascend. You can keep monitoring that tank pressure, since the SPG is on that first stage.

But what if the left-hand tank, first stage or valve fails? You close the manifold and the failed left-hand side and breathe from the right-hand tank while ascending. So you cannot monitor your pressure and have to rely on the last known pressure and an immediate end of the dive. Is that it? No redundancy on the SPG then? Or am I missing something?

The way I do my doubles is to put the SPG on the left hand side as you stated. On the right hand side I have a Perdix AI transmitter hooked into my first stage. So if the left side fails, I know what's in my right tank. If any side fails, the dive is over for me.

I like redundancy and this method serves me well.

 

[Cheizz]

The way I do my doubles is to put the SPG on the left hand side as you stated. On the right hand side I have a Perdix AI transmitter. So if the left side fails, I know what's in my right tank. If any side fails, the dive is over for me.

I like redundancy and this method serves me well.

That's what I intend to do as well, as soon as my transmitter arrives.

 

[loosenit2]

The way I do my doubles is to put the SPG on the left hand side as you stated. On the right hand side I have a Perdix AI transmitter hooked into my first stage. So if the left side fails, I know what's in my right tank. If any side fails, the dive is over for me.

I like redundancy and this method serves me well.

This is what I have always done as well. Redundant air monitoring. Does not negate the need for propper planning, but redundancy in critical things is necessary.