Rock Bottom/Turn Pressure/Rule of xths for Doubles?

Reg Braithwaite · Dec 21, 2008

I didn't want to hijack another thread, so I thought I'd post this question separately...

If you are diving doubles with a standard isolation manifold, the usual worst-case scenario is a valve malfunction that forces you to isolate the working tank and lose the remaining gas in the other.

Given that you would normally turn the dive with enough air for you and your buddy to spend a minute dealing with a failure and then return to the surface (using whatever formula you like), my question is this:

Do you plan for having that much air even if you lose a tank, in other words, that much air in each of the two tanks? Or do you live with the possibility that if your buddy goes OOA and you also lose a tank, you don't have enough gas for both of you to get home?

sea2summit · Dec 21, 2008

Short answer, no.

The rule of thirds your talking about is single tank so if I have a first stage, burst disc, etc problem I'm totally out of air.

With doubles you have an isolator (or should) and save the air from one tank so while you would turn the dive your still using your own air and your buddy should still have a substantial reserve.

Thalassamania · Dec 21, 2008

Your gas plan (Rock Bottom for the DIRs, we've always called it BINGO, after a similar aviation gas concept) should always get you and your buddy back to safety (surface, deco tanks, etc) and your buddy's should always get you back the same way.

Reg Braithwaite · Dec 21, 2008

Thalassamania:
Your gas plan (Rock Bottom for the DIRs, we've always called it BINGO, after a similar aviation gas concept) should always get you and your buddy back to safety (surface, deco tanks, etc) and your buddy's should always get you back the same way.

So are you saying that if we turn the dive at BINGO and then my buddy has a complete OOA, we do not have enough gas to get home if I subsequently lose one tank while sharing air with my buddy? To flip it around, if I lose one tank just as we reach BINGO, we obviously turn the dive on the spot. Are we now fuxxored if my buddy goes OOA on the way home?

Or is our BINGO plan to turn the dive while I have enough gas to handle losing one tank AND my buddy going OOA?

Thalassamania · Dec 21, 2008

No, sorry about the lack of clarity, in most cases the gas plan does not accommodate a double failure, at BINGO I have enough gas to get us both back and you have enough gas to get us back. If one of us has a complete failure that wipes out all of our gas ... they the other is carrying enough gas to get us home, assuming that there are no further failures.

rstofer · Dec 21, 2008

But I guess the point is multiple simultaneous failures. Do you account for losing one of your tanks and your buddy also being OOA? At what point in the dive do you plan to have this malfunction? If it's near the turn point, your buddy is in trouble if he goes OOA.

In the end, we are all going to die. I think you can go a little too far in planning multiple simultaneous failures. But it's your buddy that is OOA. You still have the remaining gas in the functioning cylinder. Send flowers.

Richard

Thalassamania · Dec 21, 2008

The probability of having both failures is the probability of your failure multiplied by the probability of your buddy's failure ... rather unlikely, if both were one in a thousand (and I expect the real value is much lower) the probability of a multiple failure would be one in a million.

Reg Braithwaite · Dec 21, 2008

rstofer:
In the end, we are all going to die. I think you can go a little too far in planning multiple simultaneous failures. But it's your buddy that is OOA. You still have the remaining gas in the functioning cylinder. Send flowers.

Now I understand the value of the seven foot hose with the quick release on the first stage. Isolate the working tank, back fin to full extension, release the hose, exit alone. Got it.

AndyNZ · Dec 21, 2008

Reg Braithwaite:
Do you plan for having that much air even if you lose a tank, in other words, that much air in each of the two tanks? Or do you live with the possibility that if your buddy goes OOA and you also lose a tank, you don't have enough gas for both of you to get home?

I don't know what is considered "standard", but generally I plan for just one failure.

For example, in a deco dive plan, I will make sure that I can complete the dive with back gas if I lose my deco gas. Similarly, I will plan that if I have a failure that loses half of my backgas, that I still have enough to make it to the first gas switch.

Answering your question with a question, where do you stop the "what if?"....?

Should I plan for a minimum reserve that enables me to have a manifold failure, that still gives me enough gas to bring my buddy up to the first switch, but unfortunately we've both lost our deco gas.... so both have to deco off a dwindling reserve of back gas.

How likely is this to actually happen??

rstofer · Dec 21, 2008

Reg Braithwaite:
Now I understand the value of the seven foot hose with the quick release on the first stage. Isolate the working tank, back fin to full extension, release the hose, exit alone. Got it.

Not being a cave diver, I had never been interested in learning to back-fin. See, you learn something every day! Shame about that $80 hose...

Richard

Rock Bottom/Turn Pressure/Rule of xths for Doubles?

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