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

[Reg Braithwaite]

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.

True provided the two events are completely independent. For a counter-example, consider the probability of a free flow when two divers are diving singles with standard valves.

The conditional probability of both divers suffering a free flow is higher than the product of the probability of either diver suffering a free flow because the act of putting two divers on the same first stage raises the probability that it will free flow.

Not that this is relevant to OOA emergencies, I imagine. But it is always wise to examine possible multiple disasters and be sure that the probabilities are truly independent.

 

[TSandM]

No matter what kind of safety reserves you plan, you can always come up with a scenario where you won't have enough gas to get home. Carrying enough gas to deal with more than one catastrophic failure rapidly becomes impractical.

Some problems are likely to come in packs -- For example, you hit the ceiling, crack your manifold, and while trying to shut valves, you lose trim and silt out the passage. You're now minus half your gas (at least) and in zero viz. This kind of thing can happen. But a circumstance that would result in your buddy (in doubles) losing access to ALL of his gas, and then have you lose access to half of yours, would be so unlikely that it is not worth planning for it.

No one I have talked to (even people with thousands of cave dives) has ever had someone go completely OOG in a cave. They HAVE had problems that have resulted in loss of half the backgas on one of the divers. Of the stories I have read, the only cases where anyone went OOG had to do with getting lost first.

 

[texdiveguy]

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.

There are some situations that just can not be anticipated totally and fully guarded against.....in the situation you have outlined with multiple failures of both divers gas supplies....one total and one partcal there may just be a point were you have to rescue yourself/assuming you are the one with remaining gas and your buddy is on his own....this is real world diving. Add to this complicated diving such as o/w deep wreck penetration diving, probably the toughest situations you will find yourself in....now add your example on top of that.....there is not always a solution for everyone to come out a survivor.

 

[Reg Braithwaite]

Thanks everyone. To explain why I was asking, I was thinking about the other thread where people were discussing double 40s as an alternative to a single 80.

 

[Thalassamania]

...
The conditional probability of both divers suffering a free flow is higher than the product of the probability of either diver suffering a free flow because the act of putting two divers on the same first stage raises the probability that it will free flow ...

Not really, since with the proper choice of gear the probability of freeflow as a result of adiabatic cooling is zero.

 

[flareless]

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

Thank You! We used to get questions like this when I was coaching skydiving. "what happens if your main fails and your reserve fails". The answer these people are ultimately looking for is;

You die (or end up wishing you were dead).

Happy?

 

[DA Aquamaster]

Thank You! We used to get questions like this when I was coaching skydiving. "what happens if your main fails and your reserve fails". The answer these people are ultimately looking for is;

You die (or end up wishing you were dead).

Happy?

There have been a couple people who bounced and fully recovered. Of course they make lottery winners look unlucky...

There are two issues here with regard to diving.

1. Planning for double failures is pointless.

You will quickly drive yourself crazy and limit your penetration to about 20' if you start planning for multiple failures. It makes a lot more sense to focus on gear maintenence and prevention/avoidance of circumstances that may lead to multiple failures.

2. In many cases diving thirds is not conservative enough.

Consider what happens if you have a failure at the point where you reach thirds and are turning the dive that results in the loss of 1/2 your gas (a blown burst disc or extruded neck o-ring for example). Assuming you isolate immediately and save half the remaining gas, you have enough to get out using your own reserve gas if and only if you can get back out on the same amount of gas you got in with. If you are diving a siphon or if the cave gets silted out that could become problematic. If you were skip breathing on the way in to extend the time before you reached thirds (a really stupid thing to do in my opinon - get bigger tanks or get a stage) and are stressed going out, it could also be problematic. In any of those cases, you may end up using some of your buddy's reserve on the exit.

Deco adds a whole other dimension, particularly if it is unplanned and is a result of a silted cave and/or slower exit due to gas sharing with a buddy. You and your buddy's remaining gas needs to be enough to get you both out and through any required deco.

So depending on the conditions and a wide variety of factors there will be times when you should plan to turn prior to reaching thirds to hedge your bets.

Once you get into intentional deco, your gas planning and lost gas contingency planning gets more complicated and a conservative rock bottom approach makes a lot more sense than "x"ths.

 

[Reg Braithwaite]

Thank You! We used to get questions like this when I was coaching skydiving. "what happens if your main fails and your reserve fails". The answer these people are ultimately looking for is;

You die (or end up wishing you were dead).

Happy?

Asking whether you plan for two major gas failures is hardly the same as asking what to do about two chute failures for several reasons. First, at least one agency demands that students demonstrate their ability to handle multiple failures on training dives.

Obviously divers are expected to know how to deal with multiple major failures, I was just asking if this particular pair of failures was part of standard planning, and furthermore as another post explained, I was most specifically analyzing the limitations of diving double 40s.

If you consider this an unreasonably stupid question to ask, that's your affair.

 

[Jimmer]

Obviously divers are expected to know how to deal with multiple major failures, I was just asking if this particular pair of failures was part of standard planning, and furthermore as another post explained, I was most specifically analyzing the limitations of diving double 40s.

The double 40's have really nothing to do with "tech" diving as you're discussing it. They are generally used for a couple reasons. One would be trim and stability. I find them more stable in the water, with less side to side rock than a single. They sit low to your back, and if used with a "tech" manifold, they provide redundant regs for cold water use, similar to a large single with an H valve.

Case in point, the vintage guys use double 40's with a single reg, just so they have the stability provided. I use them with an isolation manifold, for the redundant regs, and dont ever see myself actually closing the isolator. I really like them for things like shallow ice dives where I might have a couple hundred foot swim to the exit, but I'm not very deep so the gas will last a long time.

 

[rstofer]

Asking whether you plan for two major gas failures is hardly the same as asking what to do about two chute failures for several reasons. First, at least one agency demands that students demonstrate their ability to handle multiple failures on training dives.

Obviously divers are expected to know how to deal with multiple major failures, I was just asking if this particular pair of failures was part of standard planning, and furthermore as another post explained, I was most specifically analyzing the limitations of diving double 40s.

If you consider this an unreasonably stupid question to ask, that's your affair.

I don't think the skydiver meant any disrespect! He simply points out, as have others in this thread, that there are scenarios that don't have a happy outcome. Your proposed failure, as I understand it, also won't have a happy outcome. Some of the smartest divers on this board have told you the same thing: some scenarios won't have a happy outcome regardless of how you plan the dive.

At some point, failures can lead to death. They can be single failures or multiple failures. They can be preventable or not preventable (like heart attack). In the end, people die.

Further, there is no way to train for a successful resolution to an incident that will never have a happy outcome. Regardless of the training, the result is the same. Someone dies.

I doubt that any of the mainstream agencies teach or test multiple failures for recreational divers. Sure, TDI will do it but that's well outside what might traditionally be considered recreational diving. As is any dive that requires deco stops...

Then too, there are multiple disjointed failures that don't cascade. It is the serial nature of your gas failure and your buddy's gas failure that leads to the problem. If you have a gas failure and your buddy loses his mask, these disjoint failures do not cascade into an unhappy outcome.

Richard