pony or spare air?

[Tortuga68]

What you fail to address is the issue that is much more important to me than running out of air. I watch my tank pressure religiously, and very much doubt I'll ever face an OOA emergency. But even with regular equipment servicing as recommended by the equipment manufacturers, it's possible for things to break. If one's regulator 1st stage fails with ample air remaining in your primary tank, even your octopus will be useless. That's when you'll be glad to have a pony.

What if your pony fails, do you carry two just in case? Or three?

At some point you have to say 'this is possible, but highly unlikely'

Or take up deck coits

 

[doctormike]

What if your pony fails, do you carry two just in case? Or three?

At some point you have to say 'this is possible, but highly unlikely'

Or take up deck coits

Well, as in most cases, the devil is in the details. I have no idea what the likelihood of a first stage failure is. If it is insanely rare (one in a million) then that is OK to ignore. If it is just very rare (one in a thousand), then it should be considered in a risk analysis.

But the entire idea of redundant anything comes down to math. Redundancy is based on the product of two probabilities. The reason that we don't take a second pony is simple: while the chance of a pony failure is very low, and the chance of a first stage failure is very low, the chance of BOTH of them failing is negligible, since it is the product of those two failure rates.

So you might say that you don't want to live with a one in a thousand chance of first stage failure (0.1% or odds of 0.001). Therefore, I'm taking a pony, which also has a one in a thousand failure rate. You would make a reasonable assessment of relative risk vs. costs of the pony bottle (in terms of task loading, expense, weight, entanglement risk, etc..) and come up with a conclusion. Whether you decide that it is worth it or not is a separate issue, that's up to you, but you still need to go through this type of analysis at some level. However, the chance of needing a SECOND pony is 0.001x0.001=0.000001, or one ten-thousandth of one percent. THIS risk is negligible enough to be ignored, so you don't bring two pony bottles...

It's like the story about the guy who always carries a bomb when he travels by air, since he heard that the chance of there being a bomb on a plane was one in a million, and therefore the chance of there being TWO bombs was....

 

[Tortuga68]

Your analysis is flawed, since you should take into account the possible consequences of the failure as well as the likelihood

 

[doctormike]

Your analysis is flawed, since you should take into account the possible consequences of the failure as well as the likelihood

Hey, Tortuga...!

I'm not sure I see that omission -

Whether you decide that it is worth it or not is a separate issue, that's up to you

i.e. you calculate the likelihood of an event in mathematical terms. You then have to decide what to do about that event - that is where the consequences come in. But the failure rate analysis is purely a numerical thing - the chance of two events both happening is the product of their individual probabilities.

Clearly, a 1% chance of missing a great photo opportunity when a whale shark swims by is not treated the same way as a 1% chance of entanglement and drowning. That is why a camera is optional and a cutting tool is a must-have, even if the omission consequences of each might have the same numerical odds.

 

[halocline]

At least 62.3% of statistics are made up spontaneously and have no real merit.

Okay, wiseass, the vast majority of first stage regulator failures involve leaking or freeflow rather than sudden air stoppage.

Happy? Or do you want to try to dispute that?

 

[Tortuga68]

Hey, Tortuga...!

I'm not sure I see that omission

You went to six decimal places in your explanation of mutiple failure risk, but didn't factor consequences in at all apart from possibly via a vague statement that it's up to the individual

 

[doctormike]

You went to six decimal places in your explanation of mutiple failure risk, but didn't factor consequences in at all apart from possibly via a vague statement that it's up to the individual

Right, because consequences don't lend themselves to numerical probability, and failure rates do.

I'm not sure why this is an issue or what you are driving at here... Are you saying that three pony tanks are actually better than one? Are you saying that one pony tank is unnecessary? Seriously, all I was saying was that the reason that redundancy in critical systems is useful is because the statistical likelihood of two systems failing is much lower than that for a single system. Is that a controversial point?

I went to six decimal places not for accuracy, but to demonstrate a very small probability (one in a million), and to demonstrate the calculation of relative risks. The actual results are not accurate of course, because we don't actually have the failure rates of an average first stage. But there is no reason we couldn't have them, if there was, say, a company or agency that were to compile that data. It is a real number that actually exists. Just like I don't know how many feet it is from my home to the nearest McDonald's, if I needed that number, it is certainly discoverable.

No matter how much research money and time you were throw at the problem, however, you will not be able to conclude that death is X times worse than the bends, or that an arterial gas embolism is Y times worse than a moray eel bite, or whatever. That is why you can't precisely calculate consequences, they are always in the eye of the beholder.

Many non-divers would never SCUBA dive for one reason - fear of sharks. That outweighs any possible benefits in their minds. So their decision is clearly based on their own gut feelings, and even though they are at more risks of being killed by a bee sting if they stay above the water. Their own unique and subjective assessment of consequences would be what guides their risk-benefit analysis.

 

[NC Wreck Diver]

I mount my primary first stage on my right hand post with my primary second stage regulator on that first stage side. My octopus is mounted on the other left hand side of my isolation valve on every set of doubles I own (I don't dive single tanks). Then I sling a 30 cu. ft. pony bottle as another redundant system. Oh yeah, I just happen to carry an AIR2 also. So with three first stages, and four second stages on my regs, I think I've factored out the possibility of equipment failure to a very good statistic.

doctormike...your analysis looks OK to me.

 

[Tortuga68]

Seriously, all I was saying was that the reason that redundancy in critical systems is useful is because the statistical likelihood of two systems failing is much lower than that for a single system. Is that a controversial point?

No. I said the same thing without the math you used - At some point you have to say 'this (failure) is possible, but highly unlikely'

Many non-divers would never SCUBA dive for one reason - fear of sharks. That outweighs any possible benefits in their minds. So their decision is clearly based on their own gut feelings, and even though they are at more risks of being killed by a bee sting if they stay above the water. Their own unique and subjective assessment of consequences would be what guides their risk-benefit analysis.

The same applies to carrying a pony. I doubt anyone is doing the math on it - they just think two is better than one, that must be good. That's why I bought up the diminishing returns, not because I think any non-deco diver should be carrying 2 ponies

 

[GrumpyOldGuy]

I mount my primary first stage on my right hand post with my primary second stage regulator on that first stage side. My octopus is mounted on the other left hand side of my isolation valve on every set of doubles I own (I don't dive single tanks). Then I sling a 30 cu. ft. pony bottle as another redundant system. Oh yeah, I just happen to carry an AIR2 also. So with three first stages, and four second stages on my regs, I think I've factored out the possibility of equipment failure to a very good statistic.

doctormike...your analysis looks OK to me.

Uh... actually you have done a very good job of increasing your chance of equipment failure. What you did decrease is the chance of not having available air due to failure. Nit picky, I know.

In the end, our environment has a huge impact on failure and gear choices. Example: The odds of loosing you air to a 1st stage free flow are substantially different by orders of magnitude when comparing 40F- to 55F+ water.