On Thu, 20 Jul 1995, Kevin-Neil Klop wrote:
> In all the talks about failures, I think people are talking about two
> different things...
> 
> Rich uses examples of, say, putting a Y-valve on a tank and attaching two
> complete regulator systems to the valves.  The complexity has doubled (two
> regulators) but he asserts that the chances of a life threatening emergency
> has decreased.
> 
> There are several problems with this that makes the statement both true and
> false...
> 
> With the regulator example, if one regulator fails it may or may not affect
> the other regulator.  Thus, you do NOT have two separate systems.  For
> example, assume a blown hose in one regulator.  The other regulator, in a
> Y-Valve system, is most assuredly affected.
> 
> In addition, with two regulators, the chances of _a_ regulator failure has
> doubled.
> 
> Lastly, there is still a single point of failure - the Y-Valve.  Should the
> Y-Valve fail, all the redundancy in the world won't help you.

This is why I avoid using y valves and manifolds (which are just y valves
with more failure points) as much as possible. (Only took me ten years
to figure it out :) ). Of course, I use them sometimes. As there exists
a miriad of different site conditions, to think that any one rig is 
perfect for them all would be..dare I say.. foolish.

> Now, that's only part of the story.  A true analysis of whether the
> redundant system has to take into account the sum of the probabilities of
> _all_ failure modes of interest in both the redundant and non-redundant
> system.  Only after summing the probabilities can one state that System A
> is more or less likely to fail than System B.

I agree. In addition to all the mathematical probabilities, we must also
assign values to the failure modes themselves. Some can be easily
tolerated and others cannot be survived. To add to the complexity, different
dive site parameters will necessitate changing these values.

Thanks for an interesting contribution...Paul

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