So correct. We should be talking about "weak points" not "failure points". I know nothing of solo diving, and I'm a newer diver, but this is true of any technical industry. Diving is a failure point in and of itself. You take one of the core functions of life (breathing) and restrict it to a very specific method. Driving and flying each have millions of failure points, yet we do them safely all the time. A brick has almost no failure points. The difference is a brick can't do anything useful for us.
Excessive failure points should be limited in any engineering, but when you add functionality, you add the potential of failure. Weak points should be well known and avoided where possible.
As a retired safety professional, I can tell you where the term "failure point" comes from. In product safety, there is a technique called "Failure Modes and Effects Analysis," or FEMA. This technique looks at each failure point, and analyzes the effects of that failure "upstream" and "downstream" of that failure.
As an example, from my personal experience when diving solo, I was about to dive my Dacor Pacer Aero regulator. It has an Enduro safe second attached, an SPG computer, and a LP line to my BCD. This Pacer Aero is a plastic second stage that I had used before and cleaned by removing the cover and diaphragm, washing it out again, and replacing the two parts. For this dive, I did a pre-dive check by both using the purge button and breathing off it before entering the water. But upon entering the Clackamas River, in a current and with water about ten feet deep, I took my first breath, and all I got was water. I tried a second breath, and again, only water came into my airway. Almost choking, I switched to my Enduro safe second, used the purge, and upon breathing got good, dry air. I then completed the dive of about forty-five minutes on the Enduro second stage.
What happened? Well, when I put the cap back on, the diaphragm was not seated into the top grooves, and so it did not seal. It is a screw-on top box, and it was not possible to visually see, or tactically determine that the seal was bad. When I tested the regulator, the purge button obviously worked, and I had enough vacuum with my breathing to get a nice easy breath in air. But in water, no way!
Using FMEA, there were no effects upstream of this defect. The valve worked perfectly. Downstream effects in the air were hard to determine; there were no leaks, and I could breath off the regulator. But in the water, any inhalation brought only water into the second stage, with the possibility of drowning if there was not a safe second or octopus regulator. I had the Enduro octopus, and so there was actually no effect of the dive.
How could I have determined that there was a problem with this second stage? If I had done a negative pressure check by either putting my thumb over the first stage air inlet seal on the body, and breathing, there would have been air entering the second stage through the leak. I could then have corrected the problem by unscrewing the cover, reseating the diaphragm, and screwing the cover back on, then again checking that the seal was good.
I dive many different configurations, using both single hose and double hose regulators. I dive a snorkel too, on my mask or helmet. If I have a double hose regulator I am not sure of, I have a set of twin 45s with a Sherwood twin-post manifold, and mount a second regulator off the extra post. But many times I will dive a Mistral or other double hose regulator without a safe second, and my alternate air supply is the surface (my dive site is only about 25 feet, or 8 meters deep at the deepest). The Mistral or Healthways Scuba double hose regulators have only six moving parts, and are very reliable (which is why JY Cousteau and his team used the Mistral almost exclusively). Some of my collection, such as this Sportsways Hydro-Twin, are equipped with a safe second, SPG, and LP hose for my BCD.
SeaRat
