That's the point, they don't. People bandy about the failure point argument constantly
I have had 2 SPG failures (spools). Both resulted in gas being lost. One was on an SPG that was only a couple of weeks old. It did not pop when I turned the gas on. It popped a minute or two later, leading me to think that if I were the type to wait until the last second, turn my gas on, and roll back off the boat, it could easily have popped after I was already in the water.
In contrast, I have done many more dives with AI than I have with an SPG and I have never had an AI failure.
In reality the only additional points of failure on an SPG are, the mechanical spg itself, then teh hose and the HP spool.
You say that as if those additional points of failure are inconsequential.
I would classify SPG/AI failures into 3 categories: Results in loss of gas, results in loss of accurate pressure reading (but not gas), intermittent loss of pressure reading.
An SPG and AI both have an O-ring in the HP port, so they share that possibility for a gas-loss failure. The SPG has VASTLY more opportunity for gas-loss failure, thanks to the HP hose, the 2 O-rings on the spool, and the Bourdon tube inside the gauge itself.
SPGs are well known to have the possibility (low probability that it may be) of sticking and not giving an accurate reading. In particular, the possibility that it could be reading, say, 600 psi when you really only have 200 psi left. AI has the possibility of a battery dying, so you simply lose your reading altogether. I have not heard of a case where AI gives a wrong-but-believable reading. E.g., the pressure transducer might flake out and tell you you have 7000 psi, but I've not heard of a case where it went from working accurately to saying 600 when you only have 200. So, I *think* this failure mode means that an SPG could fool you, but AI would not fool you - it would simply tell you nothing or tell you something you would immediately recognize as wrong. I would much rather know that my cylinder pressure is "unknown" than think that I know my cylinder pressure when I really don't.
An intermittent failure is something that would only happen with AI. It shows as "LOST COMMS" or similar on your computer. I have seen that very occasionally, but when it happens, just the act of holding my computer up to read it allows it to re-establish comms, so the "error" goes away within a second or two. I don't see that as a problem of enough significance to sway someone away from using AI.
All in all, the assessment of failure modes leads me to think that AI is significantly preferable to an SPG.
But, let's do keep it real. SPGs are very reliable (with some basic due care). This is not a pitch to tell people they need to ditch their SPG and switch to AI. It is simply to say that, in MY opinion, SPGs are safe and reliable, but AI (as used by Shearwater, Oceanic, Aqualung, et al) is even more safe and reliable (with proper maintenance). Thus, unless you would use 2 SPGs, for redundancy, there is no reason to use AI AND an SPG, either.
