Ana
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Wow I wish I had the same faith you have in the accuracy of my SPG's. There are lots of situations where the needle on a mechanical gauge got stuck and was reading a higher level than actual.
Things go south, I don't know about "lots of situations" but when is your day for things to go wrong, they just go.
HOWEVER,,,,, One should know how much air one has at pretty much every time. Granted this is not applicable to the person that just started to dive this past Monday, but anyone with a couple of dozen dives under their belt should know within 100psi up/down how much air they have before looking at the gauge. If one doesn't know it, then maybe one needs to figure a way to know it.