Old SCUBA tanks didn't have gauges, they had a reserve tank with enough air to get you to the surface. You'd realize you were running low (which I'm sure was terrifying) then hit the switch and slowly surface (you don't want to surface quickly when diving).
Yeah, my dad had a tank like that. I dove with it exactly once - never again, yikes. It was coated inside and out so, despite being a steel tank, it was in excellent shape.
