Back in the seventies I did purchase a very expensive Scubapro pressure gauge, which instead of showing bars, was showings minutes of air remaining. The needle did move back on the scale according to external pressure, as the device did contain some sort of depth gauge. All mechanical!
I do not remember the name of this device, nor photos on the Internet. If you can refresh my memory I will be happy to see it again.
Of course, after a couple of dives, I found myself doing the calculation back for transforming that crap "minutes" indication on the real pressure in my bottles, so I did bring it back to the shop and swapped it with a standard pressure gauge.
The real crap thing is that the conversion between pressure and minutes was fixed, assuming a 20-liters twin bottle (which was the standard at the time) and an air consumption of 20 liters/minute.
I do not remember the name of this device, nor photos on the Internet. If you can refresh my memory I will be happy to see it again.
Of course, after a couple of dives, I found myself doing the calculation back for transforming that crap "minutes" indication on the real pressure in my bottles, so I did bring it back to the shop and swapped it with a standard pressure gauge.
The real crap thing is that the conversion between pressure and minutes was fixed, assuming a 20-liters twin bottle (which was the standard at the time) and an air consumption of 20 liters/minute.