Maybe a dumb question but I'd like to know the answer.
I don't think it's a dumb question. I am new to diving but have thought about this quite a bit, too.
In my open water diver class we used aluminum 80 tanks filled to 3000 psi and always left 500 psi in reserve and 300 for safety stop, meaning begin ascent at 800psi. Calculated time to turn dive based on 3000-800. Recently I dove with a steel tank 80 c.f. but only 2400 psi when full.
This reflects common practice but I don't think it's a sufficient reserve except for shallow dives where running out of air during ascent is acceptable. You have to decide what risks you're willing to take and how much of an obligation you have to your buddy should your buddy encounter an OOA emergency at the worst possible point in the dive.
For a deep dive, I think about the cylinder pressure at which ascent must begin (also called the "rock bottom pressure") as the:
Minimum pressure at which the first stage regulator will function at the surface + air needed to safely end the dive
I use 100 PSI as the minimum pressure at which the first stage will work, although others use lower or higher values.
I calculate the air needed to safely end the dive as the sum of these things, multiplied by two, because I plan for my buddy having an OOA emergency at the worst possible time, that is, right before planned ascent:
- air needed to identify that buddy is OOA, swim to buddy, start sharing air, and prepare for ascent
- air needed for ascent
- air needed for safety stop
- air needed on surface to reach safety
I disregard the distorting effect of depth on tank pressure gauge readings because it's 60 PSI max at recreational depths. Disregarding it makes the calculations slightly more conservative, not much.
I figure these in cubic feet and convert them to PSI based on the design capacity and pressure of the cylinder I'm going to use.
That site does a good job of automating some of the calculations. It does not include air needed on the surface, however.
Here's an article you may find helpful:
NWGratefulDiver.com
If diving low pressure tank do you leave less psi in reserve? What about high pressure steel tanks filled to 3400?
Yes, if the tanks have the same capacity when filled to their rated pressure.
With typical tank sizes, depth plays a larger role. For a 70 foot dive you need about a 50% higher reserve than a 35 foot dive.