If you are on an escorted dive, there is no real problem here. Just let the dive leader know your air level when they inquire, and if at some point you are under 1000 psi let them know.
While that's a pretty common occurrence, it also has some of the markings of a "Trust Me" dive plan. For example, if the dive leader hasn't asked and you're down to 1000psi before he's even reached his turnaround point, you could be a looooong ways from the boat (exit).
. AT the risk of heresy, surfacing with more than 0 psi is a successful dive. But I too believe building in a margin of a few hundred psi.
I agree with your intent, but I would have been more comfortable if you would have said "completed the dive" instead of merely "surface".
And for margins, we need to keep in mind that regulators will start to give up the ghost when the tank's remaining (HP) pressure falls to (or below) the first stage's designed LP pressure, which is typically in the 150-200psi range. As such, 200psig at a 15ft safety stop is, for pragmatic purposes, close enough to "empty" to become quite concerned about just how many breaths are actually left in the tank.
Of course, this doesn't really help the OP all that much, since we're talking about the end of the dive and not how to figure out how much bottom time is left for when that's going to be.
The long answer is that this is why we have SAC and its accompanying calculations. You take your air consumption at the surface and then you plot what your dive depth is going to be ... I believe I saw Bob's excellent webpage already referenced. This stuff is important when your dive plan needs to be meticulous, which for many Recreational dives isn't as much the case much anymore...with dive computers, Rec dives can often be far more ad hoc.
My recommendation is short: learn to cheat
We simply need to remember that any dive really only has two really important requirements in this area:
a) Stay good on decompression obligations
b) Stay good on air
With a dive computer, its pretty easy to not get into too much trouble on (a), particularly for novices with higher typical air consumption that makes it less likely that they'll get near the DCS limits. That means that for the most part, it is (b) that we need to watch. That's our SPG and to also understand what "Turnaround" is, and how to apply it.
Turnaround basically is assumes a simple "out and back" dive plan - - no circles, squares, triangles, etc. That basically means that your resources get budgeted as "half used on the way out and the other half on the way back in". Pretty straightforward.
As such, the idea is that you're going to use your SPG to tell you when you're "half out of gas". Since we start at 3000psi and want to end at 500psi, it isn't simply a divide by two: this is where that math of: (3000-500)/2 +500 = ~1800psi comes in ... and the good news is that all we really have to remember is that for a 3000psi tank, the default number is 1800psi.
Now applying.
Okay, we're going to go follow the DM guide and the dive plan is an "out-and-back". They're a good DM, so they actually turns back when we signal them that we've hit the agreed-upon 1800psi for turnaround. We proceed to swim back to the boat ... and hey! We had planned to get back with 500psi, but we actually have 1000psi...how did that happen?
This doesn't always happen, but it should be something close. The answer lies with other commonly applied good diving practices that we were also following: the DM started the dive by swimming into any current...so our return trip was easier & faster. Similarly, the DM did a multilevel profile with the deepest part first...so our return trip was shallower too, which also uses less air. Both of these added some conservatism into our dive plan, which showed up as a higher remaining tank pressure.
This is fine (because conservatism is good) and there's no rule that says we always have to use all our air.
What I do at this point is locate the boat (exit point), since that relieves any "where's the boat??" anxiety stress. It isn't fun to be a 600-500-400psi and not know where the boat is ... earlier is always better. If we have a ton of air & time left and feel like staying down a bit longer, I'll probably be willing to noodle around right in the shadow of the diveboat, until we decide its time to come up. Conditions permitting, of course.
Okay, fifty more dives have passed and the novice air hoovering is over ... what about that Dive Computer DCS stuff again?
Going back to (a), we can see that right after we've descend to our dive plan's nominal max depth, the computer will show our time-remaining (based on Deco), and for an out-and-back, our turn-around value is half of that number. For example, if it initially says 15 minutes, we should turn to return when it says 7 minutes. The idea here is still that we're dividing our allowable bottom time (based on deco now too) into half for "out" and half for "back", although do note that this assumes that the return swim is done at the same depth...not a multi-level which will allow more time.
-hh
