I have an Oceanic VT4.0 that I am pretty new to, but I seem to be having a problem. The last two dives I made, I got the "5 minutes of air time remaining" alarm with about 1200 pounds left in the tank. I am still a heavy breather, but not quite that bad. I got the second "0 minutes of air time remaining" with about 800 pounds in the tank. Am I missing something? Did I set some alarm perimeter that I didn't mean to?