I can't answer for the N2ition3, but I have the regular N2ition, and I've gone into deco several times with it. It sets the ceiling, and as long as you don't exceed the ceiling, it's counting down while you're ascending. The last deco dive I did with it I was at 90 fsw, and incurred 10 minutes of deco with a 10 foot ceiling. I was in Cozumel, and started slowly working my way up, since almost every dive there is multilevel. I did a half stop for one minute, then verrrry slowly ascended. The computer cleared deco well before I ever reached safety stop depth of 18-20 fsw. As DA said, it probably just counts it down slower the deeper you are......