Inside the watertight case, a battery-powered electronic computer uses a depth gauge and clock to track the depth profile as it regularly samples the current water pressure, and uses this information to estimate the current depth and the amount of nitrogen that has been dissolved in the diver's blood. An LCD display on the face of the dive computer shows the number of minutes that the diver is allowed to remain underwater at his current depth before he or she has exceeded safety recommendations. As time passes and the diver ascends and descends in the water, the display changes accordingly.
Dive computers also display the current depth, the amount of time the diver has been underwater, and the maximum depth reached during the current dive, so the diver can keep true to his original dive plan and surface after the agreed-upon dive duration. Most dive computers keep track of this information for at least the last ten dives so the diver can later write this information down in his diver's log book.
Other available dive computer features include:
Scuba instructors recommend that divers follow the limits of the dive tables they receive as part of their training, forming a dive plan and then following it -- this as opposed to simply jumping into the water, dropping down, and keeping an eye on their dive computer until it says that it's time to surface. Following a dive plan is good procedure and will help protect divers if they lose their dive computer in the water, or if it runs out of batteries or even if its software crashes.
Different brands and models of dive computers are called more "conservative" or "liberal", where "conservative" means the computer limits diving time more in order to increase the margin of safety against decompression sickness. Experienced divers may want to use a more "liberal" computer, but this may increase risk. Complicating this choice is the fact that susceptibility to decompression sickness is not well understood, and it seems to vary depending on the diver's fitness, age, weight, fatigue, and even recent illness, as well as the water temperature and the altitude.