Altitude Diving. Thinking back to Section One, you recall that as you ascend in air, pressure decreases. Dive tables and most computers give you their no decompression limits based on a dive ending at sea level: if you're under less pressure at altitude, nitrogen comes out of solution more following a given dive, making decompression sickness more likely.

You can use the Recreational Dive Planner for diving to altitudes as high as 300 metres/1000 feet. Above 300 metres/1000 feet, you need special conversion tables and procedures to account for the decreased atmospheric pressure or you can run an unacceptable risk of DCI.

The procedures for diving at altitude with a dive computer vary with the computer. Some automatically compensate for altitude, where as with others You'll need to tell the computer your altitude. There are a few older models that you can't use at altitude.

You also need to think about lowered atmospheric pressure if you plan to fly after diving. While this concern is similar to altitude diving, it's not identical. When you dive at altitude, you dive and return to reduced atmospheric pressure. When you fly after diving, you dive and return to normal atmospheric pressure, then expose yourself to further pressure reduction.