Your body ingasses nitrogen during a dive, and offgasses nitrogen while on the surface. Unfortunately, it takes several hours to fully offgas nitrogen after diving. This means that if you complete more than one dive per day, your body still has residual nitrogen from previous dives when you begin repetitive dives. As a result, nitrogen levels continue to increase with every dive, which reduces the time you can spend underwater during repetitive dives.

Surface intervals help you safely complete more than one dive per day. During the surface interval, your body offgasses a significant amount of residual nitrogen, which increases your maximum dive time for repetitive dives.

The chart to the left illustrates the benefit of surface intervals. As you can see, after the first dive the diver has ingassed nitrogen. But after a surface interval the diver has offgassed a significant amount of that nitrogen, which allows a longer dive time for the second dive.

As you can imagine, it would be impossible for this diver to complete these three dives without taking surface intervals between those dives.