During repetitive diving, what occurs to allowable bottom time as the surface interval increases?

As the surface interval increases during repetitive diving, the allowable bottom time significantly increases. This phenomenon occurs because the nitrogen that has been absorbed by the body during the first dive is gradually eliminated during the surface interval. The longer the diver remains at the surface, the more nitrogen can leave the body's tissues, allowing for longer dive times on subsequent dives without exceeding safe limits.

This principle is a fundamental aspect of dive planning and is rooted in the physiology of diving. After a dive, the body undergoes a process called "decompression," where the nitrogen that was absorbed at depth must be released to avoid issues like decompression sickness. By extending the time at the surface, divers effectively reset their nitrogen levels, enabling them to dive again with increased safety and potentially longer bottom times.

Therefore, a longer surface interval allows the diver to safely increase the allowable bottom time for subsequent dives.