How much surface interval time is required for a diver who completes a 30-minute dive to 20 meters (66 feet) before diving again to the same depth?

The requirement for surface interval time before a diver can safely conduct another dive is influenced by the first dive's depth and duration, as well as the nitrogen loading that occurs during the dive. In this case, a diver has completed a 30-minute dive to a depth of 20 meters (66 feet).

Based on standard diving tables and dive computer algorithms, after a dive that lasts 30 minutes at this depth, the diver will have absorbed a significant amount of nitrogen, necessitating an adequate surface interval to allow for its off-gassing and minimize the risk of decompression sickness.

The chosen surface interval of 60 minutes allows the diver sufficient time for nitrogen levels to decrease in the body, following the general guidelines for recreational diving. These guidelines suggest that longer dives or dives to greater depths typically require longer surface intervals before a subsequent dive to the same or similar depth to ensure safe diving practices.

Thus, a surface interval of 60 minutes is consistent with the precautionary principles of diving safety, enabling the body to off-gas nitrogen effectively and reducing the likelihood of complications on the next dive.