What surface interval is required in order to do a dive to 70' for 35 minutes followed by a dive to 50' for 35 minutes?

The requirement for a surface interval between dives is determined by the need to allow the body to off-gas nitrogen absorbed during the previous dive. The deeper and longer the previous dive, the more nitrogen is absorbed, and consequently, more time is necessary for the body to safely eliminate this nitrogen before embarking on another dive.

In this scenario, the first dive is to a depth of 70 feet for 35 minutes. This dive generates a significant amount of nitrogen absorption due to its depth and duration. Following this, the second dive is to a depth of 50 feet for 35 minutes. Although the second dive is shallower, it still requires consideration of the nitrogen taken on from the first dive.

The appropriate surface interval would ideally be based on tables or dive computer data which correspond to the specific depths and times of both dives. A surface interval of 60 minutes allows adequate time for off-gassing, as this exceeds the minimum requirements provided by no-decompression limits for both dives concerned.

This extended interval helps to ensure diver safety by reducing the risk of decompression sickness, which is caused by too much nitrogen remaining in the tissues when ascending from subsequent dives without sufficient time to eliminate it.