If a diver completes a 25-minute dive to 30 meters (98 feet), what is the required surface interval time before a second dive to 20 meters (66 feet) for 30 minutes?

To determine the required surface interval time before a second dive after a previous dive, we need to consider the concept of residual nitrogen in the body. The first dive was to a depth of 30 meters (98 feet) for 25 minutes, resulting in a certain level of nitrogen absorption. The dive to 20 meters (66 feet) for 30 minutes will also contribute to nitrogen saturation, which needs to be managed to avoid decompression sickness.

Using dive tables or a dive computer, divers can track how long they need to wait on the surface to allow their bodies to off-gas the excess nitrogen absorbed from the first dive. After a dive to depths around 30 meters, the nitrogen loading is significant, and the surface interval must be sufficient to safely reduce nitrogen levels before descending again.

The choice of a 45-minute surface interval is appropriate based on dive planning protocols that suggest longer surface intervals after deeper and longer dives. This time allows the body to adequately eliminate the absorbed nitrogen in a safe manner, aligning with the guidelines for successive dives. It ensures that the diver has a lower risk of nitrogen narcosis and decompression sickness when embarking on the second dive.