Define the RNT exception rule.

• A. Take the shallowest of the two dives and total the bottom time together as if it was one dive
• B. Take the deepest of the two dives and total the bottom time together as if it was one dive
• C. Use the average depth of the two dives to determine decompression
• D. Ignore decompression and dive as two separate dives

Answer: (B)

The idea being tested is how to handle a second dive when you’ve done a previous dive with little time on the surface. The RNT rule uses the concept of residual nitrogen from the first dive and plans the second dive as if it were a single longer dive at the depth that’s deeper of the two dives. In practice, you take the deepest depth reached in the two dives and add the bottom times together, treating the combined exposure as one dive at that depth. This approach is conservative because deeper depths require more careful decompression considerations, and adding the times reflects the total nitrogen exposure from both dives.

Using the deepest depth and summing the bottom times ensures you don’t underestimate nitrogen loading and the corresponding decompression requirements. Choosing the shallowest depth would understate the nitrogen load, while using an average depth wouldn’t accurately reflect the tissue loading from the deeper portion. Treating the dives separately ignores residual nitrogen and can lead to unsafe planning if the surface interval is short.