The minimum surface interval required to avoid decompression after a dive to 88 ft for 18 minutes, followed by a dive to 78 ft for 23 minutes is

The minimum surface interval required to avoid decompression after a dive to 88 ft for 18 minutes, followed by a dive to 78 ft for 23 minutes is

• A. 2 hr 30 minutes
• B. 4 hr 49 minutes
• C. 3 hr 40 minutes
• D. 5 hr 40 minutes

Answer: (B)

The concept here is surface interval credit—how long you must wait on the surface for nitrogen to off-gas enough from the first dive so the second dive stays within no-decompression limits. After the 88 ft dive for 18 minutes, nitrogen remains in your tissues. A follow-up dive deeper than a few feet can still add to that nitrogen load, so the water time and depth of the second dive (78 ft for 23 minutes) require a longer wait on the surface for off-gassing to reach safe levels. The minimum interval that NAUI tables indicate for these two dives to avoid decompression is about 4 hours and 49 minutes. In short, the long interval accounts for how nitrogen absorbed on the first dive must dissipate before the second dive can be conducted without decompression requirements. Actual timing can vary a bit with conditions, but the 4 hr 49 min figure reflects the table-based calculation for these dive depths and times.

The concept here is surface interval credit—how long you must wait on the surface for nitrogen to off-gas enough from the first dive so the second dive stays within no-decompression limits. After the 88 ft dive for 18 minutes, nitrogen remains in your tissues. A follow-up dive deeper than a few feet can still add to that nitrogen load, so the water time and depth of the second dive (78 ft for 23 minutes) require a longer wait on the surface for off-gassing to reach safe levels. The minimum interval that NAUI tables indicate for these two dives to avoid decompression is about 4 hours and 49 minutes. In short, the long interval accounts for how nitrogen absorbed on the first dive must dissipate before the second dive can be conducted without decompression requirements. Actual timing can vary a bit with conditions, but the 4 hr 49 min figure reflects the table-based calculation for these dive depths and times.