What determines the surface interval after diving when planning an ascent to higher altitude?

• A. The surface interval is fixed and does not depend on altitude.
• B. The surface interval depends on the planned altitude increase only.
• C. The surface interval depends on the planned increase in altitude and the highest repetitive group designator obtained in the previous 24-hour period.
• D. The surface interval depends on dive time.

Answer: (C)

The surface interval is the time you need on the surface to off‑gas inert gas before the next exposure. When planning an ascent to higher altitude, two factors drive this: how much nitrogen you still have in your tissues from recent dives, and how much altitude you plan to gain. The amount of remaining nitrogen is captured by你的 repetitive group designator—the higher the group, the more off‑gassing is required. Elevating to higher altitude changes the ambient pressure you’ll be under, which affects decompression safety, so you must ensure enough off‑gassed nitrogen before the ascent. To be practical after multiple dives in a 24‑hour window, the rule uses the highest repetitive group achieved in the previous 24 hours, ensuring the surface interval accounts for the worst residual nitrogen load from that period. Dive time alone doesn’t determine the interval, and altitude alone isn’t sufficient without considering how loaded you are from recent dives.