On a two-tank dive, the surface interval can feel like an inconvenient break between the dives you actually came to enjoy. Change cylinders, grab something to drink, have a snack, and get ready to go again. It's easy to spend more time preparing equipment than thinking about what your body is doing between dives.
Your surface interval gives your body time to eliminate nitrogen absorbed during the previous dive. It also provides an opportunity to recover from the physical demands of diving, particularly when you're making several dives in a day.
What Happens During a Surface Interval?
While underwater, your tissues absorb nitrogen under increased pressure. After surfacing, that nitrogen gradually leaves your body. The longer you remain at the surface, the more nitrogen you generally eliminate before your next dive.
Your dive computer estimates this process, accounting for residual nitrogen when calculating your next dive's no-decompression limit (NDL). A longer interval generally means more available bottom time, although the computer is modeling nitrogen loading rather than measuring what's actually happening inside your body.
Your computer cannot account for every factor influencing your risk of decompression sickness. Age, health, exertion, temperature, and individual physiology all play a role. Following your computer's limits is essential, but it doesn't guarantee you won't experience DCS.
Why Longer Intervals Are Worthwhile
Allowing more time between dives reduces residual nitrogen loading before your next descent. It can also increase your available no-decompression time, particularly when making several dives in a day. An extra half hour on the surface may give you considerably more flexibility during the next dive, depending on the profiles involved.
Diving can also be physically demanding. Cold water, strong currents, heavy equipment, and challenging conditions can leave you tired long before you've reached any decompression limit. Fatigue and discomfort affect concentration and decision-making, neither of which improves when you rush back into the water. A longer break gives you time to get warm, eat, drink, and recover.
How Long Should You Wait?
For ordinary recreational diving, I'd plan at least one hour between dives. After deeper or longer dives, or when making several dives in a day, I'd extend that to 90 minutes to two hours or longer. These are reasonable planning guidelines, not universal minimums, and I'd still verify the next planned dive against my computer.
If a dive boat schedules a shorter interval, I'd plan a shallower or shorter second dive rather than pushing my remaining no-decompression time. If I'm cold, exhausted, or feeling unwell, I'd extend the break or skip the dive altogether.
The schedule deserves some attention before getting in the water. Liveaboard divers making four or five dives a day may find it worthwhile to sit one out occasionally. Shore divers can change cylinders and re-enter surprisingly quickly, sometimes without giving themselves much opportunity to recover. Cold-water divers should be particularly attentive to whether they've warmed up sufficiently before diving again.
Make the Interval Count
I use surface intervals to get warm, drink something, eat if I'm hungry, and relax. I'll also review the previous dive and check my computer against the planned depth of the next one. If the first dive was particularly demanding, I may change the next dive's plan or extend the break.
I enjoy getting as much underwater time as I reasonably can, but I've never seen much value in rushing through a surface interval. The extra time can improve my next dive's no-decompression limits, give me a chance to recover, and leave me better prepared to enjoy the next descent.