Dive Bottom Time by Depth

Bottom Time and Pressure in Scuba Diving

Underwater, ambient pressure climbs by 1 atm with every 10 m of seawater you descend. The total pressure pushing on a diver works out to P = 1 + depth/10 in atm, and that's why lung volume and gas use shift so sharply as you go deeper. What caps your bottom time is the No-Decompression Limit (NDL) — the longest you can spend at a given depth and still head straight back up without owing any mandatory decompression stops.

Here are some typical NDLs from the PADI Recreational Dive Planner (RDP): 18 m → 56 min, 21 m → 40 min, 24 m → 30 min, 30 m → 20 min, 40 m → 9 min (the recreational limit). Once you pass 30 m, nitrogen narcosis (“rapture of the deep”) starts to dull your judgement, slow your reactions and make you clumsy. Dive computers from Suunto, Mares, Garmin, Shearwater and Scubapro keep tabs on your nitrogen loading as it happens, yet knowing how to plan a dive on tables is still a core skill that PADI, NAUI, SSI and CMAS all teach.

Applications

A recreational reef dive at 18–25 m usually runs 40–50 min and finishes with a 3 min safety stop at 5 m. Drop into the 30–40 m range for a wreck or wall and you'll want careful gas planning (the rule of thirds) plus a buddy briefing on what narcosis feels like. Below 40 m, technical dives run trimix to cut the narcosis and bring staged decompression into the picture. Log your depth, time, water temperature and surface interval every time, because repetitive dives stack up residual nitrogen.

FAQ

Why does pressure increase 1 atm every 10 m? A 10 m column of seawater weighs roughly 1 kgf/cm², and that's the same as 1 atm. Tack on the 1 atm of air already sitting at the surface and you've got the total pressure bearing down on the diver.

What happens if I exceed the NDL? You're now into mandatory decompression, which means you have to stop on the way up to let the nitrogen off-gas. Skip those stops and you're gambling with decompression sickness, the “bends”.

Do I still need tables if I have a dive computer? Yes. They're your backup, and they let you plan the dive before you ever hit the water. A computer can glitch or die mid-dive on a flat battery, and knowing the tables is what keeps you out of trouble when it does.