Bottom Time
Definition
Bottom time is the total time a diver spends at depth during a dive, from the beginning of descent to the start of the final ascent.
More on Bottom Time
Bottom time is a key factor in dive planning and decompression theory. It determines the amount of nitrogen absorbed by the diver’s body and helps calculate safe no-decompression limits (NDLs). Exceeding these limits can require staged decompression stops during ascent to avoid decompression sickness. Dive computers track bottom time automatically, but divers using dive tables must monitor it closely. Accurate bottom time tracking allows for proper gas management and conservative dive profiles, especially when doing repetitive or multi-level dives.
Frequently Asked Questions
Bottom time is measured from the moment the diver leaves the surface (starts descent) until the time they begin the final ascent. Computers track this automatically; manual divers often use a bottom timer.
No. Bottom time ends at the beginning of ascent. A safety stop is a separate, precautionary pause at shallow depth to reduce nitrogen loading.
Bottom time affects nitrogen absorption, gas usage, and decompression needs. Staying within no-decompression limits is vital to avoid required deco stops or DCS risk.
Dive tables use bottom time and depth to determine nitrogen exposure and assign pressure groups. Exceeding bottom time can shift a diver into decompression mode.
Miscalculations can lead to exceeding NDLs or running out of gas. This increases the risk of decompression sickness or emergency ascents.