Dive Tables vs. Dive Computers: How Each One Tracks Your Nitrogen
The short answer
A dive table and a dive computer both estimate how much dissolved nitrogen a dive leaves you with, but a table does it once, from the deepest depth and total bottom time you write down, while a computer does it continuously, sampling actual depth every few seconds and recalculating as you go. Because a table charges your whole bottom time at your deepest point and a computer credits the shallower parts of the profile, the computer usually shows more allowable time than the table does for the same dive. Both are implementations of decompression models — the kind documented alongside the NOAA no-decompression tables — not direct measurements of your body.
What a table does
A table treats a dive as a rectangle: one depth, one duration. You round your deepest depth up to the next row and your bottom time up to the next column, read a limit or a group letter, and carry that forward through the surface-interval and residual-nitrogen grids. The whole method is designed to be worked with a pencil, and the procedures that go with it — ascent rate, how bottom time is defined, rounding rules — are published with the table itself, as in the NOAA Diving Standards and Safety Manual.
The consequences of the rectangle assumption are worth naming:
- A multi-level dive that spent most of its time shallow is still charged at its deepest point.
- A brief excursion to the deepest depth changes which row you use for the entire dive.
- Rounding is always in the conservative direction, which stacks across a day of diving.
- The output is coarse: bands and letters, not minutes-by-minute detail.
How to read a no-decompression dive table walks through the grids, and residual nitrogen time explained covers how a table charges the second dive for the first one.
What a computer does
A computer samples pressure at short intervals and feeds that depth history into an algorithm with multiple theoretical tissue compartments, each loading and unloading gas at a different rate. From that it displays a running estimate of remaining no-decompression time, updated as the dive changes depth. It keeps calculating at the surface, so it also carries your state through surface intervals automatically.
What that buys you, and what it costs:
- Credit for the real profile. Time spent shallow is modeled as time spent shallow.
- A live countdown. The remaining-time figure responds to what you actually do, rather than to a plan you made on deck.
- Automatic repetitive bookkeeping. No letters to carry, no worksheet.
- Model-specific results. Two computers running different algorithms, or the same algorithm at different conservatism settings, can show different remaining times on the same dive. The number is the algorithm's opinion, not a reading off your tissues.
- A device to maintain. Batteries, seals, firmware, and settings are now part of dive logistics.
Why the two numbers disagree — and which is "right"
Neither is a measurement, so neither is right in the sense of being verifiable against your body. They are different amounts of information fed into different models. The table has less information about the dive and compensates with conservative assumptions; the computer has more information and applies its model to all of it.
The practical point is that they are separate systems with separate state. You cannot dive on a computer and then convert its display into a table letter, and you cannot hand a table letter to a computer. If you want to plan on paper after diving on a computer, you start the table calculation from the profile — deepest depth, bottom time, that table's rounding rules — and accept the conservative answer that produces.
Which system governs a given dive is usually set by context, not preference: a program's manual, a course's materials, or an operator's stated procedure. Scientific and working-diver programs specify it explicitly, along with dive-planning and depth-authorization requirements — see the AAUS standards for scientific diving for how that is written down.
What both systems leave out
A table and a computer model gas loading from depth and time. Neither one knows anything about the diver in front of it. Individual variation, thermal conditions, workload, hydration, fitness-to-dive questions, and post-dive considerations like flying after diving sit outside the model entirely — those belong to medical references such as the CDC Yellow Book dive chapter and the Divers Alert Network health resources library, and to the professionals who evaluate individual divers.
Understanding that boundary is most of dive-planning literacy: the numbers are a planning framework with known assumptions, and knowing the assumptions is what makes the numbers useful. If the vocabulary is still settling, residual nitrogen in diving is the place to start.