Dive No-Decompression Limit and Residual Nitrogen

A second dive to 60 ft against a table that gives 55 minutes, after a surface interval leaving 21 minutes of residual nitrogen, has an adjusted limit of 34 minutes -- residual nitrogen has taken 38% of it before the diver enters the water. A diver planning to 55 would be 21 minutes into a decompression obligation at the point they believed they still had time in hand. THE TRADE IS SURFACE INTERVAL AGAINST BOTTOM TIME. Lengthen the interval until the residual falls to 9 minutes and the adjusted limit rises to 46 -- twelve minutes of bottom time bought on the boat rather than underwater, and that trade is the whole structure of a repetitive dive plan. Plan 30 minutes on the first case and 51 minutes are credited against the table 55, leaving 4 in hand. NO FORMULA PRODUCES A NO-DECOMPRESSION LIMIT and none is attempted here. Different tables and algorithms give materially different answers for the same profile, they are validated as complete systems, and mixing a limit from one with a residual nitrogen figure from another is not valid: the limit and the residual entered must come from the same table. This does the bookkeeping around whichever one the operation uses. On a nitrox mix the plan is made by entering the air table at the equivalent air depth -- EAN32 at 60 ft enters at 47 ft -- and that is where the extra bottom time comes from, so the equivalent depth is reported alongside. The limit falls steeply with depth, which is why depth discipline matters more than time discipline on a working dive and why a few feet deeper than planned can consume the whole margin. It does not model tissue compartments, decompression obligation, omitted decompression, or repetitive group letters, and it does not correct for altitude -- sea-level tables do not apply at altitude and a separate procedure governs. The applicable dive tables or computer algorithm, the diving supervisor, and the operation diving safety manual govern.

adjusted no-decompression limit = the table's limit for the depth - the residual nitrogen time the table gives for the repetitive group and surface interval; credited bottom time = residual nitrogen time + planned bottom time; a nitrox mix enters the AIR table at the equivalent air depth ((1 - FO2) / 0.79) x (depth + 33) - 33.

The repetitive-dive bookkeeping by name. NO FORMULA PRODUCES A NO-DECOMPRESSION LIMIT and none is attempted: the limit and the residual are entered from whichever table or algorithm the operation uses, and they must come from the same one. The applicable dive tables or computer algorithm, the diving supervisor, and the operation's diving safety manual govern.

Subtraction on figures the diver reads from their own table; no dive table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: planned_depth_ft, table_ndl_min, residual_nitrogen_time_min, planned_bottom_time_min, alt_residual_time_min, oxygen_fraction, feet_per_atm, adjusted_ndl_min, credited_bottom_time_min, remaining_min, alt_adjusted_ndl_min, surface_interval_gain_min, equivalent_air_depth_ft

• No formula gives an NDL the table or algorithm does, and this is the bookkeeping around itdive table practice
• Limit and residual come from ONE table mixing sources is not validdive table practice
• Altitude needs a separate procedure sea-level tables do not applyaltitude diving procedure