Dive Surface Air Consumption, Rock Bottom, and Gas Planning

A diver measured at 0.65 cu ft/min breathes 2.23 at 80 ft, because consumption at depth is the surface rate times the absolute pressure of 3.42 -- each breath contains proportionally more gas, which is why bottom time falls so much faster with depth than people expect. Twenty-five minutes on the bottom is 55.6 cubic feet. ROCK BOTTOM IS THE NUMBER TO BUILD THE PLAN AROUND. Two divers ascending from 80 ft at 30 ft/min to a 15 ft stop, sharing gas at an elevated 1.0 cu ft/min each, need 10.6 cubic feet on the way up and 8.7 on the stop: 19.3 cubic feet, 24% of an 80 cu ft cylinder, that is not available for the dive at all. Usable gas is 60.7, which at 2.23 a minute is 27 minutes -- not the 36 the cylinder raw capacity suggests. Planning to a fraction like turn-at-a-third happens to approximate this on some dives and badly underestimates it on deep ones, where the ascent itself consumes a great deal. THE RESERVE IS COMPUTED AT AN ELEVATED RATE, not the measured one, because surface air consumption is personal and is not constant: it rises with work rate, cold, stress, and poor trim, and a rate measured on a calm dive underestimates a hard working dive. Measuring it on the actual kind of work is what makes it trustworthy. A gas volume calculation on figures the user supplies. It does not plan a decompression profile, evaluate a no-decompression limit, or account for gas needed beyond the single stop entered. It assumes an average pressure over the ascent, which is a linear approximation to a real one, and it does not model cylinder pressure against temperature, gas density and work of breathing at depth, or the failure the reserve exists for. The operation diving safety manual, the supervisor, and the applicable regulations govern.

consumption at depth = the measured surface air consumption x the absolute pressure (1 + depth / 33 seawater); rock bottom = the team's elevated rate x the ascent time at the average ascent pressure, plus the stop time at the stop pressure; usable gas = cylinder volume - rock bottom.

The standard gas-planning relations by name. The operation's diving safety manual, the diving supervisor, and the applicable regulations govern.

Volume arithmetic on the diver's own measured consumption and cylinder; no proprietary planning table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: sac_cuft_per_min, planned_depth_ft, planned_bottom_time_min, cylinder_volume_cuft, team_size, ascent_rate_fpm, stop_depth_ft, stop_time_min, stress_sac_cuft_per_min, feet_per_atm, depth_ata, rate_at_depth_cuft_min, bottom_gas_cuft, ascent_gas_cuft, stop_gas_cuft, rock_bottom_cuft, usable_gas_cuft, max_bottom_time_min, naive_bottom_time_min

• Reserve first, dive second usable gas is what the reserve leaves, not what the cylinder holdsgas planning practice
• The reserve uses an elevated rate a measured calm-dive rate underestimates a real emergencygas planning practice
• One stop only gas for a decompression profile is not computed herethe operation's dive plan