antasena: A Python Library for Nitrox Diving Calculations

Sandy Hardian Susanto Herho , Faruq Khadami, Faizal Ade Rahmahuddin Abdullah, Iwan Pramesti Anwar, Karina Aprilia Sujatmiko, Alfita Puspa Handayani, Dasapta Erwin Irawan

We present antasena, an open-source Python library for calculations that underlie diving with nitrox, oxygen-nitrogen mixtures richer in oxygen than air. It computes depth limits, equivalent depths, inert-gas uptake, no-decompression limits, staged decompression schedules, central-nervous-system and pulmonary oxygen exposure, real-gas cylinder contents, and partial-pressure blending, and its command-line and desktop interfaces
display the derivation of every result with the user’s inputs substituted. Each working equation is derived from the three-dimensional momentum balance of a fluid at rest, the advection-diffusion equation for a dissolved gas in perfused tissue, and the Helmholtz free energy of a gas mixture, with every reducing assumption stated. Because a dive plan is a sequence of segments of fixed gas and linearly varying depth, tissue tensions, oxygen
doses, and gas consumption admit closed forms without time stepping. An independent re-implementation reproduces the closed-form tensions to within relative differences of 5 × 10^{−13} against fourth-order Runge-Kutta integration and 9 × 10^{−16} against 60-digit arithmetic, whereas the textbook form loses digits to cancellation on short segments. The analysis indicates that the classical equivalent air depth is shallower than its alveolar-consistent counterpart by 6 to 15 cm, that the textbook blending formula yields fills about 0.3 percentage points richer than intended for a 32% mixture at 200 bar, and that ideal-gas planning overstates the usable gas of an air cylinder filled to 232 bar by about 5%. All 79 independent checks pass. The library serves teaching and planning and does not predict individual physiological risk.