Assumptions and limits

What it is

The pages in this section explain each modelling choice in context. This page collects them in one place (everything you trust when you read a Dive Kit schedule), followed by what the engine deliberately does not claim, and the sources it was built from.

The assumptions, in one place

The model

• Dissolved-gas Bühlmann ZH-L16C with Erik Baker gradient factors (GF, interpolated linearly with depth); no bubble model and no separate deep-stop algorithm beyond what your GF-Low implies.
• Tissue loading integrated in 1-second steps through every depth change, not only at fixed stops.
• Nitrogen and helium tracked separately per compartment, with continuously re-blended coefficients as the mix changes.

Stops and ascent

• Stops sit on a configurable grid (3 m default); a ceiling always rounds up, never shallower than your true ceiling.
• Stop times round up to whole minutes by default, with the rounding done inside the engine (one-way, hold-longer-only, so never an early ascent); precise second-resolution (MM'SS") and round-to-30-seconds are opt-in. The last stop carries a one-minute minimum in every mode.
• The last stop is a real stop, even when a coarse grid would otherwise force you deeper.
• Ascent uses a two-rate model (deep + shallow, switching at a configurable depth); descent rate is configurable.
• A level’s entered time is its at-depth time; the descent into it is accounted separately.

Gas switching (open circuit)

• Each gas’s switch depth and hyperoxia warning use one O₂%-band PPO₂ cap (lean / mid / rich, default 1.4 / 1.5 / 1.6), and the same number drives both.
• An optional gas-switch (purge) time is spent at depth on the old gas; off by default.
• On descent and at a fixed level the auto picker takes the richest legal gas; on the way up it switches in the smallest O₂ step that is legal at each stop (a smallest-step walk-up). Both paths honour the same per-gas PPO₂ cap and never breathe a gas past it.

Oxygen toxicity

• CNS (central nervous system oxygen toxicity) and OTU (oxygen tolerance units, the pulmonary “whole-body” measure) accumulate from ambient PPO₂ (no water-vapour subtraction), against NOAA limits; CNS decays on a half-life, including across surface intervals.

Environment

• A water-type metres-per-bar factor feeds both gas loading and the M-value/ceiling maths; surface pressure is configurable for altitude.

Closed-circuit rebreather (CCR)

• Three setpoints (descent, bottom, deco), with a switch depth applied only on the first descent, an optional depth-keyed deco-setpoint and diluent schedule, and any setpoint capped at the achievable ambient PPO₂.
• “Stay on the loop” by default; open-circuit bailout planned from the worst realistic failure point at an elevated stress breathing rate.

Other

• Repetitive dives carry tissue tension, CNS and OTU across the surface interval; an oxygen break re-loads the fast tissues, and that loading carries through the rest of the plan.
• Gas density uses a fixed temperature assumption: a reference reading, not a safety boundary.
• Gas volume used depends entirely on your breathing-rate setting (your surface respiratory minute volume, SRMV), a personal preference.

What we deliberately don’t claim

• No model is a guarantee. Bühlmann is a dissolved-gas model fitted to data, and decompression sickness is probabilistic. Your gradient factors are your conservatism dial and your call; a planner’s job is to honour them faithfully, which is what this one does.
• This is a planning tool, used on the surface before and between dives. It is not a substitute for training, a dive computer, a sensible personal margin, or judgement.
• We don’t invent physics. Where the literature is settled we follow it and cite it; where it leaves a genuine choice we make a defensible one, document it, and validate the result against an independent reference.

References

The engine is an independent implementation written from the published papers and equations. The open-source projects below are independent cross-checks, not code it was derived from.

• A. A. Bühlmann, Decompression: Decompression Sickness: the ZH-L tissue model and its coefficients.
• R. D. Workman (US Navy EDU, 1965): the origin of the linear “M-value” ascent-limit line.
• Erik C. Baker, “Understanding M-values” & “Clearing Up the Confusion About Deep Stops”: the gradient-factor method, the decompression zone, and the GF slope.
• Erik C. Baker, “Oxygen Toxicity Calculations”: CNS and OTU from ambient PPO₂.
• Subsurface, the open-source decompression engine: independent confirmation that water vapour enters gas loading only, never the ceiling.
• The Theoretical Diver (R. Helling): gradient-factor and oxygen-toxicity derivations.
• NOAA Diving Manual & Shearwater’s CNS oxygen clock: single-exposure oxygen limits.

CCR diluent switching (dilout)

• MultiDeco user guide (HHS Software): diluent changes inside a CCR plan (“change to air diluent, and 1.4 at 20m”): the capability Dive Kit’s diluent switches model.
• Shearwater Perdix Operations Manual, “Select Gas” / “CC Gases”: five closed-circuit diluent slots, switchable mid-dive.
• Stéphane Havard, “Understanding and safe use of PPO2 set-points for CCR” (2002): a practitioner’s deco-diluent switch (8/62 to air or EAN32 at 40 m when it saves 15 to 20 minutes), diluent-flush mechanics, and an honest framing of the debate.
• Silent Diving, “Which CCR Diluent Fits Your Dive Profile?”: diluent surface-breathability and diluent switching as “a more advanced configuration and not necessary for most dive profiles”.

For the full, grouped list (decompression theory, oxygen toxicity, gas density and CO₂, isobaric counter-diffusion, CCR diluent switching, and buoyancy), each with a short note on what it is and why it matters, see Further reading.

Related

• The decisions that shape your schedule explains each of these choices in context.
• How Dive Kit compares to MultiDeco lays out the checks and the documented differences.
• How the deco engine works is the foundation underneath the assumptions.
• Gradient factors is the conservatism dial referenced in “The model” and in what the engine does not claim.
• Hazards beyond decompression expands the gas-density assumption noted under “Other”.