Skip to content

Algo 05 Multi Gas Switching

Jump to bottom
matejhron edited this page May 10, 2026 · 3 revisions

Algo-05 — Multi-Gas Switching

Real technical dives carry one bottom gas plus one or more deco gases — EAN50 for shallow stops, pure O₂ for the last 6 m. DecoJS handles this across three layers: MOD calculation, gas-priority selection inside the deco loop, and waypoint-level switch insertion for chart rendering.

MOD calculation

// js/diveSetup.js:807-811
export function calculateMOD(o2Fraction, maxPpO2 = 1.4) {
    if (o2Fraction <= 0) return Infinity;
    const maxAmbient = maxPpO2 / o2Fraction;
    return Math.floor((maxAmbient - 1) * 10);
}

$$MOD = \left\lfloor \left(\frac{ppO_2^{max}}{f_{O_2}} - 1\right) \cdot 10 \right\rfloor \text{ m}$$

Default maxPpO2 = 1.4 for bottom gas; 1.6 for deco (relaxed because the diver is resting at a stop, not working). Math.floor rounds toward shallower — conservative, always-below-MOD.

The deco loop then snaps MOD to the 3 m stop grid:

// js/decoModel.js:944
const switchDepth = Math.max(0, Math.floor(mod / stopIncrement) * stopIncrement);

Examples at $ppO_2 = 1.6$:

Gas $f_{O_2}$ MOD (m) switchDepth (3 m grid)
EAN50 0.50 22 21
EAN80 0.80 10 9
Pure O₂ 1.00 6 6

Gas priority in the deco loop

When multiple deco gases are eligible at a depth (i.e. within MOD and not yet switched), DecoJS picks the one with the deepest MOD — the richest gas that's still safe. This enforces sequential switching: EAN50 at 21 m before O₂ at 6 m, never the other way around.

// js/decoModel.js:968-994
const switchToBestGas = (atDepth, recordSwitch = true) => {
    const eligible = gasSwitchPoints.filter(gas =>
        atDepth <= gas.switchDepth &&
        gas.n2 < currentN2 &&
        !usedGases.has(gasKey(gas))
    );
    if (eligible.length === 0) return false;

    // Pick the gas with the deepest MOD (highest switchDepth) - ensures sequential switching
    const best = eligible.reduce((a, b) => (b.switchDepth > a.switchDepth ? b : a));
    const key = gasKey(best);
    currentN2 = best.n2;
    currentGasName = best.name;
    usedGases.add(key);
    if (recordSwitch) {
        gasSwitches.push({ depth: atDepth, gas: best.name, gasId: key });
    }
    return true;
};

usedGases is a Set preventing re-selection of a gas already switched to. gasKey(g) = g.id ?? g.name (js/decoModel.js:918) handles gases without an explicit id field. The gas.n2 < currentN2 guard ensures we never "switch" to a gas with equal or worse inert-gas fraction than the current one.

Where switches are handled — three layers

1. Anchor search (findFirstStopAtGFLow)

The grid-iterating ascent simulation accepts gasSwitchPoints and applies them as it crosses each switch depth (via _simulateAscentWithGasSwitches). Without this, the dive ceiling at GF_low would be computed as if the diver stayed on bottom gas to the surface, yielding an incorrectly deep first stop. See Algo-03-First-Stop-Ramped-GF.

2. Deco scheduler (generateDecoSchedule)

Gas switches can occur either:

  • During ascent to first stop, at MOD depths that lie between currentDepth and firstStopDepth (js/decoModel.js:1044-1064). Important when a rich deco gas is usable before the first mandatory stop.
  • On arrival at a stop depth, via switchToBestGas(depth) inside the stop loop (js/decoModel.js:1086).
  • During ascent in a no-deco scenario, if the dive is within NDL but deco gases are still carried for ascent richness (js/decoModel.js:1007-1037).

3. Waypoint insertion (insertGasSwitchWaypoints)

// js/diveSetup.js:1009 (signature)
export function insertGasSwitchWaypoints(waypoints, gases, ascentRate = 10, maxPpO2 = 1.6, gasSwitchTime = DEFAULT_GAS_SWITCH_TIME)

Post-processes a complete waypoint array to add explicit switch waypoints at the correct depth/time. Pre-scans for existing stops to avoid inserting redundant waypoints where a deco stop is already scheduled at the switch depth:

// js/diveSetup.js:1040-1047
const existingStopDepths = new Set();
for (let i = 0; i < waypoints.length - 1; i++) {
    if (waypoints[i].depth === waypoints[i + 1].depth && waypoints[i].depth > 0) {
        existingStopDepths.add(waypoints[i].depth);
    }
}

When inserting, the function:

  • Rounds switch depth to 3 m grid with Math.floor(decoGas.mod / 3) * 3 (js/diveSetup.js:1071).
  • If there is no existing stop at that depth, inserts an arrival waypoint and a departure waypoint separated by gasSwitchTime minutes.
  • If there is an existing stop, merges: updates the stop's gasId rather than inserting a parallel row.
  • Marks every ascending waypoint with the currentGasId so downstream consumers (chart, tissue simulation) pick up the correct $f_{N_2}$.

Gas-switch time

// js/diveSetup.js (constant)
export const DEFAULT_GAS_SWITCH_TIME = 0;

Configurable 0–5 min. Zero matches decotengu (an instantaneous switch) — the diver is conceptually assumed to verify the regulator + clear O₂ windows within the natural stop duration. Conventions that add a minute or two at the switch as a safety margin are accommodated by setting gasSwitchTime > 0 — the time is added to the stop at the switch depth, and tissue loading is simulated for that time on the new gas (js/decoModel.js:1086-1089).

Gas fraction across a switch

Tissue pressure is continuous across a switch. What changes is the alveolar target $P_{alv}$ = $(P_{amb} - P_{H_2O}) \cdot f_{N_2,\text{new}}$, which drives the Schreiner / Haldane equations for subsequent segments. Because the new gas typically has a lower $f_{N_2}$, off-gassing accelerates immediately after the switch — the whole point of carrying deco gases.

Worked example

40 m on air + EAN50 + O₂, 25 min bottom, $GF = 30/85$:

  1. Descent to 40 m on air; bottom time 25 min on air.
  2. Ascent 40 → 21 m on air. At 21 m, switch to EAN50 (MOD 21 m on 3 m grid). usedGases = { ean50 }.
  3. Ascent 21 → 15 m (first stop) on EAN50. Stop at 15 m, 12 m, 9 m on EAN50.
  4. At 6 m, switch to O₂ (MOD 6 m on 3 m grid). usedGases = { ean50, o2 }.
  5. Stop at 6 m and 3 m on O₂.
  6. Ascent 3 → 0 m on O₂.

Each switch is recorded in gasSwitches: [{depth: 21, gas: 'EAN50', gasId: 2}, {depth: 6, gas: 'O2', gasId: 3}] for chart annotation.

Regression — gas switch at time 0

tests/gasSwitchTime-zero-regression.test.mjs guards a subtle case: a 0-minute gas switch waypoint at time = 0 used to drift tissue state by one 10-second step. The fix ensures consecutive same-time waypoints are consumed without advancing the integration clock.

Cross-references

Clone this wiki locally