This companion drives the exact same physiology model as the 3D simulator — nothing about the ODE was changed, only the view. Cabin CO₂ still obeys a first-order mass balance: crew respiration adds gas at a fixed rate, and whatever scrubbing capacity is online removes it at a rate proportional to how much CO₂ is present:
dP/dt = k_gen · n_crew − k_scrub · A(t) · P
P cabin CO₂ partial pressure (mmHg)
n_crew crew count aboard
A(t) fraction of scrub capacity online (1.0 = both units, standby fraction while primary is offline)
Steady state: P_ss = k_gen · n_crew / (k_scrub · A)
What's new here is the view, not the physics: the 3D sim only ever shows the present instant, orbited freely in a 3D cabin. This 2D companion instead renders a pannable, zoomable schematic cross-section of the same cabin (drag to pan, scroll/pinch to zoom — the same CO₂ particle-density idea, just in 2D) alongside a fixed strip chart that plots the entire pCO₂ history of the run against the 7.6 mmHg caution line and the 15 mmHg emergency line. That trend view makes the shape of the approach to steady state — and how close a given change window cuts it — visible at a glance in a way a single 3D snapshot cannot.
- Crew slider — more crew means faster CO₂ generation, so the same standby capacity depletes margin sooner.
- Standby capacity slider — the fraction of nominal scrubbing the backup unit alone can sustain; a weaker backup means a steeper CO₂ rise.
- Change window slider — how long Unit A must stay offline to complete the change; a longer window gives CO₂ more time to climb before Unit A returns.
- Start Change Request — takes Unit A offline, starts the clock, and runs the ODE above until the window closes (success) or the emergency limit is hit (abort).
- Strip chart — resets on every Reset/Start and keeps tracing pCO₂ vs. elapsed time for the current run, so you can see exactly how the curve approached (or crossed) each limit.
The 7.6 mmHg / 15 mmHg thresholds are representative of the spacecraft cabin CO₂ limits used for ISS-class habitats — a real change board would size the standby capacity and window length so the margin never gets this close in the first place.