Cabin CO2 follows a first-order mass balance between crew metabolic production and scrubber removal (identical model to the 3D version of this simulator, rendered here as three linked 2D panels instead of a rotatable 3D gauge):
dC/dt = P − k·C
P = N · p_CO2 · 1e6 / V [ppm/min produced]
k = Q_max · η / V [removal rate constant, per min]
C_ss = P / k [steady state, ppm]
ppCO2 (mmHg) = C(ppm) · 1e-6 · 760 mmHg
Where V is the cabin free volume (400,000 L, ISS-scale), N is crew size, p_CO2 ≈ 0.42 L/min is the per-astronaut resting metabolic CO2 output, Q_max is the scrubber's maximum air-processing flow (300 L/min), and η is the duty-cycle/efficiency slider. The simulator integrates this ODE every frame (clamped Δt) so the cabin fill level, particle density and strip-chart trace all trace the true exponential approach to C_ss.
Space agencies do not agree on a single safe ceiling: the Shuttle-era rule allowed up to 7.6 mmHg (~10,000 ppm) over 24h; NASA's current 7-day Spacecraft Maximum Allowable Concentration (SMAC) is 5.3 mmHg (~7,000 ppm); and post-ISS cognitive-performance research (elevated CO2 measurably impairs decision-making well below the SMAC) has driven proposals to cap long missions nearer 2.5 mmHg (~3,300 ppm). Switching the standard buttons re-evaluates the live status, the comparison bars, and the analytic time-to-breach against whichever threshold is selected — the same cabin can be "compliant" under one agency's rule and in violation of another's.
- Crew size / scrubber efficiency — set the production and removal terms of the ODE directly.
- Mission-clock speed — advances simulated days per real second so multi-week trends are visible quickly.
- Standard buttons — pick which threshold the status readout, comparison bars and time-to-breach are measured against; all three remain visible as reference lines on the cabin gauge and strip chart.
- Cabin panel drag/scroll — rotate the cutaway around its vertical axis and zoom in, purely a viewing aid over the same 2D model.