Spacecraft cabin atmosphere isn't governed by one universal rulebook — NASA-STD-3001, ESA's ECSS series and the ISS Multilateral operating limits each draw the line for CO2, O2 and total pressure slightly differently, and a vehicle that serves more than one agency has to satisfy all of them at once. This simulator integrates a real first-order mass-balance model of a cabin's CO2, O2 and total pressure driven by crew size, scrubber efficiency, O2 generator output and leak rate, then renders each standard's allowable region as a translucent 3D box and computes their live geometric intersection — the harmonized envelope a multi-agency vehicle must never leave. Toggle standards on and off, push the hardware toward failure, and watch the glowing cabin-state marker either ride safely inside the shrinking intersection or break out of it.