Every crew member adds liquid water to the sealed cabin air as vapor — roughly 0.3–0.5 L/day just from breathing, more when exercising and sweating. That vapor raises the air's water-vapor partial pressure e. The air can only hold so much before it saturates: the maximum it can hold at a given temperature is the saturation vapor pressure es(T), given by the Magnus formula. Relative humidity is simply how close the air is to that ceiling.
e_s(T) = 6.1094 · exp(17.625·T / (T+243.04)) [hPa]
e = m_vapor·R_v·T_K / V [hPa]
RH = 100 · e / e_s(T)
T_dew = 243.04·ln(e/6.1094) / (17.625 − ln(e/6.1094))
The dew point is the temperature at which the current vapor content would saturate — i.e. where condensation starts. Spacecraft avionics racks and window panes run a few degrees cooler than cabin air because they conduct heat to the cold hull, so whenever the dew point catches up to that cold-surface temperature, moisture condenses out onto the equipment: exactly the failure mode a condensing heat exchanger exists to prevent, by chilling a coil below the target dew point and draining the water it collects into the cabin's condensate loop.
- Crew size & activity — more people, or exercising instead of resting, raises the vapor generation rate feeding the cabin air.
- Cabin temperature — warmer air can hold more vapor before saturating, so RH falls even with the same vapor mass; it also sets how much colder the surfaces run.
- Condenser capacity — the heat exchanger's maximum moisture-removal rate; its duty cycle ramps up as RH climbs past ~30% and saturates near ~70% RH, mimicking real thermostatic dehumidifier control.
If generation outpaces the condenser's ramped-up removal, vapor accumulates until the dew point reaches the cold-surface temperature — at that point excess vapor condenses directly onto equipment instead of the condenser coil, which is the electronics-damage and mold-risk scenario this sim tracks.