SAM is able to function in 4 unique modes of operation:
Mode 0: Unpressurized, sealed
Mode 1: Unpressurized, flow-through
Mode 2: Pressurized, flow-through
Mode 3: Pressurized, sealed
In Mode 0 the vessel is not pressurized by the lung and pump and the blower is disabled. This is used when the lowest possible leak rate is desired and monitoring the interior, non-circulated, non-filtered air is desired, as with human CO2 generation or plant CO2 sequestration studies. Mode 0 offers the lowest leak rate as there is a minimal pressure differential between the interior of the SAM vessel and the exterior, ambient atmosphere, resulting in the least quantity of molecules transferred. The temperature difference from inside to outside will invoke a minor pressure differential, but remains minimal when compared to Mode 2 and 3.
In Mode 1 the vessel is not pressurized by the lung and pump, rather, the blower moves a continuous air stream through SAM to maintain a constant flow of fresh air. This is used for teams that do not desire to maintain the SAM structure in a pressurized state, nor manage the air quality monitor and levels. Valves at all three pressure doors will be open, assuring fresh air is circulating for the duration of the mission.
In Mode 2 the vessel is pressurized and the blower moves a continuous air stream through SAM to maintain a constant flow of fresh air. The blower is powered by a VFD (variable frequency drive) which enables fine tuning of the volume of air, such that the teams can maintain a lifted lung and thereby pressurized state while compensating for the intentional loss of air at designated, open ports (e.g. sleeping quarters). This is used for teams that desire a pressurized state, but do not wish to manage the CO2 scrubber, and was the defacto mode for all missions from 2023 through 2025.
In Mode 3 the vessel is pressurized and hermetically sealed, meaning no significant quantity of air escapes SAM. But as all pressure vessels have some leak rate, Apollo, Shuttle, ISS, and Artemis included, the blower will be activated to introduce additional, filtered air at a monitored rate and calculated volume. At SAM this function represents ISRU (in situ resource utilization), meaning the production of breathable air from lunar or martian water, or delivery of air via a docking space craft, as with the ISS over the past twenty five years. The lung will rise and fall each day with changes in temperature and exterior barometric pressure, while maintaining a continuous +0.05 PSI differential.
With the introduction of the 4-Bed CO2 scrubber in 2026, cabin air is continuously circulated from the Crew Quarters clockwise through the Engineering Bay, Test Module, and Lung and through the scrubber in EARL, where the carbon dioxide is captured for reuse or “vented to space”. While EARL is not a sealed room, the scrubber inside of EARL is a continuation of the hermetically sealed SAM.