International Space Station (ISS) Major Constituent Analyzer (MCA) On-Orbit Performance 2008-01-1971

This paper summarizes the first seven plus years of on-orbit operation for the Major Constituent Analyzer (MCA). The MCA is an essential part of the International Space Station (ISS) Environmental Control and Life Support System (ECLSS). The MCA is a mass spectrometer instrument in the US Destiny Laboratory Module, which provides critical monitoring of six major atmospheric constituents (nitrogen (N₂), oxygen (O₂), hydrogen (H₂), carbon dioxide (CO₂), methane (CH₄), and water vapor (H₂O)). These gases are sampled continuously and automatically in all United States On Orbit Segment (USOS) modules via the ISS Sample Delivery System (SDS). Continuous readout of the partial pressures of these gases is critical to verifying safe operation of the Atmosphere Re-vitalization (AR) system, Atmosphere Control System (ACS), and crew safety for Airlock Extravehicular Activity (EVA) preparation. The system also supports dedicated rapid sampling of any one location, a feature used primarily to support Joint Airlock atmosphere sampling during crew preparations for EVAs and module atmosphere sampling during re-pressurizations of the ISS atmosphere with oxygen or nitrogen.

The performance of the MCA is discussed; in particular, the sample pump life, accuracy tolerances, and calibration gas consumption. We will also discuss a study conducted to identify design changes to the MCA that would extend its life and operability. The operational life of the mass spectrometer assembly of the MCA will be presented in the Reference 1 paper.