Accuracy Assessment of the Major Constituent Analyzer 2005-01-2893

The Major Constituent Analyzer (MCA) is a mass spectrometer-based atmospheric monitoring instrument in the Laboratory Module of the International Space Station (ISS). The MCA is used for continuous environmental monitoring of 6 major gas constituents in the ISS atmosphere as well as safety-critical monitoring for special Environmental Control and Life Support (ECLS) operations such as Pre-Breathe in the Airlock for Extra-Vehicular Activities (EVAs) and oxygen re-pressurizations. For the latter, it is desirable to make most efficient use of consumables by transferring the maximum amount from O₂ re-supply tanks on board the shuttle or Progress. The upper safety limit for O₂ transfer is constrained by the MCA measurement error bands. A study was undertaken to tighten these error bands and afford NASA-Mission Operations Directorate (MOD) more operational flexibility.

The MCA has a fully self-contained calibration capability and on-orbit experience indicated the stability and precision of MCA atmospheric measurements were well within specified limits, so a more rigorous study was undertaken to quantify sources of error. This study examines on-orbit and terrestrial data and performs statistical and error analyses for various operational modes and environmental conditions. The results are compiled into a database that projects MCA partial pressure constituent accuracies under various operational modes and conditions (such as time since last self-calibration).

This study concludes that a significant margin exists between demonstrated and specified MCA accuracy. Forward work includes an assessment of the process necessary to re-certify the MCA to a tighter error tolerance band and thus provide MASA-MOD with increased operational flexibility.