Improving the Measurement Accuracy of Water Partial Pressure Using the Major Constituent Analyzer

The Major Constituent Analyzer (MCA) on board the International Space Station (ISS) is a mass spectrometer-based system designed to monitor the major constituents of the ISS internal atmosphere. Of these, nitrogen, oxygen, methane, carbon dioxide, and hydrogen are being monitored as originally designed. Obtaining accurate water vapor measurements, however, has been problematic. The primary issue for water vapor measurement is the adsorption of water onto the interior surfaces of the ISS sampling lines, MCA internal sample transport lines, and the MCA analyzer itself. Whenever the partial pressure of water in the sample stream changes, a new equilibrium between the adsorbed and gas phases must be reached before a measurement will reflect the actual partial pressure. Thus, while changes in the partial pressures of the other major constituents are detected in real-time, accurate measurement of the water partial pressure is delayed. Unfortunately, the time required to reach equilibrium is longer than current MCA operating parameters allow, so the measured values are not accurate.