Gas Standards in Support of NASA’s Space Shuttle Monitoring Program

The National Aeronautics and Space Administration (NASA) Kennedy Space Center requires accurate gas mixtures containing argon, helium, hydrogen, and oxygen in a balance of nitrogen to calibrate mass spectrometer-based sensors used around the space shuttle launch area and inside the shuttle cabin. NASA was in need of these gas mixtures to assure the safety of the shuttle launch system and shuttle cabin. In 2006, NIST certified three new sets of gas standards. These standards were compared to new primary standard mixtures (PSMs) and historic standards developed for NASA in 1993 that NIST had retained. Results between the suites of primary standards and the historic mixtures agreed, verifying the stability of the mixtures. NIST certified each component in these three new mixture suites to uncertainties better than 2 % relative.

A suite of accurate standards was needed as part of the safety monitoring system of the shuttle launch area as well as inside the shuttle cabin. These standards are used to calibrate the instruments used to “sniff” connections within the cabin air environment and on the launch platform for leaks of hydrogen and oxygen. Nitrogen-filled areas are also monitored for argon, which signals an air leak. Any leak could result in catastrophic failures in the rocket boosters or in the shuttle itself. This testing allows NASA to determine whether there are any potentially catastrophic leaks or other problems with the space shuttle before the launch.

A new suite of 22 primary standards bracketing the three concentration levels required were prepared gravimetrically. The standards used for the Level 1 and 2 NASA samples were compared to previously made standards (1993) to verify both stability and accuracy and to ensure the continuity of this program. No previously prepared standards existed for the Level 3 concentration standards (lowest concentration) and verification of stability will need to be confirmed for this mixture in the future. 

NIST uses gas chromatography with thermal conductivity detectors (TCDs) to measure He, H₂, O₂, and Ar. TCDs provide precise measurements resulting in much low uncertainties in the final certified values. Measurement of the combination of He, H₂, Ar, and O₂ in a balance of N₂ is a complex problem. The thermal conductivity is measured as the difference between the component of interest and the GC carrier gas. Because the thermal conductivity of the components of interest are different, two GC methods employing different carrier gases, He for Ar and O₂, and N₂ for He and H₂, had to be developed for these analyses. 

Major Accomplishments:

• NIST certified hydrogen, helium, argon, and oxygen, with uncertainties that were well below the required maximum for NASA (± 1 % relative for the two higher levels; ± 3 % relative for the lowest level).
• NIST now has new PSM suites and analytical methods to continue to support the NASA Space Shuttle program.