Gas Standards Development in Support of NASA’s Sensor Calibration Program Around the Space Shuttle

Abstract

The National Aeronautics and Space Administration (NASA) Kennedy Space Center (KSC) requires accurate gas mixtures containing argon (Ar), helium (He), hydrogen (H(2)), and oxygen (O(2)) in a balance of nitrogen (N(2)) to calibrate mass spectrometer-based sensors used around their manned and unmanned space vehicles. This also includes space shuttle monitoring around the launch area and inside the shuttle cabin. NASA was in need of these gas mixtures to ensure the safety of the shuttle cabin and the launch system. In 1993, the National Institute of Standards and Technology (NIST) was contracted by NASA to develop a suite of primary standard mixtures (PSMs) containing helium, hydrogen, argon, and oxygen in a balance gas of nitrogen. NIST proceeded to develop a suite of 20 new gravimetric primary PSMs. At the same time NIST contracted Scott Specialty Gases (Plumsteadville, PA) to prepare 18 cylinder gas mixtures which were then sent to NIST. NIST used their newly prepared PSMs to assign concentration values ranging from 100 to 10,000 micromol/mol with relative expanded uncertainties (95% confidence interval) of 0.8-10% to the 18 Scott Specialty Gases prepared mixtures. A total of 12 of the mixtures were sent to NASA as NIST traceable standards for calibration of their mass spectrometers. The remaining 6 AIRGAS mixtures were retained at NIST. In 2006, these original 12 gas standards at NASA had become low in pressure and additionally NASA needed a lower concentration level; therefore, NIST was contracted to certify three new sets of gas standards. NIST prepared a new suite of 22 PSMs with weighing uncertainties of <0.1%. These 22 PSMs were compared to some of the original 20 PSMs developed in 1993 and with the NIST valued assigned Scott Specialty Gas mixtures that NIST had retained. Results between the two suites of primary standards and the 1993 NASA mixtures agreed, verifying their stability. At the same time, NASA contracted AIRGAS (Chicago, Illinois) to prepare 45 cylinder gas mixtures which were then sent to NIST. Each of the 3 sets of standards contained 15 cylinder gas mixtures: set no. 1, He at 12,000 micromol/mol, H(2) at 600 micromol/mol, Ar at 100 micromol/mol, and O(2) at 600 micromol/mol; set no. 2, He at 15 000 micromol/mol, H(2) at 5000 micromol/mol, Ar at 1000 micromol/mol, O(2) at 5000 micromol/mol; and set no. 3, He at 50 micromol/mol, H(2), Ar, and O(2) each at 25 micromol/mol with a balance gas of N(2). NIST used their newly prepared primary standards to assign concentration values to each component in these three new mixture sets to relative expanded uncertainties of 0.5-2.2%. The NIST certified AIRGAS prepared mixtures were then sent to NASA to use as "working standards" to calibrate their mass spectrometers (MSs).