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A Mass-Spectrometer System for Detecting Gas Leaks

This versatile, expandable system can be controlled from a safe remote location.

The Hydrogen Umbilical Mass Spectrometer (HUMS) consists of an integrated sample delivery system, a commercial mass-spectrometer- based gas analyzer, and a set of calibration gas mixtures traceable to NIST (National Institute for Standards and Technology). The system, except for the calibration gas mixtures and the remote operator display, fits into a standard 24-in. wide, 6-ft high, 36-in. deep (0.61 by 1.83 by 0.91 m, respectively) equipment rack and is powered by 120-Vac, 30-A, 60-Hz source. It was designed to perform leak detection and measurement of cryogenic propellants (oxygen and hydrogen) from a remote location during shuttle-launch countdown. It is used specifically to sample the background gas surrounding the 17-in. (0.43-m) Orbiter-ET disconnect, looking for leakage of gaseous hydrogen. The capability to monitor shuttle purge gases and cryogenic hydrogen fill and drain line T-0 disconnect helium purge gas is incorporated into the shuttle installation on each Mobile Launch Platform (MLP).

HUMS was designed to switch rapidly between background gases (helium, nitrogen, or air) during normal operation. The operator has the ability to remotely select one of eight sample lines and between any of six calibration gas mixtures. It can measure from 0 to 100 percent hydrogen, helium, or nitrogen; 0 to 25 percent oxygen; and 0 to 1 percent argon, in any combination in either a helium, nitrogen, or air background. It has an internal cycle mode, added after installation, to cycle between various pre-set sample and calibration gas lines on a continuous basis, if desired.

Operational features include the ability to update the reading for background of each gas in the mixture, thereby avoiding performance of a complete recalibration during operation. This saves a considerable amount of time. The zero gas for the background of interest must be monitored for a couple of minutes to allow the system to stabilize and a reading taken. Only the zero coefficient in the calibration equation for the background of interest is updated. Switching between backgrounds requires only changing to the new background and updating the zero reading for each species. This flexibility is critical when testing in an environment where samples are taken from helium, nitrogen, and air during the test. After testing is complete, a sequence of readings in each background, consisting of zero, test, and span gas mixtures, in that order, provides post-test verification that the system performance remained unchanged since the initial calibration was performed, pre-test.

Selection of calibration gas mixture concentrations was critical to remote verification of performance. Three concentrations of each gas are included, each in backgrounds of helium and nitrogen. This calibration/verification technique was developed after installation of the Hazardous Gas Detection System (HGDS) in 1979 and is based on experience gained during operation of that system. The performance of the COTS multigas, multicollector magnetic sector analyzer is slightly dependent on background gas, whether helium or nitrogen dominates the mixture. Independent calibration curves are used, depending whether the unknown sample is drawn from either a helium or nitrogen/air background.

During the calibration process, linear calibration curves are generated for each gas in the mixture (hydrogen, helium, nitrogen, oxygen, argon), based on pure background (helium or nitrogen) and a span gas (1 to 10 percent of each gas, in each background). An independent test gas, containing mixtures approximating red-line levels (where action is to be taken, based on readings of the sample) is used to both verify a good calibration (test gas reading lies on the line generated by zero and span, for each species) and to compare directly with the unknown sample, if necessary, to remove uncertainty in reading the unknown mixture. The choice of calibration gases allows differentiation of leak sources of cryogenic oxygen from oxygen contained in air. The operator has the ability to measure the ratio of oxygen to argon (~20), indicative of air intruding into the purge gas. Cryogenic oxygen contains no argon.

The design of HUMS achieved two goals. The first was to provide a permanently installed replacement for the Interim-HUMS (I-HUMS), a system designed and built over a weekend (in part to support nearly simultaneous STS-35, STS-38 launch attempts) to be portable between shuttle MLPs. I-HUMS replaced the one-time installation of the Turbo Mass Spectrometer (TMS) developed for launch of STS-26R (first shuttle launch after 51-L). TMS was designed to verify the performance of the 17-in. (0.43-m) Hydrogen Orbiter-ET Disconnect during hydrogen fill and drain operations, prior to shuttle launch. TMS demonstrated the ability of a high-vacuum turbo-molecular pump to operate and survive in a high vibration environment.