The pressure at the inlet of a mass spectrometer is regulated.
The figure schematically depicts a portable helium leak detector that includes an improved sample-delivery system. This instrumentation is designed for detecting leaks in the fuel and oxidizer systems of the space shuttle main engines. In a helium leak test, helium is used as a tracer gas in place of the hazardous fuel or oxidizer: The fuel or oxidizer plumbing of interest is pressurized with helium. The aft section of the space shuttle (which section contains the plumbing) is purged with air, and the portable helium leak detector is used to monitor for any increase in helium content of the purge exhaust at concentrations down to the part-per-million level. The portable helium leak detector can also be used to perform tests to detect very small leaks in systems other than the space shuttle main engines.
The portable helium leak detector includes three instrumentation modules: a commercial helium leak detector, a control system, and a sample-delivery system. These modules, plus gas supplies, are mounted on one cart that can be wheeled to the test location.
Like other commercial helium leak detectors, the present one is essentially a mass spectrometer designed specifically for detecting helium. The inlet of the mass spectrometer is attached to the outlet of the sample-delivery system. The mass spectrometer is connected to the control system via an RS-232 serial data port.
The control system includes a laptop computer connected via serial data ports to solid-state relays and analog-to-digital converters that, in turn, are connected to the mass spectrometer and the sample-delivery system. The computer runs LabVIEW software that controls the general sequencing of events, the switching of valves, and the acquisition of data. The computer sends the valve commands to solid-state relays that turn power on and off, as commanded, to control the valves. The output signals from a temperature sensor and from the mass spectrometer are sent to the computer via the analog-to-digital converters.
The software monitors and displays long-term (a test can take several hours) leak trend data and saves all pertinent information on the hard drive of the computer. The software provides options for performing self-calibration and self-leak tests. Control menus on the computer display are grouped as pages or screens that include a setup page, a self-leak-check page, an autocalibration page, and pages dedicated to specific test setups. New pages can readily be added as needed.
The sample-delivery system includes pumps, valves, pressure transducers, and mass-flow controllers, all working together to bring a sample from the atmosphere of interest to the inlet of the mass spectrometer. The sample-delivery system includes two inlets: one for high flow and one for low flow. The high-flow inlet is for use at high volumetric purge flow rates and enables the rapid detection of leaks. The low-flow inlet is for use in tests that involve little or no purge flow.
The most novel aspect of the design of the sample-delivery system is a provision for regulating the pressure at the inlet of the mass spectrometer. The mass spectrometer effectively measures the partial pressure of helium and therefore it is desirable to regulate the total inlet pressure in order to ensure that the measured partial pressure bears a known proportionality to the concentration of helium in the sampled air. The regulation of the inlet pressure is effected by a feedback loop between a pressure transducer and a flow controller. Another unique aspect of the instrument is its ability to be calibrated and provide data as parts-per-million leakage rates. This is done to match the space-shuttle main-engine permissible leakage-rate criteria.
The entire portable leak detector (including the cart) is 23 in. (58 cm) wide, 45 in. (114 cm) high, and 30 in. (76 cm) deep, and has a weight of 346 lb (mass of 157 kg). The three modules can easily be removed to facilitate transport. All three modules are powered via a single 115 Vac outlet.
This work was done by Fredrick Adams, Carolyn Mizell, David Collins, and Gregg Breznik of Kennedy Space Center; Richard Hritz, Francisco Lorenzo-Luaces, Guy Naylor, Curt Lampkin, Timothy Griffin, and Terry Greenfield of Dynacs Engineering Co., Inc.; and Larry Lingvay formerly of I-NET, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.