The figure schematically depicts a portable microleak-detection system that has been built especially for use in testing hydrogen tanks made of polymer-matrix composite materials. (As used here, "microleak" signifies a leak that is too small to be detectable by the simple soap-bubble technique.) The system can also be used to test for microleaks in tanks that are made of other materials and that contain gases other than hydrogen. Results of calibration tests have shown that measurement errors are less than 10 percent for leak rates ranging from 0.3 to 200 cm3/min.
Like some other microleak-detection systems, this system includes a vacuum pump and associated plumbing for sampling the leaking gas, and a mass spectrometer for analyzing the molecular constituents of the gas. The system includes a flexible vacuum chamber that can be attached to the outer surface of a tank or other object of interest that is to be tested for leakage (hereafter denoted, simply, the test object). The gas used in a test can be the gas or vapor (e.g., hydrogen in the original application) to be contained by the test object. Alternatively, following common practice in leak testing, helium can be used as a test gas. In either case, the mass spectrometer can be used to verify that the gas measured by the system is the test gas rather than a different gas and, hence, that the leak is indeed from the test object.
The flexibility of the chamber makes it adaptable to test objects having a variety of simple or complex shapes. The flexible vacuum chamber includes an aluminized polyethylene terephthalate vacuum membrane that is sealed to the outer surface of the test object by a flexible, adhesive seal material. A scrim is placed between the inner surface of the membrane and the outer surface of the test object to maintain a gap to accommodate the flow of any leaking gas. A capillary tube that passes through the seal connects the gap volume with the plumbing that leads to the mass spectrometer, the vacuum pump, and a control volume described next.
The control volume has a known size and is instrumented with pressure and temperature sensors. In use, the control volume is evacuated, then disconnected from the vacuum pump, and then the pressure and temperature are measured as the leaking gas flows into the control volume. By use of the ideal-gas law, the rate of leakage can be calculated from the temperature and measured rate of increase of pressure.
An unusual feature of this system is a heating/cooling subsystem that includes a tube embedded in the flexible adhesive seal. A heating or cooling liquid can be circulated through this tube to maintain the seal at or near room temperature, where it is most effective, regardless of the temperature of the test object or the environment. The heating/cooling subsystem is essential, for example, for maintaining an effective seal for testing a tank, pipe, valve, or other object that contains liquid hydrogen or other cryogenic fluid. The heated/cooled seal enables testing at temperatures from –455 to +350 °F (about –271 to +177 °C), even in the presence of distortions caused by mechanical and thermal loads applied to the test object.
This work was done by H. Kevin Rivers and Joseph G Sikora of Langley Research Center and Sankara N. Sankaran of Lockheed Martin Space Operations.