Another consideration with pressure decay testing is the susceptibility to distortion by changes in the temperature of the air inside the test object. Temperature rises as air is compressed and the test processes must wait until the temperature stabilizes. Some pressure decay systems now employ software algorithms and thermometers that can compensate for temperature distortion to a limited degree, but it it is difficult to fully eliminate this problem.
Leak Testing With Helium Tracer Gas
Newer methods of leak detection utilize tracer gas testing methods to detect whether any tracer gas has penetrated through a leak.
The first such method, helium mass spectrometry, was first developed for the Manhattan Project during World War II to locate extremely small leaks in the gas diffusion process. Over the years, helium leak testing has been used for a number of aerospace applications, including testing the integrity of the inter-compartmental sealing technology of the International Space Station.
The helium mass spectrometer is a complex piece of equipment that requires a vacuum. One approach is to hook up the mass spectrometer to the compartment being tested and evacuate the air within the compartment. Once an acceptable vacuum is achieved, helium is sprayed on the compartment’s exterior, particularly at any suspect locations such as seals, valves, and welds. An increased level of helium detected by the mass spectrometer indicates that a leak exists near the location where the helium was sprayed.
The alternative approach is to use a “sniffer” tube to draw a stream of air (and potentially dust and other contaminants) to the mass spectrometer or ion-pump-based helium detector. The compartment being tested is filled with helium. The compartment’s exterior is scanned with the sniffer. The operator identifies the existence and location of leaks when the mass spectrometer detects an elevated level of helium in the air flowing through the sniffer tube.
The Hydrogen Method
Another tracer gas method, which utilizes hydrogen, has been used in a variety of manufacturing systems for the past two decades and is now gaining acceptance in the aerospace industry. The hydrogen tracer gas (a non-flammable and widely available mixture of 95 percent nitrogen and 5 percent hydrogen) is detected by a highly sensitive semiconductor sensor located at the tip of the hand-held device. The portability and flexibility of the probe make it particularly suitable for situations where operators must take the test apparatus to many different locations on the an aircraft where access to compartments and systems can be restricted.
Hydrogen gas also has certain properties that make it particularly effective for aerospace leak detection. It is the lightest element in the universe (15 times lighter than air), and its molecular velocity is much higher than that of air and helium. This means that hydrogen spreads easily throughout the test object (such as the odd geometries of fighter-jet fuel tanks tucked away in any available space), penetrates leaks more readily, and vents away significantly faster than any other tracer gas. Helium, in contrast, tends to stick to surfaces, thereby increasing background buildup in the areas surrounding test objects, causing false negatives or masking small leaks.
The hydrogen method offers the ability to pinpoint the exact location of leaks, with sensitivity comparable to the helium method. There is no need to create a vacuum or to employ a sniffer to remove gas samples and transport them over a distance of several yards for analysis by the helium detector.
While no single approach will satisfy the full range of leak integrity testing challenges, the advent of the hydrogen method provides a precise, cost-effective alternative to conventional leak testing methods. Differences such as size, shape, and location of the object being tested most often dictate the chosen leak testing methodology.
Maintaining a consistent level of quality through leak testing is a large part of the aircraft manufacturing and maintenance process. In order to ensure leak integrity, aerospace construction and maintenance businesses are demanding quality assurance methods which are customizable to their particular product. '