Light and Sound Reveal Gas Leaks

R. Winn Hardin

ANN ARBOR, Mich. -- Automakers are keeping an eye on a technique that uses laser light to detect gas leaks in engine and exhaust parts.
Hermetic containers traveling down an assembly line have to be tested for leaks prior to shipping, or manufacturers risk deflating a customer's expectations. Testing methods include a quick bath in a dunk tank or mass spectroscopy -- time-consuming processes. A new method that uses coherent light, trace gas and ultrasensitive microphones promises to find much smaller leaks in significantly less time.
The technique developed by David Dowling and Serdar Yonak at the University of Michigan uses a 12-W CO₂ laser from Synrad Inc. of Mukilteo, Wash. An interior grating tunes the output beam to 10.55 µm, the absorption band of sulfur hexafluorine gas. (Zinc selenide optics combine the 10.55-µm beam with a 5-mW visible beam from a HeNe laser to improve safety.)

Leaks in engine assemblies can cause serious problems for drivers. New testing methods that use coherent light could help improve quality assurance.

As the gas quickly absorbs the light, molecules vibrate. This deposition of thermal energy produces an acoustic wave that can be detected with microphones or even human ears. "It's clearly audible," assures Dowling. A leak that allows 10²² cm³/s of gas to escape will produce a sound that roughly measures 70 to 75 dB.

On the factory floor
To test the part, the gas is pumped inside, and a polygonal mirror raster scans the light across the part's surface 6250 times per second. A slower, secondary mirror completes a 2-D vertical scan every 6 s. If as little as 10²⁵ cm³/s of the sulfur hexafluorine escapes, two ultrasensitive microphones will "hear" the leak.
To put that size leak in perspective, at that rate it would take five days for enough gas to leak to fill a thimble, Dowling said. "Our goal is to get down to 10²⁶."
The limiting factor today is the scanning process itself. As the light impinges on the container, the metal creates its own acoustic sound. Future designs could eliminate this noise by scanning the beam just above the container's surface.
Other design considerations include increasing the number of microphones to better isolate and remove factory noises or sounds created by laser-induced vaporization of impurities such as oil and water on the part's surface.