Editing Helium (section)

==== Industrial leak detection ====
[[File:Ac-system 2.jpg|thumb|left|A dual chamber helium leak detection machine|alt=Photo of a large, metal-framed device (about 3×1×1.5 m) standing in a room.]]

One industrial application for helium is [[leak detection]]. Because helium [[diffusion|diffuses]] through solids three times faster than air, it is used as a tracer gas to detect [[leak]]s in high-vacuum equipment (such as cryogenic tanks) and high-pressure containers.<ref name="nostrand">{{cite encyclopedia| title = Helium|editor = Considine, Glenn D.| encyclopedia = Van Nostrand's Encyclopedia of Chemistry| pages = 764–765|publisher = Wiley-Interscience|date = 2005|isbn = 978-0-471-61525-5}}</ref> The tested object is placed in a chamber, which is then evacuated and filled with helium. The helium that escapes through the leaks is detected by a sensitive device ([[helium mass spectrometer]]), even at the leak rates as small as 10<sup>−9</sup> mbar·L/s (10<sup>−10</sup> Pa·m<sup>3</sup>/s). The measurement procedure is normally automatic and is called helium integral test. A simpler procedure is to fill the tested object with helium and to manually search for leaks with a hand-held device.<ref>{{Cite book|url=https://books.google.com/books?id=5L8uIAFm4SoC&pg=PA493|page=493|title=High-vacuum technology: a practical guide|author=Hablanian, M. H.|publisher=CRC Press|date=1997|isbn=978-0-8247-9834-5}}</ref>

Helium leaks through cracks should not be confused with gas permeation through a bulk material. While helium has documented permeation constants (thus a calculable permeation rate) through glasses, ceramics, and synthetic materials, inert gases such as helium will not permeate most bulk metals.<ref>{{Cite book|author=Ekin, Jack W.|title=Experimental Techniques for Low-Temperature measurements|url=https://archive.org/details/experimentaltech0000ekin|url-access=registration|publisher=Oxford University Press|date=2006|isbn=978-0-19-857054-7}}</ref>