The first atomic bomb used uranium isotope 235, which occurs in nature together with uranium‐238, from which it has to be separated. The separation was accomplished in 1944–1945 in a huge ‘‘diffusion’’ plant using microporous tubing as the diffusion medium. The process was such that moist ambient air could not be tolerated in the process chambers. Accordingly, all the equipment had to be ‘‘free’’ of leaks. Equipment of the size used in the diffusion plant had never been tested to such an extreme specification. A number of leak detection devices were tried and proved inadequate. Ultimately, a simplified mass spectrometer based on the Nier 60 in. spectrometer tube was chosen for leak detection. It could easily be set to detect helium which was the choice for ‘‘probe’’ gas. From an original sensitivity such that a flow of 10−6std cm3of helium could be detected, the current unit has been improved so as to detect a flow of 10−10–10−11std cm3. Size of the detector has been reduced by almost an order of magnitude and operation is highly automated.