Large cryogenic liquid hydrogen tanks are composed of inner and outer shells. The outer shell is exposed to the ambient environment while the inner shell holds the liquid hydrogen. The region between these two shells is evacuated and typically filled with a powder-like insulation to minimize radiative coupling between the two shells. A technique was developed for detecting the presence of an air leak from the outside environment into this evacuated region. These tanks are roughly 70 ft (≈21 m) in diameter (outer shell) and the inner shell is roughly 62 ft (≈19 m) in diameter, so the evacuated region is about 4 ft (≈1 m) wide.