Sealing against Helium is vastly more difficult than sealing against air, as air is made out of molecules of rather large atoms, while Helium gas consists of single small atoms. That is why divers watches often have a "helium escape valve". The watches are sealed against high pressures of water or air. But when worn in pressure chambers where divers live on a mix of high pressure helium and oxygen, the helium gets into the watch. When depressurizing at the end of the pressure chamber time, the helium pressure could blow the watch crystal from the watch if not released by the escape valve. For a similar reason pilot watches have special fastening of the watch crystal, so that underpressure doesn't lift it off.