218 Bernd Kroplin manner as the air pressure decreases during ascent and the buoyancy gas expands. During decent air has to be pumped into the ballonets in order to keep the gas pressure in the body at a certain level.The differential pressure level is typically around 500 Pascal. The ballonet principle can be used as pressure control in ground structures also as external ballonet outside the structure.A hall of his type has been built 1992 at the International Garden Exhibition in Stuttgart,see Fig.10.The full roof of the structure,which can be opened including the supporting ring-structure,is carried by buoyancy. Fig.9.Ballonet system in an airship [9] hot Fig.10.HELION-Buoyancy structure Stuttgart 1993 [10]218 Bernd Kroplin ¨ manner as the air pressure decreases during ascent and the buoyancy gas expands. During decent air has to be pumped into the ballonets in order to keep the gas pressure in the body at a certain level. The differential pressure level is typically around 500 Pascal. The ballonet principle can be used as pressure control in ground structures also as external ballonet outside the structure. A hall of his type has been built 1992 at the International Garden Exhibition in Stuttgart, see Fig. 10. The full roof of the structure, which can be opened including the supporting ring-structure, is carried by buoyancy. Fig. 9. Ballonet system in an airship [9] Fig. 10. HELION - Buoyancy structure Stuttgart 1993 [10]