Helium diffuses faster than hydrogen
As Lifting gas This is the name given to gases that are used as filling for aircraft such as airships and gas balloons in order to generate lift according to the lighter-than-air principle (Archimedes' principle).
For this purpose gases come into question, which have a lower density than air (approx. 1.293 kg / m3 under normal conditions). From a physical point of view, hydrogen with a density of only 0.0899 kg / m is ideal for this3. Since this gas is also relatively easy and cheap to produce, it was predominant as a lifting gas well into the 20th century.
Chemically, however, hydrogen has the unfavorable property of being easily flammable and even explosive when mixed with oxygen, for example from the air (oxyhydrogen). Because of these disadvantages, it was gradually replaced by the inert noble gas helium as soon as it could be produced in sufficient quantities. Before the Second World War, however, only the USA was able to do this.
Helium has 0.1785 kg / m3 a density twice that of hydrogen. Since the difference to the air density is decisive for the buoyancy, a helium filling generates only about 8% less buoyancy than a hydrogen filling.
Both hydrogen and helium have the property of diffusing through many substances (balloon envelope). It can also happen that gas has to be released to compensate for buoyancy. In the case of constantly filled balloons or airships, a small part of the gas must therefore be replaced at regular intervals.
With one cubic meter of hydrogen a lift of 1.203 kg can be generated, with one cubic meter of helium a lift of 1.1145 kg. However, these values only apply under normal conditions. At some altitude there is a lower air pressure (see: barometric altitude formula), which requires a larger gas volume for the same buoyancy force.
The stated values result in a generous rule of thumb: To counteract a weight of one kilogram, around one cubic meter of lifting gas is required.
In the past, the easily available and cheap illuminating gas was also used as a lifting gas, probably exclusively in private balloon sport. In addition to the disadvantage of lower buoyancy, the risk of fire also had to be accepted. Since the public gas supply was switched to natural gas, there is no longer any coal gas available.
Above 100 ° C, water vapor is also available as a lifting gas, which (molecular mass = 18) generates about twice as much buoyancy per gas volume as air of the same temperature, but less than a quarter of that of helium or hydrogen. In 2006 the hot steam aerostat HeiDAS UH was developed in Germany.
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