Lifting gas

A lifting gas is required for aerostats to create buoyancy. Its density is lower than that of air (about 1.2 kg/m³, 1.2 g/l).

Ammonia

Ammonia is sometimes used to fill weather balloons. Due to its relatively high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquified aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast).

Methane

Methane (the chief component of natural gas) is sometimes used as a lift gas when hydrogen and helium are not available. It has the advantage of not leaking through balloon walls as rapidly as the small-moleculed hydrogen and helium. (Most lighter-than-air balloons are made of aluminized plastic that limits such leakage; hydrogen and helium leak rapidly through latex balloons.)

Hydrogen and helium

Hydrogen and helium are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both so much lighter than air that this difference is inconsequential. Both provide about 9.8 N of lift (the force to lift 1 kg) per cubic meter of gas at room temperature and sea level pressure. Helium is preferred because it is not combustible.

The relative lifting power of hydrogen and helium can be calculated using the theory of buoyancy as follows:

The density at sea-level and 0°C for air and each of the gases is:
* Air (ρ_air) = 1.292 (g/L).
* Hydrogen (ρ_H2) = 0.08988 g/L
* Helium (ρ_He) = 0.1786 g/L

Thus helium is almost twice as dense as hydrogen. However, buoyancy depends upon the "difference" of the densities (ρ_gas) - (ρ_air) rather than upon their ratios. Thus the difference in buoyancies is about 8%, as seen from the buoyancy equation:
* Buoyant mass (or effective mass) = mass × (1 - ρ_air/ρ_gas)
* Therefore the buoyant mass for one liter of hydrogen in air as:
** 0.08988 g * (1 - (1.292 / 0.08988) ) = -1.202 g
* And the buoyant mass for one liter of helium in air as:
** 0.1786 g * (1 - (1.292 / 0.1786) ) = -1.113 gThe negative signs indicate that these gases tend to rise in air.

Thus hydrogen's additional buoyancy compared to helium is:
* 1.202 / 1.113 = 1.080, or approximately 8.0%

Hydrogen safety

Many countries have banned the use of hydrogen as a lift gas for manned vehicles. The Hindenburg disaster is frequently cited as an example of the hydrogen safety risks posed by hydrogen. The high cost of helium (compared to hydrogen) has led researchers to reinvestigate the safety issues of using hydrogen as a lift gas: with good engineering and good handling practices, the risks can be significantly reduced.Fact|date=July 2008