Lifting Gas - Hydrogen Versus Helium

Hydrogen Versus 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 (1 newton is the force required to accelerate 1 kg at 1 m/s2) per cubic meter of gas at STP.

The lifting power in air 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 kg/m3.
• Hydrogen (ρ_H2) = 0.090 kg/m3
• Helium (ρ_He) = 0.178 kg/m3

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 m3 of hydrogen in air is:
  • 0.090 kg * (1 − (1.292 / 0.090) ) = −1.202 kg
• And the buoyant mass for one m3 of helium in air is:
  • 0.178 kg * (1 − (1.292 / 0.178) ) = −1.114 kg

The negative signs indicate that these gases tend to rise in air.

Thus hydrogen's additional buoyancy compared to helium is:

• 1.202 / 1.114 ≈ 1.080, or approximately 8.0%