Data including density, flammability range, and auto ignition energy for hydrogen.

Notes:

1. NTP (normal temperature and pressure) = 20°C (68°F) and 1 atm
2. Reference state: Internal Energy U=0 at 273.16 K for saturated liquid; Entropy S=0 at 273.16 K for saturated liquid.
3. The autoignition temperature depends on hydrogen concentration (minimum at stoichiometric combustion conditions), pressure, and even the surface characteristics of the vessel.  Reported figures range from 932-1085 oF according to source [a].