Hydrogen technical data: formula H₂ density, boiling point, LEL and UEL, and why a gas has no flash point

Hydrogen is the simplest and lightest gas there is: two hydrogen atoms, a molar mass of 2.016 g/mol and a density at 0 °C of 0.0899 kg/m³, about one-fourteenth that of air. The numbers that matter for a buyer or a plant engineer are its density and how it compresses, its boiling point for anyone handling the liquid, and three fire numbers: a lower flammable limit of 4 %, an upper limit of 75 % and an autoignition temperature of about 560 to 574 °C, depending on the test method. This page collects those figures from NIST and the US Coast Guard CHRIS data sheet and explains the one figure people keep asking for that does not exist, the flash point.

Two columns in that table do most of the work. The density column explains why hydrogen rises out of a leak and collects under roofs, which is covered in hydrogen safety data explained. The thermal-conductivity and specific-heat figures, about 0.18 W/m·K and 14.3 kJ/kg·K, are the reason hydrogen is used as a coolant in large generators and as an annealing atmosphere; its uses are set out under uses of hydrogen gas.

Hydrogen is bought and invoiced by volume, so the reference conditions matter. NIST gives 0.0899 kg/m³ at 0 °C and 0.0852 kg/m³ at 15 °C, both at 1.01325 bar. From those:

A quotation for hydrogen "per m³" without the temperature can differ by 5 % between the first two rows. The conversion for ten gases, with the reference conditions stated, is on the Nm³ to kg page.

The ideal gas law says a cylinder filled to 150 bar should hold 150 times its water capacity as gas at 1 atm. Hydrogen holds less. At 15 °C NIST gives a density of 11.54 kg/m³ at 150 bar and 14.94 kg/m³ at 200 bar. Divided by the 1 atm density at the same temperature, that is:

The compressibility factor, Z, is therefore about 1.1 at these pressures, meaning hydrogen molecules resist being pushed closer together. The practical result is that raising the fill pressure by a third adds only about 30 % more gas, and that a contents estimate made by multiplying water capacity by gauge pressure overstates the contents by roughly 10 %. The full treatment of Boyle's, Charles's and Gay-Lussac's laws with Z is in gas laws and cylinder pressure.

Liquid hydrogen is a specialist product, not a cylinder gas. It boils at −252.8 °C, has a density of only about 70.8 kg/m³, a fourteenth of water, and expands about 845-fold into gas at 20 °C. Its critical temperature is −240.0 °C, so it cannot be held as a liquid at any pressure above that. The latent heat of vaporisation is about 443 kJ/kg (CHRIS gives 190.5 Btu/lb), small enough that heat leak boils it quickly. Any surface cold enough to hold liquid hydrogen will condense air out of the atmosphere, and the condensate is enriched in oxygen.

The Indian product standard is IS 1090:2002, Compressed hydrogen — Specification (third revision), reaffirmed in December 2023 with one amendment, published by the Bureau of Indian Standards' industrial gases committee, CHD 06. Its grade table and impurity limits are not reproduced here; consult the current edition of the standard before specifying a grade. Purity in the trade is usually expressed in the N-notation, where 5.0 means 99.999 %. The standards position is on the hydrogen standards page.

The left-hand thread exists so that a fuel-gas regulator cannot be screwed onto an oxygen or inert cylinder, or the other way round. The nut of a left-hand fitting usually carries a notch or groove so it can be recognised by eye and by touch. IS 4379:2021 gives the same signal red ground colour to LPG, propane, butane and coal gas, so colour tells you the cylinder holds a fuel gas; the stamped marking tells you which.