Density and Related Measures

A.2.4.1 Density

Hydrogen has the lowest atomic weight of any substance and therefore has very low density both as a gas and a liquid. Density is measured as the amount of mass contained per unit volume. Density values only have a meaning at a specified temperature and pressure since both of these parameters affect the compactness of the molecular arrangement, especially in a gas. The density of a gas is called its vapour density, and the density of a liquid is called its liquid density.

A.2.4.2 Specific volume

Specific volume is the inverse of density and expresses the amount of volume per unit mass. Thus, the specific volume of hydrogen gas is 11.9 m3/kg at 20 °C and 0.1 MPa , and the specific volume of liquid hydrogen is 0.014 m3 /kg at -253 °C and 0.1 MPa.

A.2.4.3 Specific Gravity

A common way of expressing relative density is as specific gravity. Specific gravity is the ratio of the density of one substance to that of a reference substance, both at the same temperature and pressure. For vapour, air (with a density of 1.203 kg/m3) is used as the reference substance and therefore has a specific gravity of 1.0 relative to itself. The density of other vapours is then expressed as a number greater or less than 1.0 in proportion to its density relative to air. Gases with a specific gravity greater than 1.0 are heavier than air; those with a specific gravity less than 1.0 are lighter than air. Gaseous hydrogen, of a density of 0.0837 kg/m3, has a specific gravity of 0.0696 and thus has approximately 7% the density of air. For liquids, water (with a density of 1000 kg/m3) is used as the reference substance: it has a specific gravity of 1.0 relative to itself. As with gases, liquids with a specific gravity greater than 1.0 are heavier than water; those with a specific gravity less than 1.0 are lighter than water. Liquid hydrogen, with a density of 70.8 kg/m3, has a specific gravity of 0.0708 and has thus approximately (and coincidentally) 7% of the density of water.

A.2.4.4 Expansion Ratio

The difference in volume between liquid and gaseous hydrogen can easily be appreciated by considering its expansion ratio. The expansion ratio is the ratio of the volume at which a gas or liquid is stored compared to the volume of the gas or liquid at atmospheric pressure and temperature. When hydrogen is stored as a liquid, it is vaporizes upon expansion to atmospheric conditions with a corresponding increase in volume. Hydrogen's expansion ratio of 1:848 means that hydrogen in its gaseous state at atmospheric conditions occupies 848 times more volume than it does in its liquid state. When hydrogen is stored as a high-pressure gas at 250 bar and atmospheric temperature, its expansion ratio to atmospheric pressure is 1:240. While a higher storage pressure increases the expansion ratio somewhat, gaseous hydrogen under any conditions cannot approach the expansion ratio of liquid hydrogen.

A.2.4.5 Hydrogen Content

Even as a liquid, hydrogen is not very dense. Ironically, every cubic meter of water (made up of hydrogen and oxygen) contains 111 kg of hydrogen whereas a cubic meter of liquid hydrogen contains only 71 kg of hydrogen. Thus, water packs more mass of hydrogen per unit volume, because of its tight molecular structure, than hydrogen itself. This is true of most other liquid hydrogen-containing compounds as well; a cubic meter of methanol contains 100 kg of hydrogen and a cubic meter of heptane contains 113 kg. Hydrocarbons are compact hydrogen carriers with the added advantage of having a higher energy density than pure hydrogen. When used as vehicle fuel, the low density of hydrogen necessitates that a large volume of hydrogen be carried to provide an adequate driving range.

A.2.4.6 Leakage

The molecules of hydrogen gas are smaller than those of tall other gases, and hydrogen can diffuse through many materials considered airtight or impermeable for other gases. This property makes hydrogen more difficult to contain than other gases. Leaks of liquid hydrogen evaporate very quickly since the boiling point of liquid hydrogen is so extremely low. Hydrogen leaks are dangerous in that they pose a risk of fire where they mix with air. However, the small molecular size that increases the likelihood of a leak also results in very high buoyancy and diffusivity, so leaked hydrogen rises and becomes diluted quickly, especially outdoors. This results in a localized region of flammability that disperses quickly. As the hydrogen dilutes with distance from the leakage site, the buoyancy decreases and the tendency for the hydrogen to continue to rise decreases. Very cold hydrogen, resulting from a liquid hydrogen leak, becomes buoyant after is evaporates.

Living Off The Grid

Living Off The Grid

Get All The Support And Guidance You Need To Be A Success At Living Off The Grid. This Book Is One Of The Most Valuable Resources In The World When It Comes To When Living Within The Grid Is Not Making Sense Anymore.

Get My Free Ebook


  • codey
    How biogas density is leeser than air?
    6 years ago

Post a comment