The Wobbe number, or Wobbe index, of a fuel gas is found by dividing the high heating value of the gas in Btu per standard cubic foot (scf) by the square root of its specific gravity with respect to air. The higher a gases' Wobbe number, the greater the heating value of the quantity of gas that will flow though a hole of a given size in a given amount of time. It is customary to give a Wobbe number without units–even though it has the dimensions Btu per scf–because to do so would lead to confusion with the volumetric heating value of the gas.
In almost all gas appliances, the flow of gas is regulated by making it pass through a hole or orifice. The usefulness of the Wobbe number is that for any given orifice, all gas mixtures that have the same Wobbe number will deliver the same amount of heat. Pure methane has a Wobbe number of 1363; natural gas as piped to homes in the United States typically has a Wobbe number between 1310 and 1390.
Occasionally Wobbe numbers are calculated on the basis of megajoules per cubic meter instead of Btu per scf; that this is being done is usually obvious from the great difference in the values. For example, using SI units the Wobbe number for methane is 50.7.
ASTM D 1945 (American Gas Association Bulletin No. 36)
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Last revised: 23 January 2002.