The Wobbe index is a measurement of the degree to which fuels such as natural gas and propane can be interchanged. It is an important measure for appliances that use these fuels, as a gas line to one's house can carry gas supplied from a variety of different sources and suppliers. The index provides a simple way to compare which particular fuels are directly compatible and which may require adjustments to use safely.
The index is named for Italian scientist Goffredo Wobbe. In the late 1920s, he first determined the relationship between the heat output of a burner, the specific gravity of the gas used, and its higher heating value. His work is critical given the important conceit that not all types of fuel gas are created equal.
For the purposes of the Wobbe index, fuel gasses are split up into three families, which are the manufactured or "town" gas, natural gas, and liquefied petroleum gas (LPG). Pressure, specific gravity, and calorific value make up a fuel's potential energy, and vary between families and even among gases of the same type, depending on the supplier. LPG fuels typically posses the highest index numbers, with natural gas in the middle, and town gas on the low end.
Appliances are typically equipped to handle gas from a single family with a limited degree of variance in composition. Generally, a range of five percent in the Wobbe index is acceptable for replacement fuel. Any greater differential requires at the vary least adjustment or replacement of a burner's components.
A fuel's Wobbe index number is calculated by dividing its higher heating index, the amount of energy released as heat when combusted, by the square root of the gas's specific gravity. The result is a number that can be used as a basis for comparison between different gases. British Thermal Units per standard cubic foot (BTU/scf) or megajoules per standard cubic meter (MJ/Sm3) are the common units of measure for the index, but to eliminate confusion it is common practice simply to deal with the metric calculation of Wobbe numbers and not list the unit type.
The lower a fuel's Wobbe index, the less energy it releases per volume combusted. A stovetop designed for gas with a Wobbe index number of 50, for example, will provide too much air if supplied with gas possessing a Wobbe index number of 35. This can result in an inferior burn and even an inability to ignite, which can be extremely dangerous.