The Wobbe index is a measure of the interchangeability of gases when they are used as a fuel. It compares the energy output of different gases during combustion. The Wobbe index is essential for analyzing the impact of a fuel changeover and is also a common specification of appliances that use gas and of devices that transport gas. The Wobbe index may be calculated from the higher heating value (HHV) and the specific gravity of the gas.

Define the HHV. The HHV is the amount of heat that a given quantity of fuel releases during combustion. Note that the units of measure for HHV will be in the form of energy/quantity. Common units of measure for HHV include British Thermal Units (BTU) per cubic foot or megajoules per cubic meter.

Define specific gravity. Specific gravity is the ratio of the density of a given substance with the density of water under specific conditions, typically a temperature of 4 degrees C and a pressure of 1 atmosphere. The specific gravity is therefore expressed as SG = P/H where P is the density of the substance and H is the density of water. Specific gravity is a unit-less measure since it is a ratio of densities.

Express the Wobbe index mathematically. The Wobbe index may be defined as Iw = Vc/(Gs)^1/2 where Iw is the Wobbe index, Vc is the HHV value for the fuel and Gs is its specific gravity.

Calculate the Wobbe index for a typical fuel. The HHV of natural gas is typically 1,050 Btu / cubic foot and its specific gravity is about 0.59. The Wobbe index for natural gas is therefore about 1,367 Btu / cubic foot.

Classify fuel gasses according to their Wobbe index. The Wobbe index is used internationally to create 3 families of gasses. Family 1 consists of manufactured gasses, Family 2 is natural gasses and Family 3 includes liquid petroleum.