The Wobbe Index: The One Number That Says If Two Gases Are Interchangeable

The companion calculator finds the new supply pressure needed to switch a burner between two fuel gases through the same orifice. Buried in that calculation is one of the most important concepts in gas engineering: the Wobbe Index, a single number that determines whether two different gases can be swapped through the same equipment and deliver the same heat. It explains why fuel changeover is not as simple as matching heating values, and why blending hydrogen into the gas grid is genuinely difficult.

Why Heating Value Alone Is Not Enough

You might think two gases deliver the same heat through a fixed orifice if they have the same heating value. They do not, because a lighter gas flows faster through the same hole at the same pressure than a heavier one, so the amount of gas delivered depends on the gas's density, not just its energy content. To match the heat delivered, you have to account for both how much energy the gas carries and how easily it flows through the orifice. That is exactly what the Wobbe Index does.

What the Wobbe Index Is

The Wobbe Index combines a gas's heating value and its density into one figure, essentially the heating value divided by the square root of the gas's relative density (specific gravity).

The remarkable and useful property is this: two gases with the same Wobbe Index will deliver the same heat rate through the same orifice at the same supply pressure, even if their individual heating values and densities are quite different. The higher heating value of one gas is offset by its higher density (which slows its flow), and the two effects cancel. This is why the Wobbe Index, not heating value, is the true measure of interchangeability.

Why This Is So Practically Important

Gas utilities must deliver gas that appliances and burners across a whole network can burn safely and correctly without being re-adjusted. Since the gas supply can vary in composition, from different sources, seasonal blends, added gases, keeping the Wobbe Index within a specified range ensures all that equipment keeps delivering the right heat and burning stably. Interchangeability standards are written around the Wobbe Index precisely because it, not heating value, predicts how a burner will actually behave on a given gas.

The Changeover Calculation in This Light

When the two gases do not share a Wobbe Index, the calculator's changeover formula finds the new pressure that compensates, adjusting the driving pressure so the different gas still delivers equivalent heat through the fixed orifice. This is exactly the situation when converting equipment between, say, natural gas and propane: their properties differ, so the supply pressure (and often the orifice) must change to match the heat input. The formula is the practical tool; the Wobbe Index is the principle underneath it.

Why Hydrogen Blending Is Hard

The Wobbe Index explains a very current problem. Hydrogen has a very different heating value and density from natural gas, and therefore a different Wobbe Index, so it is not freely interchangeable with natural gas through existing appliances. Blending significant hydrogen into the gas grid shifts the Wobbe Index of the delivered gas, potentially outside the range that installed burners are set up for, which can affect flame stability, heat output, and safety. This is a central technical hurdle in using the existing gas network to carry hydrogen, and it is fundamentally a Wobbe Index problem.

Using the Changeover Pressure Well

Take the calculator's changeover pressure as a theoretical starting estimate for delivering equivalent heat when switching gases through a fixed orifice, and always verify it against the manufacturer's conversion specifications before commissioning. Understand the principle behind it: interchangeability is governed by the Wobbe Index, which combines heating value and density, so gases with matching Wobbe Index swap freely while others need a pressure or orifice change. It is the same concept that makes hydrogen blending into natural gas systems a genuine engineering challenge.