https://doi.org/10.1351/goldbook.F02440
The volume rate of one component, $\ce{\textbf{X}}$ (e.g. air, $\ce{O2}$), of the unburnt gas mixture (such as $\ce{C2H2}$, $\ce{O2}$, etc.) in the burner tip. Ambient temperature and pressure must be stated.