The air ratio can be determined from the concentrations of the flue gas components CO, CO₂ and O₂, the correlations are shown in the so-called combustion chart (see image right). When there is ideal mixing of fuel and air, any CO₂ content is related to a specific CO content (in the range λ<1) or to a specific O₂ content (in the range λ>1). The CO2 value on its own is not clear due to the curve profile running beyond a maximum, which means that an additional test is required to establish whether the gas also contains CO or O₂ in addition to the CO2. For operation with excess air (i.e. normal scenario), a definitive measurement of O₂ is now generally preferred. The curve progressions are fuel-specific, i.e. each fuel has its own diagram and a specific value for CO₂ max. The connections between these numerous diagrams is often summarized in an easy-to-handle nomogramm (“fire triangle”, not illustrated here.) This can be applied to any type of fuel.

In stationary operating mode, the sum of all the energies supplied to the plant must be equal to the sum of the energies delivered by the plant; see this table :