Combustion Lambda and Air-Fuel Ratio
The lambda and air-fuel ratio a modern analyzer shows beside excess air, from the same flue oxygen: lambda = 20.9 / (20.9 - O2), excess air = (lambda - 1) x 100, AFR_actual = lambda x AFR_stoich (17.2 natural gas, 15.5 propane, 14.5 #2 oil, by mass). A gas appliance at 3% O2 is lambda 1.17, 16.8% excess air, 20.1:1 AFR against the 17.2:1 stoichiometric - the same tune three ways. Lambda = 1 + excess_air/100, the number the instrument happens to display. A tuning aid, not a certified combustion test.
Formula and source
lambda = 20.9 / (20.9 - flue_o2_pct); excess_air_pct = (lambda - 1) x 100; afr_actual = lambda x afr_stoich(fuel).
Combustion-analysis practice lambda and air-fuel ratio, by name.
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