Advantages of measuring O₂ vs CO₂  

In addition to more accurately being able to more precisely monitor burner performance, another advantage to measuring O₂ in a flue gas sample is that a CO₂ measurement does not let us know which side of the stoichiometric curve we are on.  The left side of the graph indicates insufficient combustion air resulting in carbon monoxide and/or smoke production as well as inefficient operation.   We always want to be on the excess air side of the graph.

  Relationship between O₂, CO₂ and excess air

Note that CO₂ per cent is different for each fuel.  O₂ per cent however, remains fairly constant with the most common fuels requiring generally a 3% to 9% O₂ content in flue gases.  This is entirely dependent on the type    of unit being tested.  Only testing in the field under actual operating conditions can verify whether the system is installed and operating as designed.