40 CFR § 62.15200 - What must I do if I choose to monitor carbon dioxide instead of oxygen as a diluent gas?
You must establish the relationship between oxygen and carbon dioxide during the initial evaluation of your continuous emission monitoring system. You may reestablish the relationship during annual evaluations. To establish the relationship use three procedures:
(a) Use EPA Reference Method 3A or 3B in appendix A of 40 CFR part 60 to determine oxygen concentration at the location of your carbon dioxide monitor.
(b) Conduct at least three test runs for oxygen. Make sure each test run represents a 1-hour average and that sampling continues for at least 30 minutes in each hour.
(c) Use the fuel-factor equation in EPA Reference Method 3B to determine the relationship between oxygen and carbon dioxide.