Hello all, hoping to get some clarification on CO Air Free and CO as Measured. In the 2015 IECC, Appendix RA, Worst case testing of atmospheric venting systems, table 301.1(3), acceptable carbon monoxide levels, CO air free/as measured is not specified. I watched an Efficiency Maine training series where a reputable speaker (Rick Karg) mentioned to always use as measured when taking a draft CO. In comparison, ANSI/BPI 1200 7.9.5 Table 1 nearly all combustion appliance thresholds are measured in air free.
With a little bit of time spent on google, I can see the variance can be quite a bit depending on which unit you are using on  your combustion analyzer. While Appendix RA in 2015 IECC is “informative and not part of code” I’m left wondering if there is a most prudent measurement, or if I should gain a deeper understanding of the two and use each depending on the situation and or standard. Any information will be much appreciated.