Air-Free CO Correction
Correct a combustion analyzer's carbon monoxide to an air-free basis: CO_air_free = CO_measured x 20.9 / (20.9 - O2). As-measured CO is diluted by excess and dilution air and reads deceptively low, so a dangerous appliance can look acceptable; the ANSI Z21 400 ppm limit is air-free. Sample in the flue before the draft hood. A safety-screening aid, not a certified combustion test.
Formula and source
CO_air_free = measured_co_ppm x 20.9 / (20.9 - measured_o2_pct); over_ansi = CO_air_free > 400; over_field = CO_air_free > 100.
ANSI Z21 400 ppm air-free limit / BPI field practice air-free CO correction, by name.
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