Air-Free CO Correction
Correct a combustion analyzer's carbon monoxide to an air-free basis.
Example
You enter
- Measured CO (ppm) 60
- Flue-gas O2 at the same point (%) 8
You get
- Air-free CO 97 ppm (air-free)
Details, formula, and sources
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.
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.
The air-free CO correction is standard combustion-analysis practice; the analyzer and appliance manufacturer instructions govern the actual screening.
Estimate. AHJ and licensed professional govern.
Field names used by the API: measured_co_ppm, measured_o2_pct, co_air_free_ppm
- Air-free basis the ANSI Z21 400 ppm CO limit is air-free; measured CO must be corrected up to itANSI Z21
- Sample point sample in the flue, before the draft hood or dilution air, or the correction over-inflatesBPI field practice