Draft-Hood Dilution Ratio
How much room air the draft hood pulled in, from two O2 readings: dilution_ratio = (20.9 - O2_appliance) / (20.9 - O2_diluted), and the dilution-air fraction (20.9% ambient O2 mass balance). A clean appliance at 5% O2 that reads 12% O2 at the vent has pulled in 44% dilution air - so a CO reading there is only 56% of the true flue value, which is why the air-free CO sample must be taken before the draft hood. A diagnostic aid, not a certified combustion test.
Formula and source
dilution_ratio = (20.9 - appliance_o2_pct) / (20.9 - diluted_o2_pct); dilution_air_fraction = (diluted_o2_pct - appliance_o2_pct) / (20.9 - appliance_o2_pct) x 100; co_read_fraction = 100 / dilution_ratio.
Combustion-analysis practice; ambient-oxygen (20.9%) dilution mass balance, by name.
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