Basically, the CO reading is multiplied by the ratio of the atmosphere’s oxygen percentage (20.9) to the excess oxygen percentage. The formula is
Air-Free CO ppm = Measured CO ppm x (20.9 / (20.9 – O2% in flue gas))
So if the measured CO ppm is 50 and the measured oxygen in the flue gas is 10.5%
Air-Free CO = 50 x (20.9/(20.9 – 10.5)) = 100 ppm (approximately)
If the CO reading is exactly the same, but the O2 reading is 14%
Air-Free CO = 50 x (20.9/(20.9-14))= 150 ppm (approximately)
The second furnace is producing much more CO than the first, but the CO meter reads the same because the extra excess air in the second furnace has diluted the flue gas. This is why you should use air-free readings whenever checking flue gas CO levels. Most digital flue gas analyzers will do this for you. If you are using the old hour glass bubblers, you will need to do the math yourself.
For a more detailed look at air-free CO and carbon monoxide in combustion gas, take a look at this article by Richard Karg http://www.karg.com/pdf/coairfree_article.pdf
It is actually about ovens, but the processes and science are relevant.