What is Air-Free CO

If you have used a combustion analyzer or CO detector, you may have come across the term “air-free  CO reading.” A common question is “What is the difference between the regular CO reading and the air-free CO reading?” You may have noticed the air-free reading is always higher than the “regular” reading. Basically, the “air-free” reading is calculated to determine what the CO concentration in flue gas WOULD BE if all the excess air were removed. CO readings tell how many parts per million of the sampled gas are CO. If you add a bunch of gas that  was not part of the combustion process, it dilutes the CO reading because now all those other gas “parts” are being counted, even though they had nothing to do with the combustion processs. All gas appliances are designed to operate with some excess air. Excess air is left over in the flue gas after the combustion process – thus its name; excess air. The purpose of excess air is to insure adequate combustion air by providing more than is needed. Lack of combustion air leads to CO production, so having more than enough air helps reduce CO levels. Older, natural draft furnaces have higher levels of excess air than induced draft burners; but all should have some excess air in the flue gas. Excess air is necessary for safe operation. The problem is that the excess air dilutes the flue gas, lowering the CO reading. The air-free calculation corrects for this dilution effect.

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.