Grundy

New here and just looking for a helping hand. At my job I have been charged with testing the effects of chimney caps on gas fired appliances. Simple stuff for most here I'm guessing. Anyway, question is. Can anyone explain what an air-free sample is and if I am taking one and don't know it? I'm using a California analytical dual gas analyzer, measuring CO2 and CO. The CO limit for combustion is .04% in an air-free sample, according to ANSI standards. If this question is not applicable here, can someone please point me in the right direction. No one at work has a clue as to how any of this works. So far I have set up the machine and am able to procure smaples, but need to make sense of the numbers. Thanks, Brian