For approximate determinations of the composition of the flue gases, the
Orsat apparatus, or some modification thereof, should be employed. If
momentary samples are obtained the analyses should be made as frequently
as possible, say, every 15 to 30 minutes, depending on the skill of the
operator, noting at the time the sample is drawn the furnace and firing
conditions. If the sample drawn is a continuous one, the intervals may
be made longer.