40 CFR 89.326 - Engine intake air humidity measurement.
(a)Humidity conditioned air supply. Air that has had its absolute humidity altered is considered humidity- conditioned air. For this type of intake air supply, the humidity measurements must be made within the intake air supply system and after the humidity conditioning has taken place.
(b)Nonconditioned air supply procedure. Humidity measurements in nonconditioned intake air supply systems must be made in the intake air stream entering the supply system. Alternatively, the humidity measurements can be measured within the intake air supply stream.