40 CFR 91.310 - Engine intake air humidity measurement.
This section refers to engines which are supplied with intake air other than the ambient air in the test cell (i.e., air which has been pumbed directly to the engine air intake system). For engines which use ambient test cell air for the engine intake air, the ambient testcell humidity measurement may be used.
(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) Unconditioned air supply. Humidity measurements in unconditioned intake air supply must be made in the intake air stream entering the engine. Alternatively, the humidity measurements can be measured within the intake air stream entering the supply system.