Calculate the arithmetic mean of these data points. Select a CH 4 concentration that you would use for spanning the FID during emission testing and select a C 2 H 6 concentration that is typical of the peak NMHC concentration expected at the hydrocarbon standard and the C 2 H 6 concentration typical of the peak total hydrocarbon THC concentration expected Nmc penetration fraction the hydrocarbon standard or equal to the THC analyzer's span value. Code What Cites Me. The section you Nmc penetration fraction viewing is cited by the following CFR sections. United States Code U.