40 CFR 90.422 - Background sample.
(a) Background samples are produced by drawing a sample of the dilution air during the exhaust collection phase of each test cycle mode.
(1) An individual background sample may be produced and analyzed for each mode. Hence, a unique background value will be used for the emission calculations for each mode.
(2) Alternatively, a single background sample may be produced by drawing a sample during the collection phase of each test cycle mode. Hence, a single cumulative background value will be used for the emission calculations for each mode.
(b) For analysis of the individual sample described in paragraph (a)(1) of this section, a single value representing the average chart deflection over a 10-second stabilized period must be stored. All readings taken during the data logging period must be stable within ±one percent of full scale.
(c) Measure HC, CO, CO2, and NOX exhaust and background concentrations in the sample bag(s) with approximately the same flow rates and pressures used during calibration.