40 CFR 89.420 - Background sample.
(a) Background samples are produced by continuously drawing a sample of dilution air during the exhaust collection phase of each test cycle mode.
(1) Individual background samples 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 of the test cycle modes. 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 is stored. All readings taken during the 10-second interval must be stable at the final value to within ±1 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.