§ 89.420-96 - Background sample.

(a) Background samples are produced by drawing a sample of the dilution air during the 60 second 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.