# 1066.610 Dilution air background correction.
(a) Correct the emissions in a gaseous sample for background using the following equation:
Where:
*x*<sub>[emission]dexh</sub> = measured emission concentration in dilute exhaust (after dry-to-wet correction, if applicable).
*x*<sub>[emission]bkgnd</sub> = measured emission concentration in the dilution air (after dry-to-wet correction, if applicable).
*DF* = dilution factor, as determined in paragraph (b) of this section.
Where:
*x*<sub>CO2</sub> = amount of CO<sub>2</sub> measured in the sample over the test interval.
*x*<sub>NMHC</sub> = amount of C<sub>1</sub>-equivalent NMHC measured in the sample over the test interval.
*x*<sub>CH4</sub> = amount of CH<sub>4</sub> measured in the sample over the test interval.
*x*<sub>CO</sub> = amount of CO measured in the sample over the test interval.
*a* = atomic hydrogen-to-carbon ratio of the test fuel. You may measure *a* or use default values from Table 1 of 40 CFR 1065.655.
*b* = atomic oxygen-to-carbon ratio of the test fuel. You may measure *b* or use default values from Table 1 of 40 CFR 1065.655.
(c) Determine the dilution factor, *DF*, over the test interval for partial-flow dilution sample systems using the following equation:
Where:
*V*<sub>dexhstd</sub> = total dilute exhaust volume sampled over the test interval, corrected to standard reference conditions.
*V*<sub>exhstd</sub> = total exhaust volume sampled from the vehicle, corrected to standard reference conditions.
(d) Determine the time-weighted dilution factor, *DF*<sub>w</sub>, over the duty cycle using the following equation:
Where:
*N* = number of test intervals
*i* = test interval number
*t* = duration of the test interval
*DF* = dilution factor over the test interval
*N* = 3
*DF*<sub>1</sub> = 14.40
*t*<sub>1</sub> = 505 s
*DF*<sub>2</sub> = 24.48
*t*<sub>2</sub> = 867 s
*DF*<sub>3</sub> = 17.28
*t*<sub>3</sub> = 505 s
[79 FR 23823, Apr. 28, 2014, as amended at 86 FR 34583, June 29, 2021]