{"data":{"id":"us/40-cfr-1066.610","jurisdiction":"us","citation":"40 CFR 1066.610","heading":"Dilution air background correction.","body":"(a) Correct the emissions in a gaseous sample for background using the following equation:\nWhere:\nx[emission]dexh = measured emission concentration in dilute exhaust (after dry-to-wet correction, if applicable).\nx[emission]bkgnd = measured emission concentration in the dilution air (after dry-to-wet correction, if applicable).\nDF = dilution factor, as determined in paragraph (b) of this section.\nWhere:\nxCO2 = amount of CO2 measured in the sample over the test interval.\nxNMHC = amount of C1-equivalent NMHC measured in the sample over the test interval.\nxCH4 = amount of CH4 measured in the sample over the test interval.\nxCO = amount of CO measured in the sample over the test interval.\na = 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.\nb = 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.\n(c) Determine the dilution factor, DF, over the test interval for partial-flow dilution sample systems using the following equation:\nWhere:\nVdexhstd = total dilute exhaust volume sampled over the test interval, corrected to standard reference conditions.\nVexhstd = total exhaust volume sampled from the vehicle, corrected to standard reference conditions.\n(d) Determine the time-weighted dilution factor, DFw, over the duty cycle using the following equation:\nWhere:\nN = number of test intervals\ni = test interval number\nt = duration of the test interval\nDF = dilution factor over the test interval\nExample:\nN = 3\nDF1 = 14.40\nt1 = 505 s\nDF2 = 24.48\nt2 = 867 s\nDF3 = 17.28\nt3 = 505 s","path":["Title 40—Protection of Environment","CHAPTER I—ENVIRONMENTAL PROTECTION AGENCY","SUBCHAPTER U—AIR POLLUTION CONTROLS","PART 1066—VEHICLE-TESTING PROCEDURES","Subpart G—Calculations"],"source_url":"https://www.ecfr.gov/api/versioner/v1/full/2026-08-25/title-40.xml","current_through":"2026-08-25","vintage":"","retrieved_at":"2026-08-27T02:26:04Z","sha256":"d8c4cbc43f29ef08c70bf6eca1f960f46637386e9b3d2d3363513647b352a23e","source_id":"us-cfr","stale":true,"prev":"us/40-cfr-1066.605","next":"us/40-cfr-1066.615"},"notice":"GroundRules: Original legal text. Not legal advice."}