{"data":{"id":"us/40-cfr-1066.615","jurisdiction":"us","citation":"40 CFR 1066.615","heading":"NOX intake-air humidity correction.","body":"You may correct NOX emissions for intake-air humidity as described in this section if the standard-setting part allows it. See § 1066.605(c) for the proper sequence for applying the NOX intake-air humidity correction.\n(a) For vehicles at or below 14,000 pounds GVWR, apply a correction for vehicles with reciprocating engines operating over specific test cycles as follows:\n(1) Calculate a humidity correction using a time-weighted mean value for ambient humidity over the test interval. Calculate absolute ambient humidity, H, using the following equation:\nWhere:\nMH2O = molar mass of H2O.\npd = saturated vapor pressure at the ambient dry bulb temperature.\nRH = relative humidity of ambient air\nMair = molar mass of air.\npatmos = atmospheric pressure.\nExample:\nMH2O = 18.01528 g/mol\npd = 2.93 kPa\nRH = 37.5% = 0.375\nMair = 28.96559 g/mol\npatmos = 96.71 kPa\n(2) Use the following equation to correct measured concentrations to a reference condition of 10.71 grams H2O vapor per kilogram of dry air for the FTP, US06, LA-92, SC03, and HFET test cycles:\nWhere:\nχNOx = measured NOX emission concentration in the sample, after dry-to-wet and background corrections.\nHs = humidity scale. Set = 1 for FTP, US06, LA-92, and HFET test cycles. Set = 0.8825 for the SC03 test cycle.\nH = ambient humidity, as determined in paragraph (a)(1) of this section.\nExample:\nH = 7.14741 g H2O vapor/kg dry air time weighted over the FTP test cycle\nχNOx = 1.21 ppm\n(b) For vehicles above 14,000 pounds GVWR, apply correction factors as described in 40 CFR 1065.670.","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":"19c4093ae5e357616b90879eb8698d899bad9050a56b086abfff3b1a2942a703","source_id":"us-cfr","stale":true,"prev":"us/40-cfr-1066.610","next":"us/40-cfr-1066.620"},"notice":"GroundRules: Original legal text. Not legal advice."}