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e-CFR data is current as of January 14, 2021

Title 40Chapter ISubchapter UPart 1066Subpart G → §1066.615

Title 40: Protection of Environment
Subpart G—Calculations

§1066.615   NOX intake-air humidity correction.

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.

(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:

(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:

eCFR graphic er25oc16.275.gif

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MH2O = molar mass of H2O.

pd = saturated vapor pressure at the ambient dry bulb temperature.

RH = relative humidity of ambient air

Mair = molar mass of air.

patmos = atmospheric pressure.


MH2O = 18.01528 g/mol

pd = 2.93 kPa

RH = 37.5% = 0.375

Mair = 28.96559 g/mol

patmos = 96.71 kPa

eCFR graphic er25oc16.276.gif

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(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:

eCFR graphic er19fe15.029.gif

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χNOx = measured NOX emission concentration in the sample, after dry-to-wet and background corrections.

Hs = humidity scale. Set = 1 for FTP, US06, LA-92, and HFET test cycles. Set = 0.8825 for the SC03 test cycle.

H = ambient humidity, as determined in paragraph (a)(1) of this section.


H = 7.14741 g H2O vapor/kg dry air time weighted over the FTP test cycle

χNOx = 1.21 ppm

eCFR graphic er19fe15.030.gif

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(b) For vehicles above 14,000 pounds GVWR, apply correction factors as described in 40 CFR 1065.670.

[80 FR 9121, Feb. 19, 2015, as amended at 81 FR 74207, Oct. 25, 2016]

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