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Title 40: Protection of Environment
PART 86—CONTROL OF EMISSIONS FROM NEW AND IN-USE HIGHWAY VEHICLES AND ENGINES
Subpart D—Emission Regulations for New Gasoline-Fueled and Diesel-Fueled Heavy-Duty Engines; Gaseous Exhaust Test Procedures

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§ 86.344-79   Humidity calculations.

(a) The following abbreviations (and units) are used in this section:

BARO = barometric pressure (Pa)

H = specific humidity, (gm H₂O/gm of dry air)

K = 0.6220 gm H₂O/gm dry air

M _air= Molecular weight of air = 28.9645

M H2O= Molecular weight of water = 18.01534

P DB= Saturation vapor pressure of water at the dry bulb temperature (Pa)

P DP= saturation vapor pressure of water at the dewpoint temperature (Pa)

P v= partial pressure of water vapor (Pa)

P WB= saturation vapor pressure of water at the wet bulb temperature (pa)

T DB= Dry bulb temperature (°K)

T WB= Wet bulb temperature (°K)

Y = Water-vapor volume concentration

(b) The specific humidity on a dry basis of the intake air ( H ) is defined by equation (1).

(c) The partial pressure of water vapor may be determined in two manners:

(1) A dew point device may be used. In that case:

P v= P DP

(2) A wet-bulb, dry-bulb method may be used. In that case “Ferrels equation” (eq. (2)) is used.

(d)(1) The saturation vapor pressure ( P WB) of water at the wet-bulb temperature is defined by equation (3) (Ref. Wexler and Greenspan, equation (23), National Bureau of Standards).

where:

P WBis in Pascals (Pa)

T WB= Web-bulb temperature (°K)

B = − 12.150799

F ₀= − 8.49922(10)³

F ₁= − 7.4231865(10)³

F ₂= 96.1635147

F ₃= 2.4917646(10)⁻²

F ₄= − 1.3160119(10)⁻⁵

F ₅= − 1.1460454(10)⁻⁸

F ₆= 2.1701289(10)⁻¹¹

F ₇= − 3.610258(10)⁻¹⁵

F ₈= 3.8504519(10)⁻¹⁸

F ₉= − 1.4317(10)⁻²¹

(2) The table in Figure D79–5 may be used in lieu of equation (3).

(e) The saturated vapor pressure of water at the dry-bulb temperature ( P DB) is found (if required) by using dry-bulb absolute temperature (°K) in equation (3).

(f) The percent of relative humidity ( RH ) (if required) is defined by equation (4).

(g) The water-vapor volume concentration on a dry basis of the engine intake air ( Y ) is defined by equation (5).

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