This section describes how to determine the amount of water in an ideal gas, which you need for various performance verifications and emission calculations. Use the equation for the vapor pressure of water in paragraph (a) of this section or another appropriate equation and, depending on whether you measure dewpoint or relative humidity, perform one of the calculations in paragraph (b) or (c) of this section. Paragraph (d) of this section provides an equation for determining dewpoint from relative humidity and dry bulb temperature measurements. The equations for the vapor pressure of water as presented in this section are derived from equations in "Saturation Pressure of Water on the New Kelvin Temperature Scale" (Goff, J.A., Transactions American Society of Heating and Air-Conditioning Engineers, Vol. 63, No. 1607, pages 347-354). Note that the equations were originally published to derive vapor pressure in units of atmospheres and have been modified to derive results in units of kPa by converting the last term in each equation.
Where:
xH20 = amount of water in an ideal gas.
pH20 = water vapor pressure at the measured dewpoint, Tsat = Tdew.
pabs = wet static absolute pressure at the location of your dewpoint measurement.
Example: :
pabs = 99.980 kPa
Tsat = Tdew = 9.5 °C
Using Eq. 1065.645-1,
pH20 = 1.186581 kPa
xH2O = 1.186581/99.980
xH2O = 0.011868 mol/mol
Where:
xH2O = amount of water in an ideal gas.
RH = relative humidity.
pH2O = water vapor pressure at 100% relative humidity at the location of your relative humidity measurement, Tsat = Tamb.
pabs = wet static absolute pressure at the location of your relative humidity measurement.
Example:
RH = 50.77% = 0.5077
pabs = 99.980 kPa
Tsat = Tamb = 20 °C
Using Eq. 1065.645-1,
pH2O = 2.3371 kPa
xH2O = (0.5077 · 2.3371)/99.980
xH2O = 0.011868 mol/mol
Where:
ln(pH2O) = the natural log of pH2Oscaled, which is the water vapor pressure scaled to the relative humidity at the location of the relative humidity measurement, Tsat = Tamb
Example:
RH = 39.61% = 0.3961
Tsat = Tamb = 20.00 °C = 293.15K
Using Eq. 1065.645-1,
pH2Osat = 2.3371 kPa
pH2Oscaled = (0.3961 · 2.3371) = 0.925717 kPa = 925.717 Pa
40 C.F.R. §1065.645