Measurement
The humidity of an air-water vapor mixture is determined through the use of psychrometric charts if both the dry bulb temperature (T) and the wet bulb temperature (Tw) of the mixture are known. These quantities are readily estimated by using a sling psychrometer.
There are several empirical correlations that can be used to estimate the saturated vapor pressure of water vapor as a function of temperature. The Antoine equation is among the least complex of these formulas, having only three parameters (A, B, and C). Other correlations, such as the those presented by Goff-Gratch and Magnus Tenten, are more complicated but yield better accuracy. The correlation presented by Buck is commonly encountered in the literature and provides a reasonable balance between complexity and accuracy:
where is the dry bulb temperature expressed in degrees Celsius (°C), is the absolute pressure expressed in hectopascals (hPa), and is the saturated vapor pressure expressed in hectopascals (hPa).
Buck has reported that the maximum relative error is less than 0.20% between -20°C and +50°C when this particular form of the generalized formula is used to estimate the saturated vapor pressure of water.
Read more about this topic: Relative Humidity
Famous quotes containing the word measurement:
“That’s the great danger of sectarian opinions, they always accept the formulas of past events as useful for the measurement of future events and they never are, if you have high standards of accuracy.”
—John Dos Passos (1896–1970)