Actual Vapor Pressure

on . Posted in Fluid Dynamics

Actual vapor pressure, abbreviated as e, also called vapor pressure or partial vapor pressure, refers to the pressure exerted by the vapor molecules of a substance in equilibrium with its liquid or solid phase at a given temperature.  It is a measure of the tendency of a substance to evaporate or transition from its condensed phase to the vapor phase.

The actual vapor pressure is influenced by factors such as temperature, molecular properties of the substance, and the presence of other substances in the surrounding environment.  As temperature increases, the actual vapor pressure of a substance generally increases, as more molecules gain sufficient energy to escape from the liquid or solid phase and enter the vapor phase.

In meteorology, the concept of actual vapor pressure is commonly used in conjunction with other measures of humidity, such as relative humidity and dew point temperature, to describe the moisture content of the atmosphere.  In chemistry and engineering, vapor pressure is utilized to analyze and design processes involving phase changes, such as distillation, evaporation, and vapor-liquid equilibrium calculations.  The actual vapor pressure can be measured directly using various instruments, such as vapor pressure thermometers, hygrometers, or by using mathematical models and correlations based on the properties of the substance and temperature.


Actual Vapor Pressure formula

\( e =  RH \; e_s \;/\; 100 \)     (Actual Vapor Pressure)

\( RH =  100 \; e \;/\; e_s \) 

\( e_s =  100 \; e \;/\; RH \) 

Solve for e

relative humidity, RH
saturated vapor pressure, es

Solve for RH

actual vapor pressure, e
saturated vapor pressure, es

Solve for es

actual vapor pressure, e
relative humidity, RH

Symbol English Metric
\( e \) = actual vapor pressure  \(lbf\;/\;in^2\) \(Pa\) 
\( RH \) = relative humidity \(dimensionless\)
\( e_s \) = saturated vapor pressure  \(lbf\;/\;in^2\) \(Pa\) 


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Tags: Pressure Vapor