AN607

Rev. 1.7

15

5. An Introduction to Humidity

Atmospheric air normally contains water vapor and can be thought of as a mixture of ideal gasses. Dry air (no

moisture content) is the combination of approximately (on a mole basis) 78.09% N2, 20.95% O2, 0.93% Ar and

0.03% CO2 and trace elements. The amount of water vapor found in air depends on available liquid water (or ice),

temperature, pressure and ranges from nearly zero to the point of saturation called “dew point (frost point)”. Water

vapor enters the air by evaporation due to the vapor pressure of water or ice.

5.1. Vapor Pressure

There are a few key concepts to keep in mind when discussing vapor pressure. Pure water vapor pressure, p, is

due to water vapor pressure over water or ice without the presence of other gases such as air. In combination with

air, the actual water vapor pressure is increased by a dimensionless factor referred to as a water vapor

enhancement factor, f. This factor is a weak function of temperature and pressure and is approximately 0.47% at

sea level and 20 °C. The actual vapor pressure of water vapor, p', is the pure water vapor pressure, p, multiplied by

the enhancement factor, f.

Dalton's law states that the total pressure of a mixture of gasses is equal to the sum of the partial pressures of each

component gas and assumes the combination of gasses behaves like an ideal gas. In an ideal gas mixture such as

air, the total pressure is the sum of the partial pressures of each gas. The pressure of air at any point can be

calculated as follows.

There are several useful equations when working with vapor pressure. The best one to use depends on the

available information you have about the problem you need to solve, the range of conditions for the problem the

degree of accuracy required and the computational resources available. These equations and their range of

application are discussed in detail in appendix B. Pressures referred to in this document are absolute (not gauge)

unless otherwise specified.

5.2. Temperature

The relative humidity value can change significantly with even slight variations in temperature. For example, a 1 °C

change in temperature at 35 °C and 75% relative humidity will introduce a –4% RH change. A higher temperature

increases the ability of air to absorb moisture and a lower temperature decreases the ability of air to absorb

moisture. Temperature changes can introduce moisture variations in an air mass if condensation occurs or through

secondary impacts such as changing the moisture absorption or desorption of environmental materials in an

enclosure. For humidity sensors that respond in proportion to relative humidity and not absolute humidity, the issue

of temperature measurement error is not significant unless conversion to dew point, absolute humidity or any other

measurement of water vapor in the air is required. In the case of a dew point calculation, a 1 °C error in the

measurement of the temperature will produce approximately a 1 °C error in the calculation of the dew point. This

temperature dependency not only emphasizes the importance of accurate temperature measurement, it also

highlights the necessity of thermal stability, which can be difficult to achieve. Even if the temperature and humidity

measurements are taken in relatively close proximity, there can be considerable differences in corresponding

levels of humidity and temperature. To achieve the most accurate measurement it is best if the humidity and

temperature measurements are taken as close as possible to each other—ideally co-located on the same chip.

p



pf

p 1.0047



==

pp

N2

p

O2

p

H2O

p

Ar

p

CO2

++

+

+

=