The presence of water vapor in the ambient air influences a great number of hygroscopic materials since their water content depends on the humidity level of the environment (e.g. wood, paper, textile, fibers, food products, etc).

This is a reversible phenomenon since air acts as a transfer element moving bodies of water. The most commonly used drying principle consists in creating a capillary migration of the integral water towards the material surface and providing its vaporization in dry air.

環境空氣的構成 / Ambient Air Composition
The ambient air can be defined as a variable mixture of dry air and water vapor. Dry air is also mixed with gases although in invariable proportions, at least for the major constituants (Nitrogen, Oxygen, Argon) which are molar at 99.9%.

最理想的氣體 / Perfect Gases
The thermodynamic status of air can be studied according to the perfect gas laws.
Two features are available:
-- temperature and pressure
-- pressure and volume

If dry air is to be mixed with humid air, third parameter will be required. This value is normaly used to define humidity.

空氣的混合比例 / Mixing ratio
The ratio of mixture (r) can be defined as the proportion between the water vapor volume (mv) and the dry air volume (ma).
                   mv
           r  =  ----------------
                   ma

絕對濕度 / Absolute Humidity (kg/m³)
The absolute humidity (pv) is the ratio between the water vapor volume (mv) and the actual volume (v).                   m v
                      p v =  ----------
                                     v

露點溫度 / Dew point temperature (Td in K or d in °C)
Dew temperature determines the temperature at which water vapor in gas appears as condensation in liquid phase. If the condensate is in its solid phase, it will be called freeze temperature.

Nevertheless, dew point can also be defined as the temperature at which the partial pressure of water vapor (e) equals the pressure of saturating (ew).

水份活性 / Water Activity aW
The presence of water molecules in a hygroscopic body produces a vapor pressure on its surface, which depends on body nature, absolute humidity and temperature of the body.

When this pressure equals the ambient water vapor pressure the body is balanced with the environmental conditions.

When the body has no exchange of water vapor the relative humidity is then called Equilibrium Relative Humidity (% ERH), which represents the quantitly of free water contained within the body and therefore the water activity (aW).                   ERH
              aW = ------------
                            100

結論 / Consequence:
If a hygroscopic body is placed in free atmosphere environmental conditions, the atmosphere will apply its relative humidity rate to the body after a time period. On the other hand, if the same body is placed within an atmosphere of reduced dimensions, the body will transfer its relative humidity to the atmosphere according to its water content and temperature.

K only depends on the gas nature and quantity. According to the Mariotte and Gay Lussac laws, the constant value will remain the same for all perfect gases.
                                          PV = KT
                                                      K = nR

na = number of moles of dry air.
nv = number of moles of humid air.
e' = partial pressure of humid air.
Pa = partial pressure of dry air.

Ratio of Partial Pressures
The e value can be defined as the pressure of water vapor contained in a gas at the gas temperature. eW is the pressure of the saturating vapor of humid air under pressure (P) and temperature (T) conditions.
                                     e
                     Uw =  -----------  x 100 in %RH
                                    ew

Ratio of Molar Factions       
                                           xv
                             Uw =  -----------  x 100
                                          xvw
Xv   = molar titer of water vapor
Xvw = molar titer of saturated water vapor.

Major Factors influencing RH

The Temperature influence
Relative humidity in gases depends on temperature in a very significant proportion. The relative humidity in a gas increases when the temperature decreases and viceversa.

The pressure influence
Saturating pressure is basically independent from total pressure. On the other hand, water vapor pressure, and therefore Uw, is directly proportional to the total pressure value. Any deviation in pressure will change the Uw measurement. This can be easily observed in ducts or compressed air tanks where condensation risks increase when pressure is high.

(e.g. according to the known humidity tables at 50% RH and 20°C, a temperature deviation of 1°C leads to a fluctuation of humidity of 3%, this fluctuation will be 5% at 90% RH)