(or Specific) Humidity: The ratio of the mass of water vapor in a sample
of air to the mass of air of the sample.
Absolute Pressure (psia): The pressure above the absolute
zero value of pressure that is theoretically obtained in empty
space. (Example: The absolute pressure at sea level is 14.7
Absorption: The penetration of a substance into the inner
structure of a solid or liquid where it is held in place by surface
tension effects and is readily displaced.
Adsorption: The surface retention of solid,
liquid or gas molecules, atoms or ions by a solid or liquid. The
adsorbed material is held in place by Van der Waals and other weak
inter-molecular forces and is not readily displaced.
Altitude: Height, measured as distance along the extended
earth's radius above a given datum, such as average sea level.
Angstrom (Å): A unit of length, 10-8 cm
(10-10 meter). Used
to define pore size in molecular sieves.
Ascent: Motion of an aircraft in which the path is inclined
upward with respect to the horizontal.
Atmospheres (atm): A unit of pressure equal to the pressure
exerted by a column of air one inch square and extending from sea
level to infinity. 1 atmosphere = 14.7 psi.
Breathing: Movement of gas, vapors or air in and out of an
enclosure as a result of expansions and contractions induced by
temperature or altitude change.
Condensation: The process by which vapor becomes a liquid.
of an aircraft in which the path is inclined downward with respect
to the horizontal.
Dew Point: The temperature at which air becomes saturated
when cooled without addition of moisture or change of pressure;
any further cooling causes condensation.
Diurnal: Pertaining to meteorological actions which are completed
within 24 hours and which recur every 24 hours.
Dry Bulb Thermometer: An ordinary thermometer, especially
one with an un-moistened bulb; not dependent upon atmospheric humidity.
Dunnage: Cushioning material in a container to protect shipped
goods from damage.
Equilibrium: Condition in which no change occurs in the state
of a system as long as its surroundings are unaltered.
Gas: A phase of matter in which the substance expands readily
to fill any containing vessel; characterized by relatively low
Grain (gr): A unit of mass equal to 1/7,000 of a pound, or
to 6.47989 x 10-5 kilogram (kg).
Gram (g or gm): The unit of mass in the centimeter-gram-second
system of units, equal to 0.001 kg.
Hydrophilic: Having an affinity for, attracting, adsorbing
or absorbing water.
Hydrophobic: Lacking an affinity for, repelling,
or failing to adsorb or absorb water.
Humidity: Atmospheric water vapor content.
Hygroscopic: Readily absorbs or adsorbs moisture and retains
Leakage: Undesired and gradual escape or entry of a quantity,
such as air or water vapor from an enclosure.
Micron: A unit of measure of length or distance equal to 1
x 10-6 meters, or 0.00003937 inches.
Moisture: Water that is dispersed through a gas or liquid in
the form of water vapor or small droplets, dispersed through a solid
or condensed on the surface of a solid.
Pressure: A type of stress which is exerted uniformly in all
directions; its measure is the force exerted per unit area.
Psychrometric Chart: A graph, each point of which represents
a specific condition of a gas-vapor system (such as air and water
vapor) with regard to temperature (horizontal scale) and absolute
humidity (vertical scale); other characteristics of the system, such
as relative humidity, wet-bulb or dew point temperature, specific
volume and enthalpy, are indicated by lines on the chart.
Purging: The act or process of cleaning and purifying by replacing
the current contents with a cleaner or purer medium.
restoring (through the application of heat) of the adsorption capacity
of a desiccant.
Relative Humidity (RH): The ratio of the
actual water vapor pressure of the air-water vapor mixture to the
saturation water vapor pressure, stated as a percent (dimensionless).
Saturation: The point where no more of a substance can be
absorbed, adsorbed, dissolved or retained. When referred
to desiccant, saturation occurs when the desiccant material will
no longer take on moisture for the condition it is exposed to. For
water vapor in air, when saturation occurs, liquid water precipitates
(or rains) out of the air-water vapor mixture.
Temperature: A measure of the internal energy
of a substance, usually referred to as the degree of hotness or
coldness as measured on a specific scale.
of Desiccant: Desiccants
are usually sold by performance (rate, capacity), not by weight. A
standard performance definition is the desiccant unit. One
unit of desiccant is defined as having an adsorption capacity of
6 grams of water at 77°F (25°C) and 40% RH. For instance,
a unit of silica gel weighs 26 grams, a unit of molecular sieve
weighs 32 grams and a unit of clay weighs 33 grams.
Vacuum: A space in which the pressure is far below normal atmospheric pressure.
Vapor Pressure: The partial pressure of water vapor in the atmosphere.
Water Vapor: Water in the form of a gas, especially when below the boiling
point and diffused.
Water Vapor Transmission Rate (WVTR): The rate that water penetrates through
the wall or barrier of an enclosure, usually given in g/(m2 x day)
or g/(in2 x day).
Wet-Bulb Thermometer: A thermometer having the bulb covered with a cloth,
usually muslin or cambric, saturated with water in which the temperature reading
is affected by the amount of humidity in the air.