Glossary of Terms – Desiccators

Absolute (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 psia.)


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.


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.


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.


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.


Movement of gas, vapors or air in and out of an enclosure as a result of expansions and contractions induced by temperature or pressure change (such as altitude change).


The process by which vapor becomes a liquid and energy is released.


Motion 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.


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.


Cushioning material in a container to protect shipped goods from damage.


Condition in which no change occurs in the state of a system as long as its surroundings are unaltered.

Gas (Vapor)

A phase of matter in which the substance expands readily to fill any containing vessel; characterized by relatively low density.

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.


Water, in any phase, solid (ice), liquid (water), or gas (water vapor).


Having an affinity for, attracting, adsorbing or absorbing water.


Lacking an affinity for, repelling, or failing to adsorb or absorb water.


Atmospheric water vapor content.


Readily absorbs or adsorbs moisture and retains it.


Undesired and gradual escape or entry of a quantity, such as air or water vapor from an enclosure.


A unit of measure of length or distance equal to 1 x 10-6 meters, or 0.00003937 inches.


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.


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.


The act or process of cleaning and purifying by replacing the current contents with a cleaner or purer medium.


The restoring (through the application of heat or other processes) 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) and varies based on temperature.

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) and varies based on temperature.


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.


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.

Unit 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.


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 permeates 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.

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