Absolute Pressure - The total pressure applied on a system, equal to the guage pressure plus atmosheric pressure.  Absolute pressure is designated as PSIA.  Generally pressure measurements employ a guage to record the variation between the system and the atmosheric pressure.  This is called guage pressure and is designated as PSIG.
Atmospheric Pressure - The pressure exerted by the weight of the atmosphere at sea level, equal to 14.696 PSI or 0.98692 Bar. 
Absolute Zero - The zero point of the ideal gas temperature scale designated as 0 Kelvin, -273.15° Celsius and -459.67° Fahrenheit. 
Absorption - Physical or chemical process in which the atoms or molecules of a material penetrate into another material.  The atoms or molecules are absorbed by the volume in the material, not the surface.
Adsorption - Adhesion of the atoms or molecules of a material to the surface of another material (called an adsorbent).  Usually a gas, a liquid or a dissolved substance will attach to the surfaces of a solid.
Anhydrous - The general term characterizing a material that does not contain any water molecules.
Atomic Weight - Ratio of the average mass of a chemical element's atoms to carbon-12.  The standard atomic weight of a naturally occurring element is the ratio of the weighted average of all of the naturally occurring isotopes to carbon-12, expressed in atomic mass units.
Atomic Mass Unit - 1/12th the mass of carbon-12.
Asphyxiant Gas - Non-toxic gas that can remove the oxygen contained in the atmosphere, resulting in unconsciousness and death.
ACGIH - American Conference of Government Industrial Hygienists - Professional Society devoted to the advancement of occupational and environmental health.
Boiling Point - Temperature at which the pressure of a liquid is equal to the pressure sustained by the surrounding atmosphere.  See Normal Boiling Point.
BTU - British Thermal Unit - The measure of heat required to raise the temperature of one pound of water 1° F.
Calibration Gas - A gas or gas mixture of precisely known composition used as a comparative standard.
Calorie - The measure of heat required to raise the temperature of one gram of water 1° C.
Carrier Gas - A carrier gas carries a volatile sample through a separation column normally composed of a molecular sieve, alumina or porous polymers to a detector.  The gas is called the mobile phase and is chosen by it's chemical inertness and velocity though the column packing, the stationary phase.  The gas carries or propels the sample through the packing.  The packing slows down the sample and causes a separation of the sample into it's detectable components.  The temperature, gas pressure, column length, composition and detector are carefully selected to produce a consistent and reproducible process.  The sample's components are compared to a known standard to determine actual concentration.

The gas must be clean, dry and as chemically pure as possible to assure high accuracy, consistency and to avoid contaminating the column packing.  Normally, helium or hydrogen are chosen because of their low molecular weight and size, relative inertness and ease in purification.  There are significant response differences between these gases adn the samples under analysis.  occasionally nitrogen is also used in some application. 

Catalyst - A substance that reduces the activation energy of a chemical reaction.  Catalysts enter into reactions but are not consumed by the reaction.
CGA - Compressed Gas Association - Professional Organization dedicated to the enrichment and furtherance of safety standards and safe practices in the industrial gas industry. 
CAS - Chemical Abstract Services - A division of the American Chemical Society (ACS) that provides comprehensive databases of publicly divulged research in chemistry and related sciences, including the world's largest collection of substance information, the CAS Registry SM. 
Compressed Gas - Any material or mixture with a pressure exceeding 40 PSIA at 70° F or having an absolute pressure exceeding 104 PSIA at 130°F or any flammable liquid having a vapor pressure over 40PSI at 100°F.
Corrosive - A chemical compound which noticeably destroys or irretrievably damages living tissue or chemically attacks and eats away rubber, metal or other substances.
Critical Temperature - The minimum temperature at which a gas cannot be liquefied, no matter how much pressure is applied.  At this temperature, the liquid and gas have the same density.
Critical Pressure - The pressure required to liquefy a gas at its critical temperature.
Cryogenic Liquid - A liquid with a boiling point below -60° C.
Density - The ration of the mass of a substance to its volume.
DOT - Department of Transportation - Government agency whose purview is to regulate the transportation of hazardous materials.
Dew Point - The temperature at which water vapor starts to condense into liquid water.
EPA - Environmental Protection Agency - Government agency established in 1970 for the establishment and enforcement of environmental protection standards.
Fill Density - The percent ratio of the weight of a liquefied gas in a container to the weight of the water that the cntainer will hold at 68° F.
Flammable Limits - The concentration of a flammable vapor in air at normal atmospheric pressure and temperature that will propagate a flame upon contact with an ignition source.  See LEL, UEL.
Flash Point - The lowest temperature at which a liquid gives off enough vapor to form a flammable mixture with air.
Inert Gas - A gas which does not react with other materials at NTP.
Liquefied Compressed Gas - A gas that can be liquefied in a container with increased pressure at normal temperature.
LEL - Lower Explosive Limit -  The maximum concentration of a specific flammable vapor in air at NPT that will propagate a flame upon contact with an ignition source.  Concentrations below this level will not propagate a flame upon contact with an ignition source.
Molecular Weight - The sum of the atomic weights of all atoms in a molecule.
Normal Boiling Point - The temperature at which the vapor pressure of a liquid equals 1 atmosphere (14.696 PSIA).
NTP - Normal Temperature and Pressure - 20° C and 760 torr.
OSHA - Occupational Safety and Health Administration - An agency of the Department of Labor whose mission is to prevent work related injuries and illnesses by issuing and enforcing standards for workplace safety and health.
Oxidizer - Substance that causes or contributes to the combustion of another material.  An oxidizer is not necessarily combustible in itself.
PPM - Parts Per Million - Notation for specifying precise amounts of very low concentrations of chemical elements or compounds.  For gases, denotes the number of molecules of a gas or gas mixture found in every 1 million molecules of the gas mixture. 
Specific Gravity - The ration of the weight of a given vlume of a substance to the weight of an equal volume of a reference material.  Liquids and solids are normally compared to water (sp. gr. of H2O = 1), while gaes are normally compared to air (sp. gr. of air = 1).
Specific Heat - The amount of heat required to change the temperature of a unit of mass (or a quantity, such as a mole) of a substance one degree at either constant pressure (CV) or constant temperature (CP).
Specific Volume - The volume occupied by a unit mass of a material at a given temperature.  The specific volume of a gas is normally expressed as cubic feet of volume per pound of gas. 
STP - Standard Temperature and Pressure - 0° C at 760 Torr.
Torr - Common unit of measure for vapor pressure. 1 Torr = 1 mm of mercury.
Toxic Gas - A gas that has an LC50 in air of 200 PPM or less.  Even low concentrations of a gas may cause injury or death.
UEL - Upper Explosive Limit - The maximum concentration of a specific flammable vapor in air at NTP which will propagate a flame upon contact with an ignition source.  Concentrations above this level will not propagate a flame upon contact with an ignition source.
Vapor Pressure - The pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature.  The vapor pressure of gases is normally measured at 70° F.