Pressure relative to a vacuum e.g. atmospheric pressure =1 atm (abs).Anhydrous
Describes a material which contains a minimal quantity of water.Atmosphere (atm)
Unit of pressure equivalent to atmospheric pressure at sea level (1013.25 mbar).Autoignition temperature
Minimum temperature for a combustible mixture to initiate a self-propagating reaction.Azeotrope
A liquid mixture in which the vapour produced by partial evaporation of the liquid has the same composition as the liquid.
The main gas constituent in a gas mixture.bar
Unit of pressure equal to 105 newtons per square metre (= 14.5psi or 1.02kg/cm2)Blanketing
Use of an inert gas (usually nitrogen, argon or helium) to reduce risk of flammability.Boiling point (bp)
Temperature at which the vapour pressure of a substance equals that of the surrounding atmosphere.BS 5355
British Standard relating to filling ratios and developed pressures for liquefiable and permanent gases.BS 5750
A series of British Standards, which have now been superseded by the ISO 9000 series of standards.BTCA
British Technical Council of the Motor and Petroleum Industries Fuels Committee.
A gas mixture of known composition used for the calibration of a process or laboratory instrument.Calorie (cal.)
Heat required to raise the temperature of 1g of water by 1°C (= 4.18 joules).Calorific value (CV)
A measure of ability of a substance to produce heat (especially fuel gases). Net CV is normally used; this implies that the water produced by the reaction remains as a vapour without giving up its latent heat. Atmospheric pressure, capacities and contents are normally defined in liquid litres.CFC
Chlorofluorocarbon. A fully halogenated compound of chlorine, fluorine and carbon. Substances of this type are controlled under the Montreal Protocol and EU phase out regulations.Check valve / non-return valve
An in-line device designed to permit gas flow in one direction while preventing back flow.Chromatography
Analytical technique used to separate and measure components in a mixture.Compressed gas
A collective term for gaseous chemicals (in the gaseous or liquefied state) when compressed to a pressure in excess of 500mbar in a container or system.Corrosive
A product which will attack many substances it comes into contact with, such as metals, polymers or tissue.Critical pressure
The absolute pressure at which both liquid and gas phases co-exist at the critical temperature.Critical temperature
The temperature of a gas above which it cannot be liquefied, regardless of pressure.Critical volume
The volume occupied by 1.0 gram-mole of the substance at its critical temperature and pressure.Cryogenic liquid
A liquid which boils at temperatures of less than about 110K, such as liquid hydrogen, helium, nitrogen, oxygen, air or methane.
The pressure developed by the contents of the container at a given reference temperature.Density
The weight of a substance per unit volume.Dewar
A vessel for holding cryogenic liquids, usually at atmospheric pressure. Capacities and contents are normally defined in liquid litres.Dewpoint
The highest temperature at which liquid can exist in equilibrium with its vapour at a given pressure.Dip, siphon or eductor tube
A tube fitted internally to the valve of a container for the withdrawal of liquefied gas.Doping gas
A gas or mixture used by the electronics industry to add controlled amounts of impurities to dope silicon or other semiconductors.
Gas used by the electronics industry to remove unwanted material and create necessary channels on semiconductor devices.
Flame Ionisation Detector. A detector used in gas analysis usually to determine the concentration of hydrocarbons.Filling ratio
Ratio of the mass of gas introduced into a container to the mass of water at 15°C that fills the container fitted as for use, i.e. complete with valve, dip tube etc.Filter
A device designed to capture particles in a gas stream which might otherwise harm equipment or a process down stream.Flammable gas
Gas that forms a flammable mixture with air at a concentration of 13% or less, or having a flammable range of at least 12 percentage points, determined at normal atmospheric pressure.Flammable range
The range over which a gas will form a flammable mixture with air.Flashback (flame) arrestor
Prevents flashback by quenching a flame which travels in the opposite direction to the normal gas flow.Flash point
The temperature at which a liquid gives off vapour sufficient to form an ignitable mixture with air near the surface of the liquid or within the vessel.Fractional distillation
A process used to separate the constituents of a liquid mixture due to differences in boiling points.Freezing point
Temperature at which a liquid changes to a solid at atmospheric pressure.
Pressure relative to atmospheric pressure.Gross weight
Total weight of a container and its contents.GWP
Global Warming Potential. The contribution to Global ('Greenhouse') Warming caused by a compound, relative to carbon dioxide. See also TEWI.
A halogenated hydrocarbon containing one or more of the following four halogens: fluorine, chlorine, bromine and iodine.Heat capacity
Quantity of heat required to raise the temperature of a unit mass of material by 1°C.HFC
Hydrofluorocarbon. A compound of hydrogen, fluorine and carbon (many environmentally acceptable refrigerants are of this type as they have no impact on the ozone layer).HGWP
Halocarbon Global Warming Potential. The global warming contribution of a compound, expressed as a fraction of the global warming caused by R11.Hydrocarbon
A compound containing only hydrogen and carbon.HCFC
Hydrochlorofluorocarbon. A compound of hydrogen, chlorine, fluorine and carbon. Substances of this type are controlled under the Montreal Protocol and EU phase out regulations.
A hypothetical gas which behaves according to the gas law: PV = nRT.Inert gas
A gas which will not support combustion and does not react with other compounds under normal conditions and temperatures.ISO 9000
International Quality Standard.
Change in temperature resulting from expansion of a gas vapour. A lowering of temperature is the usual result of such an expansion.
The quantity of heat absorbed or released by a unit mass when changing from one phase to another without a change in temperature.LEL
Lower Explosive Limit. The lowest concentration of a flammable gas in air that will propagate a flame from the ignition source.Liquefiable gas
A gas which, at certain conditions of temperature and pressure, remains in a liquid state in the cylinder. Normally defined as any gas having a critical temperature of 20°C or above.
Manifolded Cylinder Pallet. A number of cylinders contained within a pallet, connected together to a common outlet.MAP
Modified Atmosphere Packaging. A method of food packaging where air is removed and replaced by a variety of gas mixtures.MEL
Maximum Exposure Limit. Maximum concentration of an airborne substance, averaged over a reference period, to which employees may be exposed by inhalation under any circumstances.Mole (of a substance)
One mole is equal to 6.022 x 1023 atoms, molecules etc of a substance.Molecular weight
Weight of one molecule of a substance in units of atomic weight. Molecular weight may be computed by summing the atomic weights of the component atoms of a molecule.Montreal Protocol
An agreement controlling production and consumption of CFCs, HCFCs and halons.
Weight of the contents of a container.NIST
National Institute of Standards and Technology - American standards authority. Provides international gas standards for traceability purposes. Fomerly the National Bureau of Standards (NBS).Normal boiling point
Boiling point at 1 atm.NPL
National Physical Laboratory - an Executive Agency of the Department for Business, Innovation and Skills (BIS). In the context of calibration gas mixtures, the NPL provides international cylinder gas standards that are accepted as primary. These are known as Secondary Reference Material (SRM).
Ozone Depletion Potential - the contribution to stratospheric ozone depletion caused by organic chlorine-containing compounds. (Usually expressed as a fraction of the ozone depletion caused by R11).OEL
Occupation Exposure Limit. Recommended exposure limit to materials as specified by the HSE.OES
Occupational Exposure Standard is the concentration of an airborne substance, averaged over a reference period, at which, according to current knowledge, there is no evidence that it is likely to be injurious to employees if they are exposed by inhalation, day after day, to that concentration.
Defined by Dalton as the pressure exerted by the gas in a mixture. The sum of the partial pressures equals the total pressure.Permanent gas
A gas, which at the usual filling temperature and pressure, remains in a gaseous state in the cylinder i.e. has a critical temperature below 20°C.PPB
Parts per billion (1.0 x 10-9). A convenient means of expressing very low concentrations of a substance in a mixture, or a low level contaminant in a pure product.PPM
Parts per million (1.0 x 10-6). A convenient means of expressing very low concentrations of a substance in a mixture, or a low level contaminant in a pure product.Pressure regulator
A device for regulating a generally variable inlet pressure to a stable or constant outlet pressure.Pressure relief device
A generic term for a device designed to reduce the possibility of a failure of a pressurised container or system due to the development of excessive pressure.Propellant
A liquefiable gas giving a constant source of pressure at a specified temperature. This pressure can be used in aerosols to move or propel products.psia
Pounds per square inch absolute.psig
Pounds per square inch gauge.Purging
Operational procedure to remove contaminants from a container or system.Pyrophoric substance
A substance that self-ignites when exposed to normal atmospheric conditions.
Refers to those constituents of air which comprise less than 1% of air and are generally considered inert: argon, helium, krypton, neon and xenon.Refrigerant
The medium of heat transfer in a refrigerating system which absorbs heat by evaporating at a low temperature and a low pressure while giving up heat on condensing at a higher temperature and pressure.Residuals
Small quantities of undefined impurities.
Supercritical Fluid Chromatography. An analytical technique using a fluid above its critical temperature and pressure as the mobile phase in chromatographic separation.SFE
Supercritical Fluid Extraction. An extraction using fluid above its critical temperature and pressure as a solvent.Specific gravity
Ratio of the mass of a gas to the mass of an equal volume of air at a specified temperature. It is dimensionless. For liquids and solids it is the ratio of the mass of a body to the mass of an equal volume of water.Specific heat
Amount of heat required to raise a unit weight of substance one degree of temperature at either constant pressure or constant volume. Usually expressed in joules per kilogram per degree Kelvin.Specific heat ratio
Ratio of specific heat at constant pressure to the specific heat at constant volume at a particular temperature.Specific volume
Volume of a unit weight of substance at a given temperature. Usually expressed as cubic metres per kilogramme.SRM
Secondary Reference Material. In the context of calibration gas mixtures, SRMs are cylinder gas mixtures provided by a national standards laboratory (e.g. NPL or NIST) that are accepted as primary standards.
The weight of an empty container.TEWI
Total Equivalent Warming Impact (of a refrigerant). The sum of the direct contribution to global warming caused by a refrigerant plus the indirect global warming caused by the carbon dioxide released when fossil fuels are burned in the generation of electricity required to operate a refrigeration system during its lifetime.THC
Refers to the Total Hydrocarbon Content. Usually used to describe the quantity of a hydrocarbon impurity present, expressed as methane equivalents.Thermal conductivity
Property of a material which describes the rate at which heat will be conducted through a unit area of material for a given driving force. It is dependent on the material and upon its temperature.TLV
Threshold Limit Value - US equivalent to the UK's exposure limit (see MEL and OES).Torr
Unit of vacuum measurement in mm of mercury.Toxic gas
Gas which is injurious to health.Triple point
Temperature at which solid, liquid and gaseous forms of a substance coexist.TWA
Time Weighted Average
Upper Explosive Limit. The highest concentration of a flammable gas that will propagate a flame from the ignition source.UKAS
United Kingdom Accreditation Service.Ullage or vapour space
The space provided in a container for the expansion of liquid resulting from an increase in temperature.ULSI
Ultra Large Scale Integration.
Heat exchanger used for vaporising liquid gas as it leaves the supply vessel.Vapour pressure
The pressure of a gaseous form of a substance which is in equilibrium with the liquid or solid state.VLSI
Very Large Scale Integration.VOC
Volatile Organic Compound.VPB
Parts per billion by volume.VPM
Parts per million by volume.
Water capacity of a container
The mass of water at 15°C that fills the container fitted for use, i.e. complete with valve, dip tube, etc. as necessary.Wobbe index
Calorific value of a fuel gas divided by the square root of its relative density.
Gas used as a carrier for gaseous samples in gas chromatography and other gas analysis equipment. The zero gas gives 'zero base line response'.