Gas Detection Glossary


Needing a better idea as to what something means that you saw on our site or in your user manual for your new gas detector? We got you covered! Certified Air Safety's exclusive Gas Detection Glossary currently holds hundreds of industry related terms with clear, concise definitions. Are we missing something? Let us know and we'll get you the answer!

 Absolute Pressure

A measurement of pressure which sets a total vacuum as having a value of zero. For example the commonly used term psia stands for pounds per square inch of absolute pressure.

Absolute Zero

The lowest point in the Kelvin temperature scale. 0°K = - 459.67°F or -273.15°C in the Farenheit or Celsius temperature scales, respectively.


The physical penetration of a substance into the structure of another substance, such as the dissolution of a gas into a liquid.


Expresses the degree of agreement of a measured value when compared to the true or expected value of the
quantity of concern. This term is often confused with precision, which is the range of the confidence level that a measured
value can be considered valid.


American Conference of Governmental Industrial Hygienists, a group which makes recommendations on the exposure levels of hazardous materials in the workplace.


The adherence of molecules, ions or atoms of a gas or liquid to the surface of another substance. The adsorbed species is thought to be adhered to the surface by weak physical or chemical forces.


Describes gases that contain oxygen and which are commonly used as atmospheres for biological culture growth.


A suspension in air (or gas) of minute particles of a liquid or a solid.


American Industrial Hygiene Association (AIHA): A membership organization of occupational and environmental health professionals practicing industrial hygiene in industry, government, labor, academic institutions, and independent organizations. Founded in 1939, a nonprofit organization with more than 75 local sections. AIHA's 11,600 members are highly educated professionals; 96 percent are college graduates, 51 percent hold master's degrees, and 12 percent possess doctoral degrees.

Alarm Only Instrument

An instrument providing an alarm(s) which does not have an integral meter or other readout device indicating current concentration levels.

Alarm Set Point

The selected gas concentration level where an alarm is activated.


A registered trademark for Specialty Gas Products offered by Air Liquide Group S.A. worldwide.


A proprietary passivation process developed by Air Liquide for treating the inner walls of cylinders to render
them inert. The process prevents the adsorption for reactive compounds in the gas from adhering to the walls of the cylinder.
The process also removes any compounds on the cylinder walls that may react with components in the gas.

Ambient Air

The concentration of air (20.9% VOL) to which an Oxygen sensing element is normally exposed.

Ammonia, (NH3)

Molecular formula (NH3). CAS Number: 7664-41-7
UN1005 Normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers. Ammonia is the starting material for nitric acid manufacture; Used in calibration gas mixtures for environmental emission monitoring, petrochemical industry, industrial hygiene monitors and trace impurity analysis; Used in semiconductor manufacturing and advanced materials for the deposition of silicon nitride (Si3N4) by Chemical Vapor Deposition (CVD); Used instead of chlorofluorocarbons (freons) in refrigeration.


Describes gases that do not contain oxygen which are used for biological culture growth.


An instrument which can determine qualitatively and quantitatively the components in a mixture.


Literally means without water. The term is often used with those gases that are particularly corrosive in the presence of moisture, such as ammonia.

Annealing Gas

A gas blend used as a reducing atmosphere in the metals industry during heating to render them less brittle. A commonly used furnace gas consists of a blend of hydrogen and nitrogen.


The acceptance by an authority having jurisdiction. This term is typically considered synonymous with "listed" or "certified."

Area Monitor

A term that is often misleading. When applied to gas monitoring sensors, a true area monitor must be able to measure the concentration of a substance at any point in three dimensional space in a defined value or it must be able to indicate the total quantity of a substance that has penetrated a defined volume.

Arsine, (AsH3)

Molecular formula (AsH3). CAS Number: 7784-42-1
UN2188 A Flammable, pyrophoric, and highly toxic gas, the simplest compound of arsenic. Applications in the semiconductor industry and for the synthesis of organoarsenic compounds. Exclusively used in semiconductor manufacturing. The Arsenic atom is an n-type dopant for epitaxial silicon. Arsenic is introduced in the silicon wafer by diffusion or implantation techniques.

Asphyxiant Gas

A gas which displaces air in an enclosed space and which can cause unconsciousness or death due to lack of oxygen.


American Society of Safety Engineers, (ASSE), founded October 14, 1911, the oldest and largest professional safety organization. With more than 32,000 members, the ASSE manages, supervises and consults on occupational safety and health and environmental issues in industry, insurance, government and education. Along with other organizations including Occupational Safety and Health Administration (OSHA), The ASSE takes part in the North American Occupational Safety and Health Week (NAOSH), helping to raise awareness of occupational safety and health issues.


American Society of Testing Materials, an organization which sponsors committees which develop standards for Industrial Manufacturers and Consumers.

Avogadro's Law

One of the gas laws which states that equal volumes of gases at the same temperature and pressure contain the same number of molecules.

Avogardo's Number

The number of molecules in one mole or gram-molecular weight of a substance (6.0221367X1023 molecules/gm-mole).


A mixture of two substances that typically cannot be separated easily by simple distillation. A commonly used term to describe a liquid mixture that has a constant boiling point.


ABritish approval agency. The Electrical Equipment Certification Service (EECS) is based at the Health and Safety Laboratory's Buxton research center in Derbyshire, England. BASEEFA provides a range of testing and certification services primarily related to equipment and systems intended for use in potentially explosive atmospheres.

Benzene, (C6H6)

Molecular formula, (C6H6) CAS Number: 71-43-2 A colorless and highly flammable liquid with a sweet smell and a relatively high melting point. Because it is a known carcinogen, its use as an additive in gasoline is now limited, but it is an important industrial solvent and precursor in the production of drugs, plastics, synthetic rubber, and dyes. A natural constituent of crude oil, and may be synthesized from other compounds present in petroleum. Benzene is an aromatic hydrocarbon.

Biosystems (gas detection products)

Founded in 1981 and headquartered in Middletown Connecticut, Biosystems is a wholly owned subsidiary of "Sperian Protection". As a manufacturer of portable and fixed gas detection systems for portable, confined space and area monitoring, Biosystems is an industry leader in the field of life safety instrumentation.

Boiling Point (BP)

The temperature of a liquid at which the vapor pressure is equal to the pressure of the atmosphere above it.

Boron trichloride, (BCl3)

Molecular formula (BCl3). CAS Number: 10294-34-5
UN1741 A colorless gas, a valuable reagent in organic synthesis. Dangerously reactive. Largely used in semiconductor manufacturing. Boron atom is an p-type dopant for epitaxial silicon.

Bourdon Tube

A curved metal tube commonly used in pressure gauges. The tube flexes a known degree as pressure is applied, and that movement is translated as the physical movement of a gauge needle across a scale.

Boyle's Law

A gas law which states that for an Ideal Gas at constant temperature the volume of the gas is inversely proportional to the pressure applied.

Bromine Trifluoride, (BrF3)

Molecular formula (BrF3). CAS Number: 7637-07-2
UN1008 A toxic, colorless, and corrosive liquid, soluble in sulfuric acid, explodes on contact with water and organic compounds. A potent fluorinating agent and ionizing inorganic solvent. Used to produce uranium hexafluoride, UF6 in the processing and reprocessing of nuclear fuel.


An abbreviation for British Thermal Unit, a unit of energy defined as the quantity of heat needed to raise the temperature of one pound of water 1°F.

Bump Gas

A cylinder of calibration gas typically packaged in an Aerosol container and sprayed over the sensor housing like a spray paint. Never to be used for a calibration procedure, these cylinders often contain much higher concentrations of gas components than is required for a correct calibration procedure.

Burst Pressure

The designed test pressure at which a gas-containment device such as a cylinder, piping or pressure adjusting device will begin leaking but not violently rupture. For most gas handling equipment, the industrial standard is that the burst pressure is four times (400%) of the normal operating pressure.

Butane, (C4H10)

Molecular formula, (C4H10) CAS Number: 106-97-8 Also called n-butane, the unbranched alkane with four carbon atoms, CH3CH2CH2CH3. Butane is also used as a collective term for n-butane together with its only other isomer, isobutane. Butanes are highly flammable, colorless, odorless, easily liquefied gases.

BW Technologies

Based in Calgary, Alberta Canada. BW Technologies is a wholly owned subsidiary of Honeywell. BW Manufactures and markets a full line of gas monitoring equipment for various industries including mining, police, fire, rescue, shipping, transportation, telecom, oil and gas industries. BW offers a broad line of gas detection equipment including portable, hand-held instruments as well as fixed detectors that can be stationed in buildings, field sites or stations. BW Technologies products provide personnel with an early warning of poisonous gases such as VOCs (PID), H2S, CO, O2, SO2, PH3, NH3,NO2, HCN, Cl2, ClO2, O3, and Combustible gases. 


Calgaz is the recognized global leader for calibration gas mixtures and related equipment. Unique production methods and dedication to quality, combined with unparalleled service and support have defined Calgaz as the standard for calibration gases. Calgaz provides calibration gases in non-refillable cylinders and features a full line of regulators manufactured in Cambridge, Maryland, USA at a state-of-the-art production facility. Calgaz is a wholly owned subsidiary of Air Liquide, the world's largest industrial gas company, operating in more than sixty countries.

Calibration Procedure

The procedure for adjusting a gas detecting instrument for proper response: e.g., zero level, span, alarm and range

Calibration Gas Standard

A gas mixture that has been accurately analyzed against a known reference standard. This mixture can be used as a comparative standard for determinations on analytical instruments.


The amount of heat required to raise the temperature of one gram of water at 15°C by one degree Celsius

Capillary Cell, (O2 Sensor)

Capillary cell clectrochemical sensor technology is replete with many different types of sensing devices that detect the presence and concentration of various gases. All sensors contain components that electrically and/or chemically analyze gases that are exposed to those components. If the sensor is designed merely to detect the presence of a certain gas, the exposure of the analyzing components to the gas is typically maximized within the confines of the sensor. In some sensors, a porous barrier is placed in front of the gas analyzing components of the gas sensor. Gas diffuses through the barrier at a known rate, thereby providing a controlled gas exposure. Porous barriers
easily become clogged with dust or other air-borne debris the adversely affect the performance of the sensor. Capillary gas sensors eliminate the use of porous barriers by replacing them within solid barriers having a single precision capillary opening formed in the center. Gas passes through the capillary opening at a rate which is determined by the diameter and the length of the capillary opening. The size and length of the Capillary opening can be precisely controlled. As a result numerous barriers can be manufactured that have a gas diffusion rate within a very small range of tolerances. In addition, Oxygen sensors that are designed with this technology compensate for variances of altitude and Barometric pressure automatically, as opposed to Galvanic Electrochemical sensors which can have wide fluctuations in readings due to changes in altitude and weather patterns.

Carbon Dioxide, (CO2)

Molecular formula, (CO2) CAS Number: 124-38-9
UN1013 (gas); UN2187 (liquid refrigerated); UN1845 (solid). A chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. A gas at standard temperature and pressure and exists in Earth's atmosphere in this state. Prouced from the combustion of coal or hydrocarbons, the fermentation of liquids and the breathing of humans and animals. Found in small proportions in the atmosphere, it is assimilated by plants which in turn produce oxygen. CO2 gas has a slightly irritating odor, is colorless and heavier than air. It cannot sustain life. It freezes at -78.5 °C to form carbon dioxide snow. In an aqueous solution it forms carbonic acid, which is too unstable to be easily isolated.

Carbon Monoxide, (CO)

Molecular formula, (CO). CAS Number: 630-08-0
UN1016 A colorless and odorless, tasteless, yet highly toxic gas. Molecules consist of one carbon atom covalently bonded to one oxygen atom. Formed from the combination of a carbon atom with an oxygen atom. Flammable, highly toxic and odorless. Produced, from incomplete combustion due to lack of oxygen. Can cause poisoning, or death if heating systems are poorly maintained. Produced on a large scale in industry, in combination with hydrogen, by reforming hydrocarbons, generally natural gas. Used in large quantities to produce various intermediary organic chemicals, such as acetic acids, isocyanates, formic acid, also certain polymers such as polycarbonates and polyketones.

Carrier Gas

The gas which flows through a separation column of a gas chromatograph and propels a sample to a detector.

CAS Number

A Chemical Abstract Services numbering system assigned to each new chemical as it is reported in the
world’s literature. Virtually every commercially manufactured chemical has been assigned a CAS number that allows it to be
easily identified.


A substance which initiates or accelerates a chemical reaction. CAS Number A Chemical Abstract Services numbering system assigned to each new chemical as it is reported in the world’s literature. Virtually every commercially manufactured chemical has been assigned a CAS number which allows it to be easily identified.

Catalytic bead Sensor

A combustuible gas sensor consisting of two coils of fine platinum wire each embedded in a bead of alumina, connected electrically in a Wheatstone bridge circuit. One of the pellistors is impregnated with a special catalyst which promotes oxidation whilst the other is treated to inhibit oxidation. Current is passed through the coils so that they reach a temperature at which oxidation of a gas readily occurs at the catalysed bead (500-550°C). Passing combustible gas raises the temperature further which increases the resistance of the platinum coil in the catalysed bead, leading to an imbalance of the bridge. This output change is linear, for most gases, up to and beyond 100% LEL, response time is a few seconds to detect alarm levels (around 20% LEL), at least 12% oxygen by volume is needed for the oxidation.


A temperature scale that has been set up so that ice melts at 0° and water boils at 100°C


Continuous emission monitor, a device used to measure the emissions typically from an exhaust stack on a continuous
basis. Also refers to the gas standards used to calibrate these monitors.

Certificate of Analysis (COA)

A printed guarantee by a gas producer that a particular gas has been analyzed to the levels of purity or impurity stated.

Certified Concentration

The concentration printed on the certificate. The numerical concentration assigned to a component that is printed on the Certificate of Analysis or Accuracy. This is the concentration represented to the customer for that component and the concentration the customer is supposed to use. The certified concentration may be either the blended or analyzed concentration.

Certificate of Conformance (COC)

A printed guarantee by a gas producer that a particular gas meets a recognized standard.


Absorption and emission of light by a chemical compound. Chemiluminescence detectors function by monitoring this absorption and emission of light by a substance at certain wavelengths.

Chlorine Dioxide, (ClO2)

Molecular formula, (ClO2). CAS Number: 10049-04-4 A reddish-yellow gas, crystallizes as orange crystals at −59 °C. One of several oxides of chlorine, A potent and useful oxidizing agent used in water treatment and in bleaching.

Chlorine, (CL2)

Molecular formula, (CL2) CAS Number: 7782-50-5
UN1017A In its common elemental form (Cl2 or "dichlorine") under standard conditions, it is a pale green gas about 2.5 times as dense as air. Has a disagreeable, suffocating odor that is detectable in concentrations as low as 1 ppm, and is choking and poisonous. Chlorine is a powerful oxidant, used in bleaching and disinfectants. A common disinfectant, chlorine compounds are used in swimming pools to keep them clean and sanitary.


An analytical method where a mixture is physically separated into its individual components.

Coefficient of Flow (CV)

Rate of flow through a regulator or other gas handling device measured in U.S. gallons per minute at 60°F with a pressure differential of 1 psig

Colormetric Gas Sampling Tube

Colormetric sampling tubes are glass tubes filled with reagents that change color in reaction to certain chemicals. A Wide variety of gases can be measured and they can measure many gases that cannot be measured by direct-reading instruments. The disadvantages of colormetric tubes are a level of accuracy of ±25% under ideal conditions. Must use hand pump (or electronic pump ) and wait for reaction to take place. Cannot provide continuous monitoring or sampling. Examples of Tubes Vendors: GasTec, Dräger Safety, Kitagawa (Matheson), Sensidyne.

Compressed Gas

A gas in a container which meets one of the following criteria: Contained at pressures exceeding 40 psia at 70°F. Contained at pressures exceeding 104 psia at 130°F. A flammable liquid having a vapor pressure exceeding 40 psia at 100°F as determined by ASTM D-323-72

Compressed Gas Association (CGA)

A nonprofit technical organization which develops and promotes industry standards for the safe handling, transport and storage of compressed gases.

Confined Space

A Confined space is any space that: (1) that has limited or restricted means of entry or exit; (2) is large enough for a person to enter to perform tasks; (3) is not designed or configured for continuous occupancy and (4) is any covered space of depth more than 4 feet. A utility tunnel, the inside of a boiler (only accessible when the boiler is off), the inside of a fluid storage tank, a septic tank that has contained sewage, and a small underground electrical vault are all examples of confined spaces. The exact definition of a confined space varies depending on the type of industry. That is, confined spaces on a construction site are defined differently than confined spaces in a paper mill. Confined spaces that present special hazards to workers, including risks of toxic or asphyxiant gas accumulation, fires, falls, flooding, and entrapment may be classified as permit-required confined spaces depending on the nature and severity of the hazard.
In the U.S., entry into permit-required confined spaces must comply with regulations promulgated by the Occupational Safety and Health Administration. These regulations include developing a written program, issuing entry permits, assigning attendant(s), designating entrants, and ensuring a means of rescue.

Correction Factor, (PID)

A "Correction Factor" adjusts Sensitivity when Measuring Pure Compounds Correction Factors are scaling factors used to adjust the sensitivity of the PID to directly measure a particular
gas compared to the calibration gas. For example, a PID is nearly twice as sensitive to benzene (CF=0.53) as it is to its calibration gas of isobutylene (CF=1.00). So if we are measuring 1 PPM of benzene, after calibrating on isobutylene, we have two options:
• We will see approximately 2 PPM on the display of the PID. If we multiply this reading by 0.53 we will get
the true reading of benzene.
• We can apply the CF of 0.53 during calibration so that the display will automatically read in PPM benzene.
By using a CF of 0.53 we can reset the internal scale of the PID to read 1 PPM of benzene even though we calibrated on isobutylene


The ability of a chemical to attack another substance, causing irreversible damage. The term applies to substances which attack human tissue and other materials it may come in contact.

Cracking Pressure

The inlet pressure at which a gas begins to flow through a regulator, valve or other pressure-control device.


The slow increase in the outlet pressure of a regulator which may be caused by changes in inlet pressure or failure of the regulator seat.

Critical Density

The density of a pure substance at its critical point.

Critical Point

The point of a temperature vs. pressure curve of a pure substance above which a gas cannot exist in both gas and liquid phases.

Critical Pressure

The pressure at the critical point above which a pure gas cannot be liquefied.

Critical Temperature

The temperature above which a gas cannot be liquefied by pressure alone.

Cryogenic Liquid

A liquid having a normal boiling point below -240°F. (-151.1°C).

Cryogenic Vessel

An insulated container for the storage, transport and dispensing of liquids having a boiling point below -130°F.


Canadian Standards Association An approval agency based in Canada.

Custom Certified Standard

Custom Certified Standards are prepared gravimetrically but must be certified against traceable reference standards. Gravimetric numbers may be used in place of analytical numbers but only if they are determined to have less uncertainty. The method of determination of the reported number is stated on the Certificate of Analysis.


A container designed to safely hold compressed gases and which is designed and tested to meet government specified standards of construction. 

Dalton's Law of Partial Pressure

One of the gas laws which states that for ideal gases the pressure of a gas blend is equal to the sum of the pressures of each of its components.


The mass of a substance divided by its volume.

Department of Transportation (DOT)

Federal agency that regulates the transport of hazardous materials per Title 49, Code of Federal Regulations.

Dew Point

The temperature at which a gas vapor begins to condense as a liquid.


A vessel that is usually portable and is used to contain cryogenic liquids.

Diameter Index Safety System (DISS)

Type of valve designed with metal to metal seals for high leak integrity, generally used for high purity, corrosive or toxic gases

Diborane, (B2H6)

Molecular formula (B2H6). CAS Number: 19287-45-7
UN1911 A colorless gas at room temperature, repulsively sweet odor. Diborane mixes well wit+B61h air, easily forms explosive mixtures. Will ignite spontaneously in moist air at room temperature. Synonyms include boroethane, boron hydride, and diboron hexahydride. In tetrahydrofurane, diborane is a powerfull reducing agent in organic synthesis. Largely used in semiconductor manufacturing. The Boron atom is a p-type dopant for epitaxial silicon. Introduced in silicon wafers by diffusion or implantation techniques. Also used for the production of borophosphosilicate glasses (BPS).

Dichlorosilane, (H2SiCl2)

Molecular formula (H2SiCl2)
CAS Number: 4109-96-0
UN2189 Commonly known as DCS, usually mixed with ammonia (NH3) in LPCVD chambers to grow silicon nitride in semiconductor processing. In semiconductor manufacturing, dichlorosilane is a silicon-precursor gas used in chemical vapor deposition processes with oxygen, nitrogen or metallic (tungsten) compounds to deposit layers of silicon dioxide, silicon nitride or oxinitride or metal (tungsten) silicide layers.

Discharge Ionization Detector (DID)

A universal detector used in gas chromatography where the species detected is ionized by electrical discharges between plates.


An impurity added to a pure substance in small amounts to alter its properties.

DOT Numbers

Product identification numbers assigned to chemicals for shipping purposes that helps in the rapid identification by emergency response teams. The prefix used, UN, designates the United Nations, meaning that these numbers are recognized worldwide.

Dual Certified Primary Standard

Dual Certified Primary Standards are recommended where the greatest accuracy is required. These mixtures are prepared gravimetrically to close tolerances and analyzed against NIST traceable standards. The reported concentrations are tied to both the gravimetric numbers and the analytical results to determine the reported uncertainty. Both numbers should fall within 1% of the reported value. Such dual analysis ensures the greatest accuracy. 

Eductor Tube

A tube inside a cylinder which allows for liquid withdrawal from the bottom of the cylinder when the valve is opened.

Effluent Splitter

The part of an analytical instrument which divides the effluent stream into smaller segments and diverts them to different detectors.

Electron Capture Detector

Chromatographic detector used commonly for halogenated compounds. Electrons are generated by a radioactive source and are captured by the species being monitored. The current drop across two plates is measured.

Environmental Protection Agency (EPA)

The governmental agency responsible for environmental standards in the United States.

EPA Protocol Mixture

Standard gas mixture prepared and analyzed following EPA-600/R-97/121 guidelines. These standards are required for calibration purposes when EPA analytical methods must be followed.

Ethylene Oxide, (C2H4O)

Molecular formula, (C2H4O). CAS Number : 75-21-8
UN1040 A colorless flammable gas with a faintly sweet odor is the simplest epoxide, a three-membered ring consisting of two carbon and one oxygen atom. Commonly handled and shipped as a refrigerated liquid. The chief precursor to ethylene glycol and other high volume chemicals but is also used for medical sterilization.

Exposure Limits

Concentration of substances under which it is believed that nearly all workers can be repeatedly exposed on a daily basis without adverse effects.


A temperature scale that has been set up so that ice melts at 32° and water boils at 212°.

Flame Ionization Detector (FID)

One of the most commonly used detectors for measuring organic compounds in a gas stream. Organic species are decomposed by a hydrogen flame and measured by electrodes near the flame.

Flammability Limits

The extremes of the range at which a gas mixed with air can be ignited with a source of ignition. The lower number is referred to as the lower explosive limit (LEL), and the upper number is called the upper explosive limit (UEL).

Flammable Gas

DOT definition of any gas which either will form a flammable mixture with air at concentrations of 13% or less by volume or have a flammable range wider than 12% regardless of the lower explosive limit (LEL).

Flash Point

The lowest temperature at which a flammable liquid will give off enough fumes to form an ignitable mixture with air directly above the liquid surface.

Fluorine, (F2)

Molecular formula, (F2) CAS Number: 7782-41-4
UN1045 A supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the most chemically reactive and electronegative of all the elements. Will readily "burn" hydrocarbons at room temperature, in contrast to the combustion of hydrocarbons by oxygen, which requires an input of energy with a spark. Molecular fluorine is highly dangerous. Uranium hexafluoride (UF6) is produced by direct fluorination of uranium oxide or UF4 using fluorine. Sulfur hexafluoride (SF6) or sodium fluoride are produced from fluorine.

Fuel Cell Sensor, (ElectoroChemical)

Electrochemical Fuel Cell Sensors are electric batteries which consume gas from outside rather than solid/liquid materials inside them. (Their original application was in space vehicles where hydrogen is consumed to provide electrical power). In
addition to consuming the fuel gas they also consume oxygen. Fuel cell sensors are miniaturized fuel cells which react to low (parts per million) concentrations of gas. They consume minute amounts of gas. The electrochemical reactions produce current (uA) which is linearly proportional to the concentration of gas in air. In theory, because fuel cell sensors consume no internal ingredients, they should have an infinite life. In practice they last 3 or 4 years. 

Galvanic Elecrochemical Sensor

Galvanic Electrochemical Sensors are not fuel cells because electrodes or electrolyte are used up. Oxygen sensors is are
not fuel cells but metal oxygen cells. The metal is gradually consumed and this governs the sensor's life. Ammonia and hydrogen cyanide are measured by consumable or galvanic sensors. The life of these sensors is governed by the amount of gas which they absorb so their life can be shortened when exposed to continuous high levels of gas. Galvanic Fuel cells for Oxygen typically last 18-24 months.


A state of matter in which the individual molecules are almost totally unrestricted by cohesive forces. An ideal gas is one which obeys the gas laws under standard conditions.

Gas Detector

A device used for detecting the presence of various gases; explosive gases such as Methane, toxic gases such as Hydrogen Sulfide and/or for insufficient amounts of oxygen within an area, these units are designed to provide a udible and visual alarms warning about gases which are harmful to humans or animals.

Gas Processing Association (GPA)

Organization consisting of both member companies and suppliers to the Gas Processing Industry. This organization was established to exchange technology related to the industry and to develop standards applying to the processing of gas products.


The Gastec Corporation is a manufacturer of gas sampling products for personal protection and Industrial Hygiene. Founded in 1970 and based in Kanagawa, Japan. Gastec manufactures, instruments for detecting and measuring gases and water quality. Products include detector tubes and instruments for detecting and measuring oxygen, carbon monoxide, hydrogen sulphide, and other toxic gases.


A wholly owned subsidiary of Thermo Science. Gastech provides Industrial Hygiene instruments that are used by a variety of people from industrial hygienists to first responders, in a wide range of situations from occupational health to safety applications. GasTech provides gas and dust detectors for personal monitoring, confined spaces and alerting of potentially dangerous environments. GasTech products continuously monitor for the presence of irritating particulates, combustible gases, and toxic gases.

Gay-Lussac's Law

One of the gas laws which states that for an ideal gas under constant pressure, the volume increase is proportional to the increase in temperature.

Germane, (GeH4)

Molecular formula, (GeH4)
CAS Number: 7782-65-2
UN2192 The simplest germanium hydride and one of the most useful compounds of germanium. Like the related compounds silane and methane, germane is tetrahedral. It burns in air to produce GeO2 and water. Used for the deposition of epitaxial and amorphous silicon-germanium alloy layers (MOCVD).

Gravimetric Certified Standard

Gravimetric Certified Standards are lower cost standards that are suitable for routine analysis or peak identification. The reported
concentrations are determined using gravimetric numbers for each component. Only qualitative laboratory analysis may be
done on these mixtures to confirm each component. Since the concentration of reactive components may change inside the
cylinder, the gravimetric numbers for reactive components may not be a good indication of actual concentration in the blend.
Therefore, the uncertainty of reactive components will not be reported.

Gross Weight

The total weight of both the container and the contents therein.


A family of compounds made up of a hydrocarbon combined with one or more halogens from the group VIIA elements in the Periodic Table. This name is commonly attributed to those compounds in the family which are used for refrigeration systems.

Heat of Adsorption

The total heat generated from the initial adsorption of a compound on an adsorbate to the point at which equilibrium conditions are met and no more adsorption can take place.

Heat of Fusion

The heat energy required to convert one mole of substance from the solid phase to the liquid phase at one atmosphere of pressure.

Heat of Vaporization

The heat energy needed to transform one mole of substance from the liquid phase to the gas phase at one atmosphere of pressure.

Hexane, (C6H14)

Molecular formula: (C6H14) CAS Number: 110-54-3 An alkane hydrocarbon with the chemical formula CH3(CH2)4CH3 or C6H14. A common constituent of gasoline and glues used for shoes, leather products, and roofing. Also used in solvents to extract oils for cooking and as a cleansing agent for shoe, furniture and textile manufacturing. In laboratories, hexane is used to extract oil and grease from water and soil before determination by gravimetric analysis or gas chromatography.


An organic compound which contains both carbon and hydrogen in its molecular structure.

Hydrogen Chloride, (HCl)

Molecular formula, (HCl) CAS Number: 7647-01-0
UN1050 At room temperature, a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry. The formula HCl is often used to refer, somewhat misleadingly, to hydrochloric acid, an aqueous solution derived from hydrogen chloride. The chemical industries use hydrogen chloride to produce a large variety of organic chlorinated compounds (methyl or ethyl chloride, benzene chlorides…). Chlorinated metals (such as aluminium or silicon chlorides) are produced with HCl. Hydrometallurgy processes used hydrogen chloride to enhance the separation coefficient of ores. The hot galvanizing process can use HCl. Hydrogen chloride is used in calibration gas mixtures for environmental emission monitoring. Hydrogen chloride is used in semiconductor fabrication for etching of native oxide, CVD reactor cleaning or moisture getter. Used with xenon in "excimer" lasers, hydrogen chloridel can produce wavelengths which vary as a function of operating conditions.

Hydrogen Cyanide, (HCN)

Molecular formula, (HCN) CAS Number: 74-90-8 A colorless, extremely poisonous, and highly volatile liquid that boils slightly above room temperature at 26 °C (78.8 °F). Has a faint, bitter, almond-like odor. Weakly acidic and partly ionizes in solution to give the cyanide anion, CN–. The salts of hydrogen cyanide are known as cyanides. A highly valuable precursor to many chemical compounds ranging from polymers to pharmaceuticals.

Hydrogen Fluoride, (HF)

Molecular formula, (HF) CAS Number : 7664-39-3
UN1052 Widely used in the petrochemical industry and a component of many superacids. Boils just below room temperature. HF is lighter than air and its odour is particularly penetrating. Aqueous solutions of HF, called hydrofluoric acid, are strongly corrosive. Hydrogen fluoride is used to calibrate environmental emission monitoring, and trace impurity analyzers. Cement and glass industries are more particularly concerned by the analysis of HF in their releases.

Hydrogen Sulfide, (H2S)

Molecular formula, (H2S) CAS Number: 7783-06-4
UN1053 A colorless, toxic and flammable gas, partially responsible for the foul odor of rotten eggs and flatulence. It often results from the bacterial break down of sulfates in organic matter in the absence of oxygen, such as in swamps and sewers (anaerobic digestion). It also occurs in volcanic gases, natural gas and some well waters. The odor of H2S is commonly misattributed to elemental sulfur, which is in fact odorless. Hydrogen sulfide is used to calibrate environmental emission monitoring, industrial hygiene monitors and trace impurity analyzers and as balance gas for some calibration mixtures.

Hydrogen, (H2)

Molecular formula, (H2) CAS Number: 1333-74-0
UN1049 (gas); UN1966 (liquid refrigerated) A colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H2. Atomic weight, 1.00794, hydrogen is the lightest element. Discovered by Henry Cavendish in 1766, hydrogen owes it name to Lavoisier, who combined the Greek hydor, water, and genen, to engender. It is the lightest gas in the world and therefore is not held by the earth’s gravity. Hydrogen is only found in the atmosphere at trace levels ; it is synthesized from hydrocarbons (petroleum and petroleum by-products) and from water where it constitutes the lightest fraction of the H2O molecule. Hydrogen gas is colorless, highly flammable, very light, cannot sustain life and reacts easily with other chemical substances. Used in a fuel cells, combines with oxygen to efficiently produce electricity and doesn’t emit anything… except water.

Inductively Coupled Plasma (ICP)

An instrument used in atomic emission spectroscopy primarily for the quantitative analysis of trace metals in solids or liquids.

Inert Gas

A gas which is considered stable and does not react with other materials at normal temperatures and pressures.

Infra Red, (CO2) Sensor

Infra Red, ( CO2 ); Gases which contain more than one type of atom absorb IR radiation. Therefore gases such as carbon
dioxide, carbon monoxide, methane, sulfur dioxide etc, can be detected by this means but gases such as oxygen, hydrogen and chlorine cannot. Specific gases are detected by measuring their absorption at particular frequencies of IR which correspond to the resonance of the molecular bonding between dissimilar atoms. In the simplest infrared gas detectors, the sample is drawn into an optical chamber by convection or by means of a
pump. At one end of the chamber is an IR source, normally a filament lamp and at the other end is an IR sensor.
The IR is tuned to the absorption frequency of the gas being measured by means of an optical filter. As the concentration of gas being measured increases, the output signal from the sensor reduces, in an approximately logarithmic fashion. This means that linearising circuitry needs to be built into the electronics.

Inorganic Compounds

Substances which do not contain carbon in their molecular structure.


A substance which causes inflammation of living tissue but does not cause irreversible damage.


International Safety Equipment Association. The trade association in the United States for companies that manufacture safety and personal protective equipment. Its member companies are world leaders in the design and manufacture of protective clothing and equipment used in factories, construction sites, hospitals and clinics, farms, schools, laboratories, emergency response and in the home.

Isobutylene, (C4H8)

Molecular formula, (C4H8) CAS Number: 115-11-7 UN1055 Isobutylene (or 2-methylpropene): A hydrocarbon of significant industrial importance. It is a four-carbon branched alkene (olefin), one of the four isomers of butylene. At standard temperature and pressure it is a colorless flammable gas. Used as an intermediate in the production of a variety of products. It is reacted with methanol and ethanol manufacture of gasoline oxygenates methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE), respectively. Also used in the production of methacrolein. Polymerization of isobutylene produces butyl rubber (polyisobutylene). Antioxidants such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) are produced by Friedel-Crafts alkylation of phenols using isobutylene. 100 ppm of Isobutylene is the default calibration gas standard for most PID (Photo Ionization Detector) sensors.


Forms of an element that have the same structure but differ from each other only in atomic mass. These slight changes in atomic mass often lead to instability and radioactivity. 


A temperature scale related to the triple point of water.

Level of Detection

In chromatography, the amount of sample in a stream necessary to produce a peak height two to three times the baseline noise height.

Liquefied Compressed Gas

A gas which under charged pressure is partially liquid at 70°F (21.1°C).

Liquified Petroleum Gas (LPG)

A term generally used to describe those hydrocarbon gases which exist as liquids at normal temperature and pressure.

Lower Explosive Limit (LEL)

The minimum percent by volume of a gas in air which forms a flammable mixture at normal temperatures and pressures. 


A device having a single outlet but several inlets to which cylinders can be connected for multiple usage at the same time.

Material Safety Data Sheet (MSDS)

A data sheet for a particular substance describing the characteristics and hazards associated with the handling and use of this product.

Melting Point

The temperature at which the solid and liquid phase of a substance are at equilibrium (normally given for 1 atmosphere of pressure).

Metal Oxide Semi-Conductor Sensor, (MOS), AKA, Solid State

The sensing material in TGS (MOS) gas sensors is metal oxide, most typically SnO2. When a metal oxide crystal such as SnO2 is heated at a certain high temperature in air, oxygen is adsorbed on the crystal surface with a negative charge. Then donor electrons in the crystal surface are transferred to the adsorbed oxygen, resulting in leaving positive charges in a space charge layer. Thus, surface potential is formed to serve as a potential barrier against electron flow. Inside the sensor, electric current flows through the conjunction parts (grain boundary) of SnO2 micro crystals. At grain boundaries, adsorbed oxygen forms a potential barrier which prevents carriers from moving freely. The electrical resistance of the sensor is attributed to this potential barrier. In the presence of a deoxidizing gas, the surface density of the negatively charged oxygen decreases, so the barrier height in the grain boundary is reduced. The reduced barrier height
decreases sensor resistance.

Methane, (CH4)

Molecular formula, (CH4) CAS Number: 74-82-8
UN1971 (gas); UN1972 (liquid refrigerated) Mixed with argon, methane is used in Geiger counters and for the detector in X Ray Fluorescence (XRF) as a quenching gas.
When mixed with other hydrocarbons, methane is used as reference point in calorimetric measurements for the measurement of PCI of hydrocarbons or coal. Methane is also used in calibration gas mixtures for petrochemical industry; environmental emission monitoring, industrial hygiene monitors and trace impurity analyzers.

Methanol, (CH3OH)

Molecular formula, (CH3OH) CAS Number: 67-56-1 Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits. A chemical compound with chemical formula CH3OH (often abbreviated MeOH). It is the simplest alcohol, and is a light, volatile, colourless, flammable, toxic liquid with a distinctive odor that is very similar but slightly sweeter than ethanol (drinking alcohol). At room temperature it is a polar liquid and is used as an antifreeze, solvent, fuel, and as a denaturant for ethanol. Also used for producing biodiesel via transesterification reaction.

Methyl Ethyl Ketone, (see n-Butanone)

MEK = Butanone; Molecular formula, CH3C(O)CH2CH3 CAS Number: 78-93-3 A colorless liquid with a sharp, sweet odor reminiscent of butterscotch and acetone. It is a ketone, better known as methyl ethyl ketone or MEK. It is produced on a large scale industrially. Butanone occurs in only trace amounts in nature.

Methyl Mercaptan, (Methanethiol, CH4S)

Molecular formula, (CH4S) CAS Number: 74-93-1 Methanethiol, also known as methyl+B119 mercaptan. A colorless gas with a smell like rotten cabbage. A natural substance found in the blood, brain, and other animals as well as plant tissues. It is disposed of through animal feces. Occurs naturally in certain foods, such as some nuts and cheese. It is also one of the main chemicals responsible for bad breath and the smell of flatulence. The chemical formula for methanethiol is CH3SH.


A unit of length equivalent to 1 x 10-6 meters.


Mass equivalent to the molecular weight of a substance. It is commonly expressed as grammole, the molecular weight in grams.

Molecular Weight

The sum of all the atomic weights of the atoms which make up a single molecule of a substance.

Nanogram (ng)

Mass equivalent to 1 X 10-9 grams.

Nanometer (nm)

Length equivalent to 1 X 10-9 meters.

National Formulary (NF)

A supplement to the United States Pharmacopoeia (USP).


National Industrial Standards Testing (NIST). A U S Government Metrology Agency.

Nitric Oxide, (NO)

Molecular formula, (NO) CAS Number: 10102-43-9
UN1660 An important signaling molecule in the body of mammals, including humans. An extremely important intermediate in the chemical industry. A toxic air pollutant produced by cigarette smoke, automobile engines and power plants. Nitric oxide is used in calibration gas mixtures for petrochemical industry; environmental emission monitoring, industrial hygiene monitors and trace impurity analyzers.

Nitrogen Dioxide, (NO2)

Molecular formula, (NO2) CAS Number: 10102-44-0
UN1067 An intermediate in the industrial synthesis of nitric acid. A reddish-brown toxic gas with a characteristic sharp, biting odor, a prominent air pollutant. Nitric dioxide is used in calibration gas mixtures for petrochemical industry; environmental emission monitoring, industrial hygiene monitors and trace impurity analyzers.

Normal Temperature and Pressure

A reference base for the gas industry of 70°F temperature and 14.696 psia pressure.

Nuclear Magnetic Resonance Spectrometer (NMR)

An analytical instrument normally used for the qualitative identification of compounds containing hydrogen. The device measures the absorption of radio frequency waves by hydrogen molecules as they are electromagnetically excited. 

Occupational Safety and Health Administration (OSHA)

Organization within the Department of Labor which sets standards for employers to ensure a safe working environment for its employees.

Oxidizing Agent

A substance that supports or causes combustion of other materials.

Oxygen, (O2)

Molecular formula, (O2) CAS Number: 7782-44-7
UN1072 (gas); UN1973 (liquid refrigerated) The third most abundant element in the universe by mass after hydrogen and helium. The most abundant element by mass in the Earth's crust. Diatomic oxygen gas constitutes 20.9% of the volume of air. Oxygen is used in calibration gas mixtures for petrochemical industry; environmental emission monitoring, industrial hygiene or safety monitors and trace impurity analyzers.

Ozone, (O3)

Molecular formula, (O3) CAS Number: 10028-15-6 Ozone or trioxygen (O3) is a triatomic molecule, consisting of three oxygen atoms. An allotrope of oxygen that is much less stable than the diatomic O2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals. The ozone layer in the upper atmosphere filters potentially damaging ultraviolet light from reaching the Earth's surface

Parts per Million (PPM)

A method of expressing low concentrations of impurities in a mixture. The unit can be expressed in moles, volume or weight per million of the same units. Lower concentration may be expressed in parts per billion (ppb) or parts per trillion (ppt).

Pellistor, Catalytic Bead Sensor

Most commonly known as a "Wheatstone Bridge Sensor", Catalytic bead Pellistor sensors consist of 2 coils of fine platinum wire each embedded in a bead of Alumina, connected electrically in a bridge circuit. One of the beads is impregnated with a special catalyst which promotes oxidation while the other bead is treated to inhibit oxidation. Current is passed through the coils so that they reach a temperature at which oxidation readily occurs at the catalyzed bead ( about 940° F ). This raises the
temperature which increases the resistance of the Platinum coil in the catalyzed bead, leading to an imbalance in the bridge.

Pentane, (C5H12)

Molecular formula, (C5H12) Any, or one of the organic compounds with the formula C5H12. This alkane is a component of some fuels and is employed as a specialty solvent in the laboratory. Its properties are very similar to those of butane and hexane. It exists in three structural isomers, the branched isomers are called isopentane and neopentane.

Permissible Exposure Limit (PEL)

Maximum routine exposure levels for different substances in the work environment as set by OSHA.

Phosphine, (PH3)

Molecular formula, (PH3) CAS Number: 7803-51-2
UN2199 Highly toxic; kills at low concentrations. Used for pest control by fumigation. For farm use it is sold in the form of aluminium phosphide, calcium phosphide, or zinc phosphide pellets. In semiconductor industries, phosphine is a N-type dopant for silicon based material (selected regions of silicon wafer such as emitters, source-drains, or collector contacts). Phosphine is used for doping polycrystalline or amorphous silicon and for forming borophosphosilicate glasses (BPS) by PECVD.

Photo Ionization Detector (PID)

A PID is a type of gas detector that measures volatile organic compounds and other gases in concentrations from sub parts per billion to 10 000 parts per million (ppm). A PID is capable of giving instantaneous readings and monitoring continuously. PID's are widely used in military, industrial, and confined working facilities for safety. PIDs are used as monitoring solutions for Ammonia detection, Hazardous materials handling, Arson investigation, Industrial hygiene and safety, Indoor air quality, Environmental contamination, remediation and Cleanroom facilities maintenance.


A substance that in small dosages can cause death or serious impairment to organs when entering a living organism by either ingestion, injection, absorption or inhalation.

Primary Reference Material (PRM)

A Calibration standard individually analyzed by NIST.

Primary Standard

Primary Standards are prepared gravimetrically to close tolerances. They are analyzed against reference standards if available to confirm the gravimetric numbers. The methodology used to determine each concentration and the uncertainty is reported on the Certificate of Analysis.

Propane, (C3H8)

Molecular formula, (C3H8) CAS Number: 74-98-6
UN1978 Propane, a three-carbon alkane, normally a gas, compressible to a transportable liquid. Derived from other petroleum products during oil or natural gas processing. Commonly used as a fuel for engines, barbecues, portable stoves and residential central heating. When used as vehicle fuel, Propane is commonly known as liquified petroleum gas (LPG or LP-gas), a mixture of propane with small amounts of propylene, butane, and butylene. The odorant ethanethiol is also added so that people can easily smell the gas in case of a leak. Propane is used to calibrate environmental emission monitoring, industrial hygiene monitors and trace impurity analyzers, particularly for total hydrocabons measurements (THC). Propane is can be used as a fuel in atomic absorption analyzers (AAS).

Propylene, {Propene} (C3H6)

Molecular Formula (C3H6) Propene, is also known as propylene, an unsaturated organic compound having the chemical formula C3H6. Has one double bond, and the second simplest member of the alkene class of hydrocarbons, it is also second in natural abundance. Uses of propene include blending for gasoline (80% of volume), a copolymer for polypropylene, synthesis of isopropanol, trimers and tetramers for detergents, propylene oxide, cumene, and glycerine. Propylene is used in calibration gas mixtures for petrochemical industry; environmental emission monitoring, industrial hygiene monitors and trace impurity analyzers.


A substance that can spontaneously ignite when exposed to air at temperatures of 130°F or below. 

Rare Gas

Those constituents of air that make up less than 1% of air and are generally considered inert. Examples include the gases in the far right column of the Periodic Table.

Restrictive Flow Orifice (RFO)

A safety device place in the outlet of a valve that limits the release rate of a hazardous gas to a maximum specified range in the event of accidental opening of the valve or failure of the gas containment system downstream.

RKI Instruments

Partnered with the Riken Keiki Company, Ltd., RKI Instruments is the world leader in gas detection and sensor technologies. Celebrating over 65 years in the gas detection business, Riken has over 150,000 points of detection world wide in the semiconductor industry alone. RKI Instruments is the exclusive North American supplier of Riken products.

Safety Relief Device

A device usually incorporated into the valve of a cylinder, actuated by either pressure or temperature at predetermined limit to prevent rupture of the vessel.

Self Venting Device

A device on certain types of regulators that relieves the outlet pressure as the regulator pressure is reduced.

Silane, (SiH4)

Molecular formula, (SiH4) CAS Number: 7803-62-5
UN2203 Used in semiconductor manufacturing as a source of silicon in polycrystalline silicium deposition for interconnects or masking; growth of epitaxial silicon; chemical vapor deposition (CVD) of silicon dioxide, silicon nitride, silicon carbide, and refractory metal silicides. Also used for the manufacturing of amorphous silicon devices for photosensitive drums or solar cells.

Span Gas

A calibration gas that is used to set the maximum reading on the scale of an analyzer.

Specific Gravity

The ratio of the mass of one substance to that of a standard substance. For gases the reference is air (air = 1).

Specific Heat

The amount of heat required to raise that temperature of a unit mass of a substance one degree at either constant temperature or volume.

Standard Reference Material (SRM)

A Reference standard certified by NIST.

Standard Temperature and Pressure (STP)

An internationally recognized reference of a standard temperature of 0°C and standard pressure of 14.6960 psia.


The direct passage of some substances from the solid state to the gaseous state without going through the liquid state first.

Sulfur Dioxide, (SO2)

Molecular formula, (SO2) CAS Number: 7446-09-5
UN1079 Produced by volcanoes and various industrial processes. The combustion of Coal and petroleum generates sulfur dioxide. Oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid rain. SO2 is used as a fumigant, preservative, bleach, and steeping agent for grain.
Small amounts of sulfur dioxide are commonly used as preservatives in winemaking to protect grapes and wine from microbial attack or further oxidation. Sulfur dioxide is used in calibration gas mixtures for the petrochemical industries, environmental emission monitoring, industrial hygiene monitors and trace impurity analyzers.

Tare Weight

The weight of an empty cylinder without a valve or cap.

Thermal Conductivity (TC Sensor)

Operates by comparing the thermal conductivity of the sample with that of a reference gas (usually air). A heated sensor is mounted so that it is exposed to the sample gas while a reference sensor is enclosed in a sealed compartment full of the reference gas. If the sample has a higher thermal conductivity than the reference gas, heat is lost from the exposed element and its temperature decreases, while the thermal conductivity
is lower than that of the reference, the sensor heats up. The temperature changes are normally monitored by electrical resistance changes in the sensing element. Because these sensors run quite hot to increase sensitivity, they are normally mounted in a flameproof sintered block like Pellistors. The technique sounds crude but is remarkably sensitive and is often used to measure hydrogen, helium, methane, neon, freon, and carbon dioxide. These are normally measured in a background gas of air, but the sensors operate just as well in a background of nitrogen or when monitoring two inert gases (often referred to as a binary mixture) such as the concentration of methane in carbon dioxide. This is very useful because many other sensor types need at least a trace of oxygen present to work. In addition the simplicity of the technique, which requires no complex signal processing or precision machining such as in Infrared sensors, means it is very costeffective and the sensors are extremely robust. Once calibrated, they rarely fall out of calibration and need very little maintenance. There are no chemicals to denature or optics to misalign.

Thermocouple Detector (TCD)

One of the earliest detectors used in gas chromatography. This detector operates as one leg of a wheatstone bridge that detects slight changes in conductivity as the exposed wire changes temperature. Also, sometimes this type of detector is referred to as a hot wire.

Threshold Limit Value (TLV)

Maximum standards set by ACGIH for airborne hazardous substances below which workers can be routinely exposed without adverse effects.

Threshold Limit Value/Short Term Exposure Limit, (TLV/STEL)

Refers to a 15 minute time weighted average exposure for substances which should not be exceeded at any time during a workday.

Threshold Limit Value/Time Weighted Average, (TLV/TWA)

Refers to the time weighted average over a normal 8 hour workday and a 40 hour week to which all workers may be repeatedly exposed without adverse effect.

Threshold Limit Value-Ceiling (TLV-C)

Airborne concentration of a substance which should not be exceeded.


A substance which has the ability to produce injurious or lethal effects through its chemical interaction with the body.

Triple Point

The defined pressure and temperature for a pure substance at which the three phases all exist in equilibrium.

Ultra Zero Air

Usually made from mixing components of Oxygen and Nitrogen together under laboratory condtitions. Manufactured Ultra Zero Air is guaranteed to be free of all contaminants. 

Vapor Pressure

The pressure exerted by the vapor above a liquid when the two phases are in equilibrium. 

Zero Air

Ambient air filtered to contain less than 0.1 parts per million (PPM) of total hydrocarbons.

Zero Gas

Calibration gas used to set the minimum reference point on the scale of an analyzer.