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Gas Detection Glossary

Learn key terms and concepts associated with gas detection.




An audible sound, visual signal, or vibration that alerts users of dangerous levels of combustible gases, toxic gases, or oxygen-deficient atmospheres.

Ambient Temperature

The temperature existing in immediate surroundings.

Approval Agencies

The agencies that have developed a set of electrical standards generally accepted in their country or field of activity as mandatory. Such agencies include Factory Mutual and Underwriters Laboratories in the U.S. and the Canadian Standards Association in Canada. Standards for various industries are also set by specific approval agencies (i.e., Coast Guard for instruments used at sea).

Aromatic Hydrocarbons

A group of hydrocarbon compounds of the closed ring formation and are derived from benzene. The most common ones are toluene and xylene.


To draw in, as gas, by suction.



The constituents in the sample or space to be tested other than the specific gas being monitored.


An opening made from the ground level to a pipeline to allow entry of a probe and hose to check for gas leaks.

Bump Test (Functional Test)

The application of gas to an instrument to verify functionality. It is used to verify sensor and alarm response.



Cubic centimeters per minute.


Cubic feet per minute.


A method of adjusting a measurement device to correct for inherent inaccuracies. A known gas concentration is used as a calibration standard to verify and adjust the output of the unit.

Calibration Curve

A graph depicting the relationship between a meter scale reading and a concentration of gas. The abscissa (horizontal axis) usually depicts gas concentration and the ordinate (vertical axis) meter reading. Calibration curves are used to interpret readings of certain gases that do not normally correspond to the initial calibration of the instrument.


A substance that accelerates or retards a chemical change but itself remains stable. In gas analysis, the most used catalyst is platinum. The platinum is used in wire form in the detection element (filament). When a mixture of air and combustible gas or vapor is brought in contact with a hot platinum wire (catalyst), a rapid combination of the combustible with the oxygen of the air takes place at the surface of the wire (catalytic reaction), raising the temperature of the wire and increasing its electrical resistance.


The framework of an electronic instrument.


An electrical network in which there is at least one conducting path that can be closed.

Circuit Board

A substrate upon which electronic components are configured so that electrical signals may be passed from one component to the next. Usually, the substrate is made of a fiberglass board onto which conducting traces are deposited. The traces form a pathway between electronic components.

Class I Locations

Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient enough to produce explosive or ignitable mixtures. A National Electrical Code classification.

Class II Locations

Class II locations are hazardous due to the presence of combustible dust. A National Electrical Code classification.

Class III Locations

Class III locations are hazardous due to the presence of easily ignitable fibers or flyings. A National Electrical Code classification.


The process of combining a substance with oxygen, usually with the liberation of heat.


The amount of a gas per unit volume, usually expressed in percent, percent LEL, or parts per million (ppm).

Combustible Gas

A gas that has a low enough flash point and wide enough explosive range to make it potentially ignitable.


The normal reduction of vapor or gas to a liquid by cooling or a change in pressure.

Confined Space

Any space that 1) has limited or restricted means of entry or exit; 2) is large enough for a person to enter to perform tasks; or 3) is not designed or configured for continuous occupancy.

Cross Sensitivity

A sensor’s reaction to an interfering gas. Exposing a sensor to a gas that is not the target of the sensor can cause an undesirable response, either positive or negative.


Data Logger

A memory device that stores information inside an instrument electronically.


The sensing element of a gas detection device, sometimes referred to as a filament or a sensor. The term detector is also applied to the assembly housing the sensing element.


A spreading out and permeation of an occupied space. For a diffusion gas detector, this means a sensing element that is exposed continuously to the atmosphere it is to monitor, and over which the sample flows by the natural movement of the gas rather than by the action of a pump.


The act of weakening by a mixture. In the case of gas detection, a gas concentration can be weakened or diluted by the introduction of air.

Direct Current (DC)

Unidirectional current which flows continuously. Most portable gas indicators are powered by direct current, supplied by a battery of one type or another.

DualSense® Technology

DualSense Technology uses two of the same type of sensors to detect a single gas. Readings are then passed through a proprietary algorithm and a single instrument reading is displayed. This lessens the likelihood of an instrument not being able to detect gas out in the field. In turn, this makes instruments with DualSense Technology safer than single-sensor instruments.



A substance that produces a conducting medium when dissolved in a suitable solvent, usually water. The sulfuric acid in a storage battery is deemed an electrolyte. The potassium acetate used in some oxygen cells is also deemed an electrolyte.


A conductor through which a current passes in or out of a liquid, gas, or insulating material. Electrodes are used in toxic gas sensors. Their function in this connection is to pass a current from the electrolyte to the instrument’s measuring circuit.

Electromagnetic Interference (EMI)

Any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronics and electrical equipment.

Explosive Limits

The explosive range of gases, vapors, and dust when presented with an ignition source, measured in percent volume. Gases, vapors, and dust have lower explosive limits (LEL) and upper explosive limits (UEL). The LEL is the lowest or minimum concentration or mixture range in which an explosion can take place, whereas a UEL is the upper or maximum range. Gases in quantities below the LEL are too lean to explode. Gases in quantities above the UEL are too rich to explode. See LEL and UEL in Glossary.

Explosion Proof Enclosure

An enclosure capable of withstanding an explosion.


Factory Mutual (FM)

FM Approvals certify industrial and commercial products and services for companies worldwide. When a product or service meets the standards of FM Approvals, it is issued the “FM APPROVED” mark to signify it will perform as expected and support property loss prevention.


The sensing element used in a combustible gas detector, usually in the form of a coil of platinum wire.


A device for cleaning and purifying the gas sample being drawn. For example, a cotton pad or cylinder is often used for trapping dust.


An explosive gas, capable of being ignited and burned.

Flame Arrestor

A barrier for the active filament in an instrument operating on the catalytic reaction principle. The flame arrestor is often made of fire-resistant, porous material—such as sintered bronze, stainless steel, or Monel®—and its purpose is to contain any flame that may result from a combustible gas mixture passing over the filament.

Flame Propagation

The spread of flame from the source of ignition through a flammable mixture.

Flash Point

The temperature of a liquid at which it gives off enough vapor to form an ignitable mixture with the air near the surface of the liquid or within the vessel used.

Flow Meter

A device to measure the rate of sample flow.

Functional Test (Bump Test)

The application of gas to an instrument to verify functionality. It is used to ensure sensor and alarm response.



A fluid form of matter that is compressible within limits and which—owing to the relatively free movement of its molecules—readily diffuses in similar forms of matter and is capable of indefinite expansion in all directions.

Guaranteed for Life

The Guaranteed for Life™ warranty covers every component of the product for as long as the product is supported by Industrial Scientific. This does not include consumable items such as sensors, batteries, water barriers, and filters.


Hot Work

The act of welding or grinding, in which a source of ignition is normally present.


The amount of water vapor in the air. There are three types of humidity: absolute, relative, and specific.


One of a large group of compounds that primarily contain hydrogen and carbon. Most gases that are detectable by combustible gas indicators fall into this category.


Ignition Point

The temperature at which a substance will ignite and burn without the introduction of a separate source of ignition.

Immediately Dangerous to Life and Health (IDLH)

Defined by the U.S. National Institute for Occupational Safety and Health (NIOSH) as exposure to airborne contaminants that are “likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment.” Examples include smoke or other poisonous gases at sufficiently high concentrations.

Inert Gas

A gas devoid of active chemical properties—neutral. In this group are helium, neon, and argon. In combustible gas analysis, nitrogen and carbon dioxide—although not strictly so—are treated as inert gases because they do not enter into or support combustible reactions.


The substances that reduce the effectiveness of the catalyst action on the catalytic bead LEL sensor. Inhibitors only hinder the working of a catalyst.


All elements and components, other than hydrocarbons or organic substances.

Intrinsically Safe

The term applied to an instrument or device that is incapable of being a source of ignition because only low energy is available from its electrical circuits. Many applications exist in which the atmosphere is so hazardous that no possible source of ignition can be allowed. In cases such as this, instrumentation used in the location shall be approved by one of the approval agencies (FM or UL) as intrinsically safe.


The atoms or groups of atoms that have picked up a positive or negative electrical charge.


The breaking up of molecules into electrically charged atoms or groups of atoms.

Ionization Potential (IP)

The amount of energy required to split a chemical into electrically charged particles. Ionization potential is measured in units called electron volts (eV).





A gas mixture with air containing too little combustible gas to be ignited.


The closeness to which a given percentage increase in gas produces the same percentage increase in reading.

Lower Explosive Limit (LEL)

The minimum concentration of a combustible gas mixed with air in which an explosion may occur. This concentration is expressed in percent of volume. For combustible gas instruments used to detect explosive atmospheres, the concentration is expressed as a percentage factor of the LEL point. A reading of 100 percent LEL corresponds to the percent of volume concentration in which combustion can occur.



An inert material that has pores of uniform size. These pores allow the transfer of molecules in the gas state. Generally, the transfer of liquids is inhibited due to their density.


National Electric Code (NEC)

A set of standards governing the design and installation of electrical equipment in the U.S. ensuring safe installation. The NEC is maintained and reviewed by the NFPA.

National Fire Protection Association (NFPA)

A non-profit organization that aims to improve methods of fire protection and prevention, publish information on these subjects, and secure the cooperation of its members and the public in establishing proper safeguards against the loss of life and property.



Of or pertaining to the nature of organisms, meaning animals or plants. Also, any chemical compound that is typical of those formed by life processes (e.g., hydrocarbons).


A small opening into a cavity. In gas measurements, a small hole of controlled dimensions is placed in the flow stream and a predictable pressure difference is developed for a given flow.


The act of uniting or causing to be united chemically with oxygen, or the state of being united (e.g., rust).


A triatomic form of oxygen that is bluish in color and pungent in odor. It is formed naturally in the upper atmosphere by a photochemical reaction with solar ultraviolet radiation or generated by electric discharge in normal oxygen.


Parts Per Million (ppm)

A unit of measurement used for small proportions or concentrations. It expresses the volume of a gas present in terms of its relationship to a whole of one million parts of air. One percent of volume = 10,000 ppm, 100 percent of volume = 1,000,000 ppm.

Parasitic Pump

A pump that is powered by attaching to a gas monitor and relying on the monitor’s battery as a source of power.

Peak Reading

The highest detected toxic gas and LEL gas reading and the lowest oxygen reading.


The quality or condition of allowing passage, especially of liquids and gases.

Permissible Exposure Limit (PEL)

PELs are regulatory limits on the amount or concentration of a substance in the air. PELS are based on an eight-hour, time-weighted average (TWA) exposure. (OSHA, 2010)

Photoionization Detector (PID)

An air monitoring instrument that detects hydrocarbons and some inorganic vapors through photoionization.


The treatment of chemicals with ultraviolet light to convert their molecules into electrically charged ions.


In gas detection, the desensitizing action of certain gases on the detection element. An example of this is the sensitivity loss that occurs on a platinum filament in the presence of silicone vapors.


A three-terminal variable resistance device in which a slider can move from one extreme to the other, picking off various values of resistance. In instrumentation, potentiometers are used as zero adjusts, volt adjusts, and calibration controls.


A rigid, hollow extension for the gas sampling line to reach gas samples and withdraw them from spaces that might be otherwise inaccessible.

Propagation of Flame

The spread of flame from the source of ignition through a flammable mixture.


The act of removing flammable, combustible, and toxic gases from a confined workspace before entry or performing hot work. An instrument should read zero when properly purged.



Reaction Chamber

A cavity or enclosure in which a reaction or a conversion of gas occurs during a gas test.

Radio Frequency Interference (RFI)

The electromagnetic radiation that is emitted by electrical circuits carrying rapidly changing signals. It is emitted as a by-product of their normal operation, causing unwanted signals (e.g., interference or noise) to be induced in other circuits.

Recommended Exposure Limits (REL)

The NIOSH “TWA” indicates a time-weighted average concentration for up to a 10-hour workday during a 40-hour workweek. A short-term exposure limit (STEL) is designated by an “ST” preceding the value. Unless noted, the STEL is a 15 minute TWA exposure that should not be exceeded at any time. A ceiling REL is designated by a “C” preceding the value. Unless noted, the ceiling value should not be exceeded at any time.


An instrument capable of making graphic records of gas response.


The closeness to which successive readings with the same instrument on the same sample agree with one another.


The tendency of all materials to resist the flow of an electrical current and to convert electrical energy into heat.


An indication on the readout device resulting from a change in the composition of the gas mixture being sampled.

Rich Mixture

An excess of combustible gas in relation to the LEL or range of the detector.



A representative portion of the atmosphere being tested.

Sample Drawing

The method used in gas detection whereby the sample is drawn to the detection portion of an instrument through hoses or tubing using suction.


In gas detection, the smallest change in gas composition that can be observed on the readout device.

Short-Term Exposure Limit (STEL)

A 15-minute, time-weighted average (TWA) exposure that can only be exceeded four times during a workday with a minimum of one hour of fresh air in between.

Span Reserve

The amount of remaining sensor life, assuming normal usage. Before testing the span reserve, the sensor should be properly zeroed and stable.


Threshold Limit Value (TLV)

The maximum concentration of a substance a workman may be exposed to without ill effects during a normal eight-hour, five-day week. The commonly accepted values are found in the set of guidelines published by the American Conference of Governmental Hygienists (ACGIH) that indicate the limit of safe airborne concentrations of toxic substances.

Time-Weighted Average (TWA)

The average gas exposure over time. Usually averaged over an eight-hour period.

Thermal Conductivity

The ability of a substance to carry or transfer heat between two locations of different temperatures. In gas analysis, it is the ability of a gas sample to conduct heat away from or toward a coil or heated filament. Each gas has its own thermal conductivity, with some gases being much more conductive than others.


Poisonous. In industrial health, toxicity is defined as having some adverse effects under certain conditions of exposure.


Underwriters Laboratory (UL)

An independent testing and approval agency that examines electrically operated equipment and accessories, primarily from the standpoints of safety and freedom from hazard. Approved equipment is listed by the UL and carries the UL label. A follow-up procedure assures that manufacturers do not make unapproved changes. The UL does not have any connection with the federal or other governments, but is recognized by many local government regulations that require UL labels on certain classes of electrical equipment.

Upper Explosive Limit (UEL)

The maximum concentration of combustible gas, when mixed with air, in which an explosion may occur. Also expressed as an Upper Flammability Limit.



Any substance in a gaseous state that—in ordinary conditions—is usually a liquid or a solid (i.e., a gasified liquid or solid).

Volatile Organic Compound (VOC)

Organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, They can have a chronic toxic effect, and can be man-made or naturally occurring chemicals.


Substances that evaporate rapidly at ordinary temperatures upon exposure to air, and that are capable of being vaporized at room temperature.


A difference in electric potential expressed in volts.

Volume Percent

A percentage of a 100 percent gas and air mixture, usually applied to the percentage of a certain gas of interest.


Water or Moisture Trap

A container installed in the incoming sample line to a gas analyzer in which liquid can collect. Used to prevent water or other liquids from being drawn into the gas analyzer. Also used when there is the danger of liquid or condensable vapor entering the sample line.

Wheatstone Bridge

A four-leg electrical bridge circuit, in which all the legs are predominantly resistive. Used for measurement of resistance. In gas detection instruments, an active and a reference filament usually occupy two legs of the bridge. The reference and two other resistive elements offer a fixed resistance, whereas the active detection elements act as a variable resistance when exposed to a gas sample, thus unbalancing the bridge and giving a reading on the meter.





The term applied to the reference level of an instrument, indicating no detection activity.

Zero Drift

The condition that occurs when the meter gradually shifts upscale or downscale when no gas is present.