General Gas Education


Alarm: An audible sound, visual signal, or vibration that signals and alerts of dangerous levels of combustible gases, toxic gases, or oxygen deficient atmospheres.

Ambient Temperature: Temperature existing in immediate surroundings.

Approval Agencies: Agencies which have developed a set of electrical standards generally accepted in their country or field of activity as mandatory. Such agencies includes in the USA - Factory Mutual and Underwriters Laboratories, in Canada - Canadian Standards Association. Standards for various industries are also set by the specific approval agencies; ie. 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.

Aspirate: To draw in, as gas, by suction.


Background: Constituents in the sample or space to be tested other than the specific gas being monitored.

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

Bump Test (Functional Test): Application of gas to an instrument to verify functionality. It is used to verify sensor and alarm response.


CCM:Cubic centimeters per minute.

CFM: Cubic feet per minute.

Calibrate: 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 which do not normally correspond to the initial calibration of the instrument.

Catalyst: A substance which accelerates or retards a chemical change but itself remains stable. In gas analysis the most often 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)
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.

Chassis: The framework of an electronic instrument.

Circuit: An electrical network in which there is at least one conducting path which 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 comprised 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 to produce explosive or ignitable mixtures. A National Electrical Code classification. 

Class II Locations: Class II locations are hazardous because of the presence of combustible dust. A National Electrical Code classification.

Class III Locations: Class III locations are hazardous because of the presence of easily ignitable fibers or flyings. A National Electrical Code classification.

Combustion: The process of combination of a substance with oxygen, usually with liberation of heat.

Concentration: The amount of a gas per unit volume, usually expressed in %, % LEL, or ppm.

Combustible Gas: A gas which has a low enough flash point and a wide enough explosive range to make it potentially ignitable.

Condensation: The reduction of vapor or gas to a liquid normally by cooling or a change in pressure.

Confined Space: Any space: 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.

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 ornegative.


Data Logger: A memory device that stores information electronically inside an instrument.

Detector: The sensing element of a gas detection device sometimes known as a filament and also sometimes called a sensor. The term detector is also applied to the assembly housing the sensing element.

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

Dilution: 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 sensors for detection of 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.


Electrolyte: A substance which produces a conducting medium when dissolved in a suitable solvent, usually water. Thus the sulfuric acid in a storage battery would be termed an electrolyte. The potassium acetate used in some oxygen cells would also be termed an electrolyte.

Electrode: A conductor by means of which a current passes into or out of a liquid, a gas, or an insulating material. Electrodes are used in toxic gas sensors. Their function in this connection is to pass 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 dusts, measured in % volume, when presented with an ignition source. Gases, vapors, and dusts have Lower Explosive Limits (LEL) and Upper ExplosiveLimits (UEL). The LEL is the lowest or minimum concentration or mixture range where an explosion can take place. UEL is the upper or maximum concentration or mixture range where an explosion can take place. 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 certifies 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.

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

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

Flammable: 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 which may result from 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 vapor sufficient 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): Application of gas to an instrument to verify functionality. It is used to verify sensor and alarm response.


Gas: Fluid form of matter which is compressible within limits and which, owing to the relatively free movement of its molecules, diffuses readily in other like 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 the consumable items such as sensors, batteries, water barriers, and filters.


HotWork: Welding or grinding where a source of ignition is normally present.

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

Hydrocarbon: One of a large group of compounds that contain primarily hydrogen and carbon. Most gases to be detected by combustible gas indicators fall into this category.


Ignition Point: The temperature at which a substance will ignite and burn without necessity for introduction of a separate source of ignition.

Immediately Dangerous to Life and Health (IDLH): Defined by the US 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 combustible reactions or support them.

Inhibitors: Substances which reduce the effectiveness of the catalyst action on the catalytic bead LEL sensor. Inhibitors only hinder the working of a catalyst.

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

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

Ions: Atoms or groups of atoms that have picked up a positive or negative electrical charge.

Ionization: Breaking up 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).


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

Linearity: 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 where an explosion may occur. This concentration is expressed in % 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% LEL corresponds to the % of volume concentration where combustion can occur.


Membrane: 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 are inhibited due to their density.


National Electric Code (NEC): A set of standards governing design and installation of electrical equipment in the USA, so as to ensure safe installation. The NEC is maintained and reviewed by the NFPA.

National Fire Protection Association (NFPA): A non-profit organization whose aims are to improve methods of fire protection and prevention, to 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.


Organic: Of or pertaining to the nature of organisms, of animals or plants. Any chemical compound typical of those formed by life processes, primarily hydrocarbons.

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

Oxidation: The act of uniting or causing to be united chemically with oxygen, the state of being so united, ie., rust.

Ozone: A triatomic form of oxygen that is bluish in color gas with a pungent odor. 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 1 million parts of air. 1% of volume = 10,000 ppm, 100% of volume = 1,000,000 ppm.

Parasitic Pump: A pump that is powered by attaching to an instrument and relying on the instruments battery as a source of power.

Peak Reading: The highest detected toxic gas and LEL gas reading, and the lowest oxygen reading.

Permeability: 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 8 hour time-weighted average (TWA) exposure. (OSHA, 2010)

Photoionization Detector (PID): An air monitoring instrument that detects hydrocarbons and some inorganic vapors by photoionization.

Photoionization: Treating chemicals with ultraviolet light to convert their molecules into electrically charged ions.

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

Potentiometer: 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.

Probe: A rigid hollow extension for the gas sampling line to reach gas samples to be withdrawn from spaces which might be otherwise inaccessible.

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

Purge: The act of removing flammable, combustible, and toxic gases from a confined work space prior to entry or performing hot work. Instrument should read zero when properly purged.


Reaction Chamber: A cavity or enclosure in which a reaction or a conversion of gas occurs in the course of making a gas test.

Radio Frequency Interference (RFI): Electromagnetic radiation which is emitted by electrical circuits carrying rapidly changing signals, as a by-product of their normal operation, and which causes unwanted signals (interference or noise) to be induced in other circuits.

Recommended Exposure Limits (REL): 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) designated by “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 “C” preceding the value; unless noted otherwise, the ceiling value should not be exceeded at any time.

Recorder: An instrument capable of making graphic records of gas response.

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

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

Response: 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 detector.


Sample: 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 by means of suction.

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

Short-Term Exposure Limit (STEL): A 15 minute time weighted average (TWA) exposure which can only be exceeded four times during a work day with a minimum of one hour of fresh air in between.

Span Reserve: The amount of sensor life left assuming normal usage. Prior to testing the span reserve, the sensor should be properly zeroed and stable.


Threshold Limit Value (TLV): The maximum concentration of a substance to which a workman may be exposed without ill affects during a normal 8 hour, 5 day week. The commonly accepted values are found in the set of guidelines published by the American Conference of Governmental Hygienists to indicate limit of safe level of airborne concentrations of toxic substances, (ACGIH).

Time-Weighted Average (TWA): Average gas exposure over time. Usually averaged over an 8 hour period.

Thermal Conductivity: Ability of a substance to carry or transfer heat between two locations of different temperature. In gas analysis the ability of a gas sample to conduct heat away from or to a coil or heated filament. Each gas has its own thermal conductivity, some gases being much more conductive than others.

Toxic: Poisonous. In industrial health, toxic is defined as having some adverse effect under some conditions of exposure.


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

Upper Explosive Limit (UEL): The maximum concentration of a combustible gas, when mixed with air, where an explosion may occur. Also expressed as Upper Flammability Limit.


Vapor: Any substance in the gaseous state which in ordinary conditions is usually a liquid or a solid; 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. VOCs can be man made or naturally occurring chemicals.

Volatile: Evaporating rapidly at ordinary temperatures upon exposure to air; capable of being vaporized at room temperature.

Voltage: A difference in electric potential expressed in volts.

Volume Percent: A percentage of 100% gas/air mixture, usually applied to the percentage of one certain gas of interest.


Water or Moisture Trap: A container installed in the incoming sample line to a gas analyzer, where liquid can collect, used to prevent water or other liquids from being drawn into the gas analyzer. Used when there is 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 while 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.


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

Zero Drift: The condition which occurs when the meter gradually shifts upscale or downscale when no gas is present.

TRAINING DISCLAIMER: These training videos and other training materials are intended to assist employers, workers, and others as they strive to improve workplace health and safety, by educating them on the practical use of Industrial Scientific® gas detection equipment. While we attempt to thoroughly address the specific topics, it is not possible to include discussion of everything in a presentation of this nature. Thus, this information must be understood as a tool to assist you in the operation of your Industrial Scientific® gas detection equipment. Finally, over time, Industrial Scientific Corporation may update the training materials and videos in light of new technology, information, or circumstances; to keep apprised of such developments, or to review a complete and up to date library of our training materials, you can visit our website at or download our app.