Gases and Related Definitions to Gas Processes

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Adsorption refers to the condensation of gas or vapor on the surface of a solid.

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

ANSI (American National Standards Institute)


Area Monitor is a term that is often misleadingly applied to gas monitoring sensors that are installed in a regular grid pattern throughout an area requiring monitoring. A true area monitor must be able to measure the concentration of a substance at any point in three dimensional space in a defined volume or it must be able to indicate the total quantity of a substance that has penetrated a defined volume.

Alarm is an audible, visual, or physical presentation designed to warn the instrument user that a specific level of a dangerous gas/vapor concentration has been reached or exceeded.

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

Alarm Set Point is the selected gas concentration level where an alarm is activated.

Analyzer is an instrument which can determine qualitatively and quantitatively the components in a mixture.

Ambient Air is air to which the sensing element is normally exposed.

ASNT (American Society for NonDestructive Testing)

ASTM (American Society for Testing of Materials)

Atmospheric Pressure
The pressure of the atmosphere at a specified place and time.

The environment against which an indication must be evaluated.

Black Light
Light in the near ultraviolet range, just short of visible light. Fluorescent penetrants absorb this ultraviolet radiation and emit light in the visible region. The darker the surroundings, the brighter this emitted light appears to be. Extended exposure to black light can be harmful unless protective eyewear is used. Black lights are also used when leak testing with fluorescent tracer dyes.

Certain conditions can cause a sensor not to function. When this happens, normal gas sensing is blocked until the conditions are removed. The most common block is lack of oxygen.

The action of the developer in "drawing out" the penetrant from a surface discontinuity causing the maximum bleed-out for increased contrast and sensitivity.

Bubble Immersion Test
A form of leak test of gas containing enclosures in which a leak is indicated by the formation of a bubble at the site of a leak.

Burst Test
The burst test method requires the filling with liquid of a container or part and pressurizing it until it bursts thus establishing its tolerances. This is usually a destructive test.

Calibration is the procedure used to adjust the instrument for proper response.

Calibration Gas is a gas of known concentration(s) used to set the instrument span or alarm level(s).

Capillary Action
The tendency of certain liquids to travel, climb or draw into tight crack-like interface areas due to such properties as surface tension, wetting, cohesion, adhesion and viscosity.

Capillary Leak
A leak through a barrier that has a hole or a discrete passage.

Carbon Monoxide

Catalysis is a phenomena in which a relatively small amount of material augments the rate of reaction without itself being consumed.


Colorimetric Leak Testing
Colormetric detectors are rapid and inexpensive leak detectors which react chemically with minute leaks causing a visible color change in the developer.

Combustion is the rapid oxidation of a material evolving heat and generally light.

Combustible Hydrocarbon (CHC) is any organic gas or vapor which when mixed with air or oxygen is capable of the propagation of flame away from the source of ignition when ignited.

Consumables are those materials or components which are depleted or require periodic replacement through normal use of the instrument.

A discontinuity whose size, shape, orientation, location or properties make it detrimental to the useful service of the part in which it occurs or which exceeds the accept/reject criteria for the given design. A rejectable discontinuity (an unacceptable leak).

Developers are used to enhance the visibility of small amounts of inspection penetrant bleeding from small discontinuities. Developers draw or absorb penetrant materials from a surface discontinuity to allow the inspection penetrant to be visible under natural or black light.

Diffusion is a process by which the atmosphere being monitored is transported to the gas-sensing element by natural random molecular movement. This movement is accelerated by thermal energy.

Discontinuity A break or interruption in the normal structure of an object.

DOT is the Department of Transportation, a government agency which regulates many of the procedures, uses and methods for the transport, storage and handling of Compressed Gas and other Hazardous Materials.

Dwell Time
The time in which an inspection penetrant or developer is in contact with the surface of the part. Drain time is considered part of dwell time.

An emulsifier is used with certain types of inspection penetrants to make oil in the penetrant water dispersible and therefore water washable.

Ethylene Oxide

Explosion is an uncontrolled chemical reaction which generates a large amount of heat and gas in a short period of time.

Fail Safe. Any system that cannot fail in any mode without providing a directly observable indication of failure. Consider an electrical relay with a set of contacts that are open when it is unpowered. If a power source and a light bulb are connected in series with the contacts, the lamp will glow when the relay is energized. If the goal of this system is to insure that the relay has power, then this system is said to be fail safe. If the lamp, relay contacts, lamp power source relay coil, or the relay coil power supply fail, then the lamp extinguishes itself providing a directly observable foolproof indication of failure.

Fixed Installation. The terminology commonly used to indicate that a gas monitor is permanently installed, such as in the control panel of a control room. Occasionally gas monitors are mounted in vehicles, such as fire trucks or tankers. These are also generally referred to as fixed installation monitors.

Flashpoint is the minimum temperature at which a liquid gives off a sufficient vapor to reach 100% LEL (sufficient vapor to form an ignitable mixture with the air near the surface of the liquid).

Flammable (Explosive) Limits. For gases or vapors which form flammable mixtures with air or oxygen, there is a minimum concentration of vapor in air or oxygen below which propagation of flame does not occur on contact with a source of ignition. There is also a maximum proportion of vapor or gas in air above which propagation of flame does not occur. These boundary-line mixtures of vapor or gas with air, which if ignited will just propagate flame, are known as the "lower and upper flammable limits" (LFL and UFL) or the "lower and upper explosive limits" (LEL and UEL), and are usually expressed in terms of percentage by volume of gas or vapor in air. LEL and LFL are different terms for the same concept and can be used interchangeably. In popular terms, a mixture below the lower flammable limit is too "lean" to burn or explode and a mixture above the upper flammable limit too "rich" to burn or explode.

Flammable (Explosive) Range. The range of flammable vapor or gas-air mixture between the upper and lower flammable limits is known as the "flammable range", also often referred to as the "explosive range". For example, the lower limit of flammability of acrylonitrile at ordinary ambient temperatures is approximately 3 percent vapor in air by volume, while the upper limit of flammability is about 17 percent. All concentrations by volume of acrylonitrile vapor in air falling between 3 percent and 17 percent are in the flammable or explosive range.

An imperfection in an item or material which may or may not be harmful. If it is harmful, it is a defect. (Used loosely to mean either discontinuity or defect.)

Leak Detection: A system which while being leak tested becomes so filled with a tracer gas as to make impracticable further leak testing.
Gas Detection:Sensor flooding occurs when a gas concentration at the sensor exceeds its stoichiometric mixture. The signal from the sensor reverts to zero because the mixture in the air is too gas-rich to burn.

Gas is a phase of matter which expands indefinitely to fill a containment vessel. Characterized by a low density.

Gas Detection Instrument is an assembly of electrical, mechanical and chemical components (either a single integrated unit or a system comprised of two or more physically separate but interconnected component parts) which senses and responds to the presence of gas in air mixtures.

Hydrostatic Leak Testing is a method of leak testing components by pressurizing them inside with water. Not to be confused with a Burst Test. See Hydrostatic Leak Testing

Hydrogen Sulfide is interchangeable for: H2S, dihydrogen sulfide, and hydrogen sulfide gas. See Hydrogen Sulfide.

IDLH (Immediately Dangerous to Life and Health) represents the maximum concentration level of a substance from which one could escape within 30 minutes without escape-impairing symptoms or any irreversible effects (For instance 300 ppm for Hydrogen Sulfide).

Ignition Temperature is the minimum temperature necessary to initiate combustion (oxidation) and have self-sustained combustion of the solid, liquid, gas, or vapor of interest.

Ignitable Mixture A mixture within the flammable range (between the lower and upper flammable/explosive limits) that, when ignited, is capable of the propagation of flame away from the source of ignition.

Immersion Leak Testing
A leak testing method where an object is immersed in a fluid and a leak indicated by the escape of air in the form of bubbles. Especially useful for difficult shaped objects.

Test response that requires interpretation and evaluation.

Inspection Penetrants
Test objects or material is coated with visible or fluorescent dye solution. Excess dye is then removed from the surface, and a dry developer is applied. The developer acts as blotter, drawing penetrant out of imperfections open to the surface. With visible dyes, vivid color contrasts between the penetrant and developer make "bleedout" easy to see. With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be seen readily.

In-Leakage Rate
Combined semi-quantitative leakage rates of individual leaks. Not accurate.

An interferent is any gas other than the target gas that will cause a gas detecting sensor to give a signal. In the case of a combustible sensor, any combustible gas or vapor will cause a signal.

Lag Time
The time in a test between input and observable meter response.

Technically, a leak is a hole or porosity in an enclosure capable of passing a fluid from the higher pressure side to the lower pressure side. Leaks are often conceived of being simply a round hole, however, this is almost never the case. A leak normally has an involved geometry sometimes extending quite a distance from beginning to end. As a result, leakage repair may require locating both the start and end of the leak.

The prevailing fluid flow through leak at existing conditions.

Leakage Rate
Quantity or measure of leakage per unit time (leak rate).

Leak Tight
Leakage that is acceptable for a particular component or system.

Liquid is a phase of matter which is free to conform to a shape of a vessel but has a fixed volume and has a greater density than a gas.

Lower Explosive Limit (LEL)
Lower Flammable Limit (LFL)

The lower explosive limit (LEL) or lower flammable limit (LFL) of a combustible gas is defined as the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and ignited. In gas-detection systems, the amount of gas present is specified in terms of % LEL: 0% LEL being a combustible gas-free atmosphere and 100% LEL being an atmosphere in which the gas is at its lower flammable limit. The relationship between % LEL and % by volume differs from gas to gas. For data on other gases, refer to the most recent edition of the Handbook of Chemistry and Physics published by the C.R.C. Press. Typical settings for the alarm circuit are 20% for the low alarm, 40% for the high alarm and 60% for the high-high alarm.
The LEL of a gas is affected by the temperature and pressure: as the temperature increases, the LEL decreases and hence the explosion hazard increases; the relationship between LEL and pressure is fairly complex, but at approximately one atmosphere a pressure increase usually lowers the LEL. The LEL of a gas is not significantly affected by the humidity fluctuations normally encountered in the operation of a gas-detecting system.

Minimum Detectable Leak Rate
Magnitude of the smallest leak rate that can be detected by a given method/technique.

Mobilerefers to a continuous-monitoring instrument mounted on a vehicle such as, but not limited to, a mining machine or industrial truck.

Monitor is an instrument used for continuous measurement of a condition which must be kept within prescribed limits. Monitors are not the same as analyzers. An analyzer is capable of determining the quality, quantity and/or type of specific substance or substances in a mixture. A monitor continuously measures a condition which must be kept within prescribed limits.

NDT (Nondestructive Testing)
Nondestructive testing is the examination of an object or material with technology that does not affect the object's future usefulness.

Nonincendive circuits are those which may spark under normal operating conditions, but which may not release enough energy to cause ignition. Circuits may contain enough energy for potential ignition should both the equipment fail and the process fail (two concurrent failures).

Leak location for a particular method/technique, no measured leak rate except as to tests sensitivity on a go/no-go relationship.

Nonsparking Nonsparking circuits are those which contain no contacts or in which contacts are isolated from the surrounding atmosphere such as by hermetic sealing.

Nominal Voltage is the voltage given by the manufacturer as the recommended operating voltage of their gas detection equipment. If a range (versus a specific voltage) is given, the nominal voltage shall be considered as the midpoint of the range, unless otherwise specified.

OSHA: Occupation Safety & Health Administration, a government agency.

In its most basic form Oxidation is a chemical reaction with oxygen. Example: the oxidation of Methane (CH4). In this example, molecular Oxygen is the oxidizing agent and the substance reacting with oxygen (methane) is called the reducing agent. Confusingly the reducing agent (Methane) can also be called an oxidizable gas. Compounds containing oxygen can yield oxygen in a reaction and are also called oxidizing agents. Oxidation is a reciprocal process in which one agent is reduced and one oxidized. A more complete way of describing oxidation is through the transfer of electrons. The substance oxidized loses electrons. The substance reduced gains electrons. Under suitable conditions, the oxidation-reduction reaction produces a flow of current.


Permeation Leak
A leak through a barrier that has no hole or discrete passage.

Parts Per Million

Gas detecting sensors can be quickly destroyed (or poisoned) by certain materials. Even low concentrations of poisoning substances can cause serious problems. The two most common phenomena are coating and etching of catalytic beads.

Portable refers to a self-contained, battery-operated or transportable gas monitor worn or carried by the person using it. A gas detector that can be carried.

Post-Emulsification Penetrant
A type of inspection penetrant containing no emulsifier but which is cleaned from a surface with water after applying an emulsifier as a separate step. (Can be abbreviated as PE)

Quantitative (Leak Measurement)
Overall leakage measurement for a complete component or system, but with no location. See also Semi-Quantitative.

Range is the series of outputs corresponding to values of concentrations of the gas of interest over which accuracy is ensured by calibration.

Leakage rate for a particular located leak, no overall measurement.

Sensitivity of Leak Test
Smallest leak rate that the technique used (instrument, equipment, system, method etc) is capable of detecting under a specified set of conditions (pressure, temperature, etc.)

A gas detecting sensor converts the presence of a gas or vapor into an electrically measurable signal. The sensor is the heart of a gas monitor.

Soak Time
Time for a tracer gas to penetrate boundary walls.

Soap Solution
A primitive form of bubble leak testing since replaced by synthetic solutions.

Solid is a phase of matter characterized by a definite volume and definite shape. A solid resists external forces to change shape.

Standard Leak
A device that permits a tracer gas to be introduced into a leak detector or leak testing system at a known rate to facilitate calibration of the leak detector.

Stationary refers to a gas detection instrument intended for permanent installation in a fixed location.

Stoichiometric. The exact percentage of two or more substances which will react completely with each other leaving no unreacted residue. For example, a 7% mixture of methane by volume in air will react completely with the oxygen present leaving only CO2 and H2O as residue. If the methane concentration here is less than 7%, there would be oxygen left over. If the methane concentration were greater than 7%, there would be methane left over.

Test Gas is a known amount of the gas to be detected diluted with a known amount of clean air.

Thin Film Leak Testing
A leak test using a solution which bubbles upon finding a leak.

Threshold Limit Value Time-Weighted Average (TLV-TWA) is the time-weighted average concentration of a substance for a normal 8-hour work day and a 40-hour work week, to which nearly all workers may be repeatedly exposed, day after day. (OSHA)

Toxic Gas or Vapor. Any substance which causes illness or death when inhaled or absorbed by the body in relatively small quantities. H2S is a highly toxic gas.

Tracer Dye
A dye usually red or fluorescent which when added to a fluid (water, oil etc) will visually show the path the fluid is travelling and therefore reveal a leak if there is one.

Tracer Gas
A gas which passing through a leak, can then be detected by a specific leak detector and thus disclose the presence of a leak.

Ultrasonic Leak Detector
Translates inaudible ultrasonic frequencies into a variety of recognizable sounds and meter readings.

Dictionary: Empty space, devoid of matter.
Practical: A condition in which the quantity of atmospheric gas present is reduced to the degree that, for the process involved its effect can be considered negligible.

Vapor is the gaseous state of a material below its boiling point.

Vapor Density relates the molecular weight of a gas to the molecular weight of air.

Vapor density is the weight of a volume of pure vapor or gas (with no air present) compared to an equal volume of dry air, at the same temperature and pressure. This information assists in determining the optimum location of a gas detecting sensor. A vapor density figure of less than 1 indicates that the vapor is lighter than air and will tend to rise in a relatively calm atmosphere. A figure greater than 1 indicates that the vapor is heavier than air and may travel at low levels for a considerable distance to a source of ignition and flash back (if the vapor is in the flammable range). Note that some gases such as ethane have a vapor density of 1 and may be present at low levels or may rise significantly, dependent upon ambient conditions.

Virtual Leak
Semblance of a leak caused by slow release of trapped gas (wall surface or intermittent area - usually under vacuum).

Wet Developer (also called Aqueous Developer)
A mixture of developing (inspection penetrant) powder and water that is used to draw the penetrant indications to the surface. See Developer.

Zero Gas
Zero gas is clean air, and is an excellent way of insuring that a small release of gas is not near the sensor while zeroing the sensor signal during calibration.

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