American Conference of Governmental Industrial Hygenists
refers to the condensation of gas or vapor on the surface of a
Aerosol is a
suspension in air (or gas) of minute particles of a liquid or a
(American National Standards Institute)
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
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.
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.
is air to which the sensing element is normally exposed.
(American Society for Non-Destructive Testing)
(American Society for Testing of Materials)
The limiting factor is the available oxygen. The minimal oxygen
content is 18% by volume, the maximum is usually set to 22%. Some
axphyxiants present an explosion hazard. (OSHA
The pressure of the atmosphere at a specified place and time.
The environment against which an indication must be evaluated.
Light in the near ultraviolet range, just short of visible light.
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
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.
(in Inspection Penetrant
The action of the developer in "drawing out" the penetrant from a
surface discontinuity causing the maximum bleed-out for increased
contrast and sensitivity.
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. See
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.
is the procedure used to adjust the instrument for proper response.
is a gas of known concentration(s) used to set the instrument span
or alarm level(s).
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.
A leak through a barrier that has a hole or a discrete passage. (See
also Permeation Leak)
Catalysis is a
phenomena in which a relatively small amount of material augments
the rate of reaction without itself being consumed.
Maximum concentration for short period (usually between 5 and 30
minutes, each gas is different) usually four such exposures are
allowed per day and average exposures must still be within
See TLV-C. (OSHA Federal
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.
For complete explanation.
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. See CHC
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).
See Flaw Finder
Developers are used to enhance the visibility of small amounts of
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.
See also Wet Developer
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.
A break or interruption in the normal structure of an object.
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
An emulsifier is used with certain types of
to make oil in the penetrant water dispersible and therefore water
Explosion is an
uncontrolled chemical reaction which generates a large amount of
heat and gas in a short period of time.
can be measured in two ways:
ppm (1% = 10,000ppm)
mg/m3 (mg/m3 = ppm x compound
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.
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.
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.
(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
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
See Flaw Finder
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.
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 (most often with water). Not to be
confused with a Burst Test. See
Hydrostatic Leak Testing
is interchangeable for: H2S, dihydrogen sulfide, and
hydrogen sulfide gas. See
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).
Temperature is the minimum temperature necessary to initiate
combustion (oxidation) and have self-sustained combustion of the
solid, liquid, gas, or vapor of interest.
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
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.
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.
Combined semi-quantitative leakage rates of individual leaks. Not
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
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. See
Leak Testing Primer.
The prevailing fluid flow through leak at existing conditions.
Quantity or measure of leakage per unit time (leak rate).
Leakage that is acceptable for a particular component or system. See
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.
See Inspection Penetrants
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
Mobile refers 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.
National Institute for Occupational Safety and Health
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
Leak location for a particular method/technique, no measured leak
rate except as to tests sensitivity on a go/no-go relationship. (See
leak testing solutions)
Nonsparking circuits are those which contain no contacts or in which
contacts are isolated from the surrounding atmosphere such as by
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.
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.
"Permissible exposure limit" or "Time Weighted Average".
This is the cumulative average concentration over an 8 hr/day, 40
hr/wk to which a worker can be safely exposed. (OSHA
Maximum one-time exposure, usually 10 minutes. No
other exposure is allowed even below TWA. (OSHA
Federal Standard - also NIOSH)
See Inspection Penetrants
A leak through a barrier that has no hole or discrete passage. (See
also Capillary Leak)
Parts Per Million (1% exposure = 10,000ppm)
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
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. See CGT-501
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)
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
Time for a tracer gas to penetrate boundary walls.
A primitive form of bubble leak testing since replaced by synthetic
solutions. For a discussion see
Solid is a phase of
matter characterized by a definite volume and definite shape. A
solid resists external forces to change shape.
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.
refers to a gas detection instrument intended for permanent
installation in a fixed location.
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.
"Threshold Limit Value" for the Time Weighted Average 8
hour day. (ACGIH Standard)
15 minutes "Short Term Exposure Limit" which should not be exceeded
at any time during the working day and not be repeated more than
four times per day. STELs should exceed three times the TWA for no
more than a total of thirty minutes and never more than five times
the TWA even if the TWA is never exceeded. there should be at least
60 minutes between STEL exposure periods and TWA should not be
exceeded. (ACGIH Standard)
Ceiling which should not be exceeded during any part of the working
day. (ACGIH Standard)
This is the concentration which could be "immediately dangerous to
life or health" and represents the maximum level from which one
could safely escape within thirty minutes. (ACGIH
"Time Weighted Average" or "Permissible exposure limit".
This is the cumulative average concentration over an 8 hr/day, 40
hr/wk to which a worker can be safely exposed. (OSHA
Test Gas is a
known amount of the gas to be detected diluted with a known amount
of clean air.
Film Leak Testing
A leak test using a solution which bubbles upon finding a leak. See
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.
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. See
A gas which passing through a leak, can then be detected by a
specific leak detector and thus disclose the presence of a leak.
Also called Search Gas. See
Ultrasonic Leak Detector
Translates inaudible ultrasonic frequencies into a variety of
recognizable sounds and meter readings. The
Sonic 3000 has been
set to translate frequencies between 30 and 50 kilohertz.
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.
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.
Semblance of a leak caused by slow release of trapped gas (wall
surface or intermittent area - usually under vacuum).
Wet Developer (also called
A mixture of developing (inspection
penetrant) powder and water that is used to draw the penetrant
indications to the surface. See Developer.
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.