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ACGIH:
American Conference of
Governmental Industrial Hygienists
Accuracy:
Correctness.
How close is a measured value to the known "true value".
Acute Toxicity:
The ability of a substance to cause
poisonous effects resulting in severe biological harm or death soon after a
single exposure or dose. Also, any severe poisonous effect resulting from a
single short-term exposure to a toxic substance.
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.
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 Contact:
A switch that operates when some
pre-set low, high or abnormal condition exists
Alarm Only Instrument:
Is
an instrument providing an alarm(s) that 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.
Ambient Air:
Is
air to which the sensing element is normally exposed.
Ambient Pressure:
Pressure of the air surrounding a transducer.
Ambient Temperature:
The average or mean temperature of the surrounding air that comes in contact
with the equipment and instruments under test.
Ampere (amp):
A unit used to define the rate of flow
of electricity (current) in a circuit; units are one coulomb (6.25 x 108
electronics) per second.
Amplifier:
A device which draws power from a source other than the input signal and which
produces as an output an enlarged reproduction of the essential features of its
input.
Analog Ground:
In high-speed acquisition
applications, system ground is generally physically separated into analog and
digital grounds in an attempt to suppress digital switching noise and minimize
its effect on noise-sensitive analog signal processing circuitry. Input signal
conditioners, amplifiers, references, and A/D converters are usually connected
to analog ground.
Analog Output:
A voltage or current signal that is a continuous function of the measured
parameter.
Analog-to-Digital
Converter (A/D or ADC):
A device or circuit that outputs a binary number corresponding to an analog
signal level at the input.
Analyzer:
An instrument that provides
quantitative and qualitative measurements of the composition of a mixture or
compound.
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.
Asphyxiant:
The limiting factor is the available oxygen. The minimal oxygen content is
18% by volume, the maximum is usually set to 22%. Some asphyxiants present
an explosion hazard. (OSHA Federal Standard)
Atmospheric Pressure:
Weight of the earth’s atmosphere
over a unit area of the earth surface, measured with a mercury barometer at sea
level, which corresponds to the pressure required to lift a column of mercury
760 mm.
Atomic Number:
(symbolized Z): The number of protons in a nucleus. It determines the chemical
properties of an element.
Atomic Weight:
The nominal atomic weight of an isotope is given by the sum of the number of
neutrons and protons in each nucleus. The exact atomic weight differs
fractionally from that whole number because neutrons are slightly heavier than
protons and the mass of the nucleus is also affected by the binding energy.
Auto-Zero:
An automatic internal correction for
offsets and/or drift at zero voltage input.
Automatic Reset:
1. A feature on a limit controller that automatically resets the controller when
the controlled temperature returns to within the limit bandwidth set.
2.
The integral function on a Gas Monitoring controller that resets the alarm
relays once the gas level has returned to a normal reading.
AWG:
American Wire Gage.
Background:
The environment against which an indication must be evaluated.
Background Noise:
The total noise floor from all sources
of interference in a measurement system, independent of the presence of a data
signal.
BIAS Current:
A very low-level DC current generated
by the panel meter and superimposed on the signal. This current may introduce a
measurable offset across a very high source impedance.
Bias Voltage:
DC (no load or quiescent) output level
of a low impedance transducer powered by constant current excitation.
Blocking:
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.
Burn-In:
A long term screening test (either vibration, temperature or combined test) that
is effective in weeding out infant mortalities because it simulates actual or
worst case operation of the device, accelerated through a time, power, and
temperature relationship.
Calibration Procedure:
A procedure that is performed to determine and set the parameters affecting an
instrument’s performance in order to ensure its designed function within
prescribed limits.
Calibration Curve:
A
plot, generally drawn on semi-logarithmic graph paper, of electrode potential
versus ion level in two or more standardizing solutions. Unknown samples are
determined by converting measured electrode potential to ionic activity or
concentration using the curve. Gran's Plot Paper can also be used to draw
calibration curves.
CAS Registration Number:
A number assigned by the Chemical
Abstracts Service to identify a chemical.
Catalysis:
is a phenomena in which a relatively small amount of material augments the rate
of reaction without itself being consumed.
Ceiling:
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
TWA.
See TLV-C. (OSHA Federal Standard)
CNG:
ACRONYM - Compressed natural gas, primarily methane.
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.
Concentration:
The
actual amount of a substance in a given volume of solution. When measuring ionic
concentrations by electrode, a distinction is made between the concentration of
the free, unbound ion, and total concentration, which includes ions bound to
complexing agents.
Consumables:
Are
those materials or components which are depleted or require periodic replacement
through normal use of the instrument.
Current-Loop:
A form of communications wherein a pair of wires is used to transmit the signal
as a current. Levels of 4 to 20 mA are often used to indicate the minimum and
maximum signal level, respectively. Sometimes, for digital applications, various
magnitudes of mA current are used to indicate a logical 1 and 0. The current
loop is often characterized by a maximum impedance of the device that is
connected to the loop.
Current:
The rate of flow of electricity. The unit of the ampere (A) defined as 1 ampere
= 1 coulomb per second.
Dead Band:
1. For chart records: the minimum change of input signal required to cause a
deflection in the pen position.
2.
For Gas Detection Transmitters: the “smart style transmitters” uses a band
where no reading is shown up to a certain detection point.
This eliminates flashing readings due to electrical and other
interference(s) not gas related.
Detection limits:
The smallest amount of a substance that can be measured. Usually defined as a
signal that is at least three times the standard deviation of the background
signal. The measurement of any signal is not perfect; there is always a certain
amount of noise in the signal. Three standard deviations are a measure in terms
of a range, which, in a large number of measurements, would include 99 % of all
the measurements. So when we say that in order to be a detectable amount it must
be a signal which measures higher than 3 standard deviations we mean that that
signal is higher than 99 % of the noise in the background.
Differential Pressure:
The difference in static pressure
between two identical pressure taps at the same elevation located in two
different locations in a primary device.
Diffusion:
The process by which molecules in a single phase equilibrate to a zero
concentration gradient by random molecular motion Brownian motion. The flux of
molecules is from regions of high concentration to low concentration and is
governed by Fick's Second Law.
Drift:
A change of a reading or a set point value over long periods due to several
factors including change in ambient temperature, time, and line voltage.
Electrolyte:
A solution through which an electric
current may be carried by the motion of ions.
Environmental Rating:
A rating given (usually by agencies and regulatory bodies) to indicate the
severity of the environment in which the unit will function reliably.
Explosion:
Is
an uncontrolled chemical reaction that generates a large amount of heat and gas
in a short period of time.
Exposure:
Can
be measured in two ways: {1} ppm (1% = 10,000ppm)
{2} mg/m3 (mg/m3 = ppm x compound molecular
weight/24.45)
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 un-powered. 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.
Fail-safe Operation:
A feature designed to alert the operator via display, and to bring a process to
a safe shutdown via output, in the event of a particular control system or
process 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.
Flammability Range:
Flammable gases/vapors have limits below, and above, which flame propagation
does not occur. The volume below which flame propagation does not occur is
called the Lower Explosive (or Flammable) Limit (LEL). Below this concentration
the mixture is said to be too 'lean' for a flame to propagate. The volume of
gas/vapor in air above which a flame does not propagate is called the Upper
Explosive Limit (UEL)
Flashpoint:
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).
Flooding:
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.
Flow Rate:
Actual speed or velocity of fluid movement.
Flowmeter:
A device used for measuring the flow or quantity of a moving fluid.
Full Range Output:
The span of electrical output between the maximum positive and the maximum
negative end points of the calibration curve.
Gas:
A state of matter, in which the
molecules move freely and consequently the entire mass tends to expand
indefinitely, occupying the total volume of any vessel into which it is
introduced. Gases follow, within considerable degree of fidelity, certain laws
relating their conditions of pressure, volume and temperature. Gases mix freely
with each other, and they can be liquefied through compression or temperature
reduction.
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) that senses and responds to the presence of gas
in air mixtures.
Hydrophobic:
Tending not to combine with water, or incapable of dissolving in water:
insoluble or immiscible in water. A property exhibited by non-polar organic
compounds, including the petroleum hydrocarbons.
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).
IEEE:
Institute of Electrical and Electronics Engineers.
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.
Ignition Temperature:
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.
Impedance:
The total opposition to electrical flow (resistive plus reactive).
Inert Atmosphere:
A gaseous atmosphere that is not conducive to chemical reactions, such as helium
or nitrogen.
Infrared:
An area in the electromagnetic
spectrum extending beyond red light from 760 nanometers to 1000 microns (106
nm). It is the form of radiation used for making non-contact temperature
measurements.
Instrumentation:
Apparatus (often electronic), which is used for measurement or control; and for
display of measurements or conditions.
Interference:
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.
Intrinsically Safe:
An instrument that will not produce any spark or thermal effects under normal or
abnormal conditions that will ignite a specified gas mixture.
Leak:
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.
LED:
Light Emitting Diode
Linearity:
The maximum deviation of the calibration curve from a straight line between zero
and full scale expressed as a percent of full scale output and measured on
increasing measured only.
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):
The concentration of a gas below which the concentration of vapors is
insufficient to support an explosion. LELs for most organics are generally 1 to
5 percent by volume.
Long term stability:
Usually measured as the amount of drift an instrument exhibits, per hour, as a
percentage of the calibrated value. If you calibrate at a value of 10 ppm and
after an hour the instrument reads back a value of 10.4 ppm the drift is 0.4 ppm
/hour or 4 %/hr.
Manual Reset (Switch):
The switch in a limit controller that manually resets the controller after the
limit has been exceeded.
Membrane:
A
thin piece of material covering a structure or separating solutions and
permitting selective transport of a chemical species between the two solutions.
1.
Liquid ion exchange electrode membrane: a porous plastic disk, permeable to the
ion exchanger and impermeable to water, which allows the ion exchanger to
contact the sample solution and separates the internal filling solution from the
sample.
2.
Gas electrode membrane: a plastic film, permeable to gases but impermeable to
water, separating the electrode from the sample solution.
Milliamp:
One thousandth of an amp, 10-3 amps, symbol mA.
Millivolt:
Unit of electromotive force. It is the
difference in potential required to make a current of 1 milliampere flow through
a resistance of 1 ohm; one thousandth of a volt, symbol mV.
Minimum Detectable Leak Rate:
Magnitude of the smallest leak rate that can be detected by a given
method/technique.
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 that must be kept within
prescribed limits.
NEMA-12:
A standard from the National
Electrical Manufacturers Association, which defines enclosures with protection
against dirt, dust, splashes by non-corrosive liquids, and salt spray.
NEMA-4:
A standard from the National
Electrical Manufacturers Association, which defines enclosures intended for
indoor or outdoor use primarily to provide a degree of protection against
windblown dust and rain, splashing water, and hose-directed water.
NEMA-7:
A standard from the National Electrical Manufacturers Association, which defines
explosion-proof enclosures for use in locations classified as Class I, Groups A,
B, C or D, as specified in the National Electrical Code.
NIOSH:
National
Institute of Occupational Safety and Health
NIST:
National Institute of Standards & Technology.
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.
Ohmmeter:
An instrument used to measure
electrical resistance.
Open circuit:
Lack of electrical contact in any part of the measuring circuit (which consists
of the sensing electrode, instrument, reference electrode and solutions). An
open circuit is characterized by rapid large jumps in displayed potential,
followed by an off-scale reading. Frequent large erratic changes in potential
indicate an intermittent open circuit.
OSHA:
Occupational and Safety Hazard Organization.
Oxidation-Reduction:
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.
PEL/TWA:
"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 Federal Standard)
PLC:
Programmable Logic Controller.
PPB:
An abbreviation for Parts Per Billion. 1 part in 1,000,000,000.
PPM:
An abbreviation for Parts Per Million. 1 part in 1,000,000.
Peak:
Maximum one-time exposure, usually
10 minutes. No other exposure is allowed even below TWA. (OSHA Federal
Standard - also NIOSH)
Permeation Leak:
A leak through a barrier that has no hole or discrete passage.
Poisons:
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.
Precision:
A measure of the reproducibility of a determination usually measured as the
standard deviation of the measurements.
Range:
Is
the series of outputs corresponding to values of concentrations of the gas of
interest over which accuracy is ensured by calibration.
Response time:
The length of time necessary to obtain a stable electrode potential when the
electrode is removed from one solution and placed in another of different
concentration. Response time depends on the electrode type, the magnitude and
direction of the concentration change, temperature and the presence of electrode
interferences, if any. After exposure to a change in concentration, values of
apparent concentration asymptotically approach the true concentration. Under
most conditions, an electrode will exhibit a value of at least 90% of the final
value within one minute. For this reason, timed readings may be taken to speed
analyses. If electrode response is prolonged, as is often the case in very
dilute solutions, Time Response Paper can be used to estimate the final
electrode potential.
Sample Draw:
Sample Draw refers to a method to cause deliberate flow of the atmosphere being
monitored to a gas-sensing element.
Sensor:
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.
Sensitivity:
In American usage, sensitivity is a synonym for precision. In most European
countries, the sensitivity of an electrode or measurement refers to the lower
limit of detection.
Short term stability:
The ability of the instrument to read back the value you told the computer was
the true value after one hour of operation. Usually around 2%.
Span:
The algebraic difference between the
limits of the range from zero to full scale.
Span Adjustment:
The ability to adjust the gain of a
process or strain meter so that a specified display span in engineering units
corresponds to a specified signal span.
Specifications:
The group of error limits within which each device will operate.
Stationary:
Refers
to a gas detection instrument intended for permanent installation in a fixed
location.
Test Gas:
Test Gas is a known concentration of the gas to be detected diluted with clean
air or Nitrogen.
Troubleshooting:
Determining which part of a system is responsible for a problem. In working with
electrodes, it must be remembered that the system has six components: sensing
electrode, reference electrode, instrument, solution, measuring technique ...
and operator. Unexpected solution chemistry, incorrectly prepared standardizing
solutions, improper plotting of data, unsuitable reference electrodes, operator
error and poor choice of method account for many more problems than do
instrument or electrode failure.
TLV TWA:
Threshold Limit Value Time-Weighted Average (TLV-TWA) is the time-weighted
average concentration of a substance for a normal 8-hour workday and a 40-hour
workweek, to which nearly all workers may be repeatedly exposed, day after day.
(OSHA and ACGIH standard)
TLV/STEL
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)
TLV-C
Ceiling limit which should not be exceeded during any part of the working day.
(ACGIH Standard)
TLV-IDLH
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 Standard)
TWA/PEL
"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 Federal Standard)
Time Stamp:
Information added to a unit of data to indicate the time at which it was
processed.
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 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. Also called Search Gas.
Transmitter:
A transducer that has a 4-20 mA two wire output.
Underwriters Laboratories
Incorporated:
American association chartered to test and evaluates products, including power
sources. The group has four locations so an applicant can interact with the
office closest in the country to his/her own location.
UPS:
Uninterruptible Power Supply.
Vacuum:
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.
VOC:
Volatile organic compound - any
organic compound that evaporates readily to the atmosphere.
Voltmeter:
An instrument used to measure voltage.
Wafer:
Round disc of pure silicon that is
used in manufacturing integrated circuits.
Warm up Time:
The time required after a power supply
is initially turned on before it operates according to specified performance
limits.
Wheatstone
Bridge:
A network of four resistances, an emf source, and a galvanometer connected such
that when the four resistances are matched, the galvanometer will show a zero
deflection or "null" reading.
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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