מושגי יסוד בתחום גילוי גז ובתעשייה
מונחים: עקרונות לגילוי גז
מונחים: תחזוקה ותפעול גלאי גז
מושגי יסוד בתעשייה ובתחום גילוי גז
Clean Air/ Fresh Air: An atmosphere which contains < I ppm Total
Hydrocarbons in an oxygen atmosphere of 20.9% Volume without any toxic
gases. Nitrogen makes up the balance of air with few other gasses.
Please Note: A Clean/ Fresh air: Is the only NON-toxic gas known!!!
Oxygen Deficiency: An atmosphere that contains less that 19.5% oxygen by
volume. You can suffocate to death in lower concentrations of oxygen.
Oxygen Enrichment: An atmosphere that contains more than 23.5% by volume of
oxygen. The hazard of enriched oxygen is fire; it makes it much easier to ignite with
high levels.
Toxic Gas: Any chemical or material that has evidence of an acute or chronic health
hazard and is listed in the NIOSH registry provided the substance causes harm at
any dose level.
Combustible Gas: A gas or material that will burn or ignite with the proper mixture of
air and heat source. Each combustible gas (hydrocarbons) has a Lower-Explosive-
Limit and will have a different flash point or volatility. Combustible gases are detected
with the LEL sensor (catalytic combustion).
% Volume: Percent-by-Volume is the full or 100 % area of a measurement.
TWA (TLV): Time Weighted Average or Threshold Limit Value of a gas. The
Government has set levels of toxic gas workers can be exposed to over a period of
time (is the average exposure a worker can perform work safely for a normal 8-hour
workday 40 hour week without any harmful effects.
TLV: Threshold-Level-Value is the same as TWA
IDLH: Immediately-Dangerous-to Life-and Health is the maximum concentration from
one that can escape within 30-minutes without any escape-impairing symptoms or
irreversible health effects. This is also used to determine the proper selection of a
respirator.
STEL: Short Term Exposure Limit is a 15 minute average that should never be
exceeded at any time during a workday…Shot-Term-Exposure-Limit is the exposure
limit or maximum concentration for a continuous exposure period of 15-minutes. This
has a maximum of four such periods per day, with at least I -hour between exposure
periods, provided the daily TLV-TWA is not exceeded.
LEL: Lower-Explosive-Limit is the lowest concentration of gas or vapor (% by volume
in air) that bums or explodes if an ignition source is present at ambient (room)
temperatures (usually 10/20…50% of the LEL is typical set point(s) for warning/
action).
LEL (Lower Explosion Limit): A Percentage of the Lower Explosion Limit of a
combustible gas (i.e., 20% LEL is 1/5 of the level of gas required for an explosion -
also referred to as LFL or Lower Flammability Limit)
UEL: Upper-Explosive-Level is the highest concentration of a material in air that
produces an explosion in fire or ignites when it contacts an ignition source (high heat,
electric arc, spark or flame). The higher the concentration of material in a smaller
concentration of air may be too rich to be ignited (this term is usually less important
then the LEL, when dealing with Gas Detection).
Organic Vapors: Compounds composed of carbon, hydrogen, and other elements
with a chain or ring structure.
PEL: Permissible Exposure Limit as set by OSHA, carries the weight of law.
PEL: OSHA establishes Permissible-Exposure-Limit. This may be expressed as
TWA limit or as a ceiling exposure limit that LEGALLY must never be exceeded
instantaneously even if the TWA exposure is not violated. OSHA PEL's have the
force of law!
PPM Parts Per Million: (i.e., 10PPM H2S is 10 parts of a million)
Oxygen: Percent of oxygen in the air - measured in % volume. Normal is 20.9% Vol.
Volume CH4: The percentage by volume of combustible methane gas in an area. At
5% by volume the mixture of methane is at 100% LEL and will explode if combined
with oxygen and a spark.
VOLUME Percent: Volume (i.e., 20.9% V of oxygen is the standard percentage of
oxygen by volume in air)
Absorption: To suck up. The gas penetrates the solid surface by chemical reaction.
Adsorption: Means to collect gas or liquid molecules on the outside or surface of
another material.
Bump Gas: This is a term used in the industry just recently to check the sensors if
they are respond to a gas (alive and kicking). This gas IS NOT certified and is most
likely unknown in concentration.
Difference between % LEL and % by Volume: The difference between % LEL and
% Volume is that the % LEL is a portion of the Volume range. (I.e. 100 % LEL for
methane is 5 % by Volume).
מונחים: עקרונות לגילוי גז
Electrochemical Cell: Toxic sensor in your instrument that will detect your "toxic
gases". The electrochemical sensor will produce an output when it comes in contact
with a toxic gas that the sensor has been designed for.
Fuel Cell: Oxygen sensor is a "fuel cell". It generates a constant electrical output just
like a battery. The oxygen sensor (self-depleting) can be consumed in or out of the
instrument.
Catalytic Combustion: A method used in LEL sensors to detect combustible gas.
Made up of a Whetstone bridge with two (2) matched pair sensing elements, one
active and one passive. These elements are heated to a temperature of 1000
degrees F and balanced to zero without gas applied. When gas comes in contact
with the elements, the active element heats to a higher temperature that changes the
resistance in the circuit and is displayed on the meter.
IR: Infra Red, these sensors are one of the most specific sensors in the industry,
however, they are still some interfering gases that can cause a false reading.
This is the preferred sensor for detecting C02 in the atmosphere from the ppm to %
volume along with certain hydrocarbons in inert (without oxygen) atmospheres.
M.O.S.: Metal-Oxide-Semiconductor is used in the CGM series and will pick up most
of your organic vapors in the atmosphere. This sensor can be a lifesaver when used
properly in a confined space instrument as general detector for unknown gases. Also
known as a "broadrange sensor" and will detect more gases than the now promoted
"P.I.D." instrument.
P.I.D.: Photo-ionization-Detector is a gas detector typically used to detect low
concentration of gases from 0.1 to 2000 ppm. Used as a leak detector for plant surveys to identify
potential problem areas and to determine the need for PPE. These instruments are
also nonspecific if more than one compound is in the air and have more limitations
מונחים: תחזוקה ותפעול גלאי גז
Zeroing: Zero means there should be NO GAS present in the area. When you "zero"
the instrument, you bring the instrument reading to display "Zero". You must make
sure that the atmosphere is free from gas, if not, use a Zero Gas that provides just air
with total hydrocarbons less than I ppm (in few applications, Oxygen deficiency
zeroing, is reversed to the zeroing term for other gasses, meaning: Zero is in fresh
air, typically 20.9% O2 in fresh air)
Calibration: This is a term used in the industry when an instrument is calibrated with a "certified" calibration gas.
Threshold limit: a value that an instrument is set to activate an external device/
Alarm (a HORN, Closing valve etc.) by using Contact Relay (CR).
Alarm Setpoint: similar as Threshold limit.
Several more terms: Alarm activation, Alarm deactivation, Threshold Hysteresis,
Service mode, General alarm, Fault ( 4>4<20 etc.), Cross sensativity, Trouble
shooting, Channel, False alarm, Delta Alarm (slope), Function test, Alarm 1,2,3.
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