Firefighter Safety Depends on Gas Detector

Continuing Education
Firefighter Safety
Depends on Gas
Detector Accuracy
To earn continuing education credits, you must successfully complete the course examination.
The cost for this CE exam is $25.00. For group rates, call (973) 251-5055.
Firefighter Safety
Depends on Gas
Detector Accuracy
Educational Objectives
On completion of this course, students will
1) Review a case study involving complacency towards equipment maintenance.
2) Discover common types of monitoring and detection equipment carried by fire apparatus.
irefighters are not perfect. They have shortcomings just like other mortals. Some common sins
include complacency toward mainte1
nance and the “we’ll figure it out on scene”
mentality. Together, these two liabilities can
produce disastrous results. On May 7, 2009,
eight firefighters were injured as a result of a
natural gas explosion inside a Maryland strip
mall. The National Institute for Occupational
Safety and Health (NIOSH) investigation
report recommended that “fire departments
should ensure gas monitoring equipment
is adequately maintained and firefighters
are routinely trained on proper use.” Fire
departments can reduce the odds of repeating
similar incidents by maximizing the accuracy
of their gas detection equipment and by training personnel how to accurately interpret gas
Today, first-due apparatus often
carry monitoring and detection equipment. Modern technology has reduced
the size, cost, and complexity of gas
monitoring devices. First-due companies are commonly equipped with one
of two popular gas detection devices:
the single-sensor gas monitor, usually
equipped with a carbon monoxide (CO)
(1) Photos by author.
3) Review the importance of calibration.
4) Determine several causes of “False Positives” while
monitoring atmospheres.
sensor; or the multisensor gas monitor, usually equipped with
lower explosive limit (LEL), oxygen (O2), CO, and hydrogen
sulfide (H2S) sensors. The LEL sensor detects flammable gases,
the O2 sensor detects oxygen enrichment or deficiency, and
the CO and H2S sensors detect toxic gases.
Regardless of the brand or model that your
fire department uses, the technology found
inside all of these popular devices is basically the same. Catalytic bead or electrochemical sensor technology converts the presence
of flammable or toxic gas into an electrically
measurable signal, which is usually displayed as a number. Teams with strong fundamentals tend to be more successful than
others. The fundamentals of gas detection
involve consistent maintenance and accurate
interpretation of the numbers.
Poorly maintained gas monitors will produce inaccurate readings, which may contribute to bad decisions affecting civilians
and firefighters, such as unnecessary
evacuation or exposure to a dangerous atmosphere. Firefighters can easily
ensure accurate readings by calibrating
the gas monitor and by avoiding activities that harm gas sensors.
Gas sensors start losing accuracy
soon after they are manufactured as a
normal result of exposure to air and
other gases. The loss of accuracy is
called “sensor drift.” Older sensors and
those that have been exposed to high
Gas Detector accuracy ●
concentrations of their target gas (the
gas that the sensor is designed to detect)
have greater sensor drift.
Calibration is one way to compensate
for sensor drift. During the calibration, a
sensor is exposed to a specific concentration of target gas. Because of sensor drift,
the amount of gas the sensor detects will
differ from the amount of target gas to
which the sensor was actually exposed.
Calibration determines the difference and
adjusts the instrument readings up or
down to compensate. Regular calibration
is necessary to ensure accurate readings.
Most manufacturers recommend that their
gas sensors be calibrated monthly and after a significant gas exposure. Check your
operations manual for the recommended
calibration schedule.
The span reserve of a gas sensor is the
amount of life remaining in it; sensors
with less than 50 percent of their span reserve remaining will
fail calibration and must be replaced. Sensor span reserve
values can only be determined through calibration. The values
are displayed on the monitor at the end of each sensor’s
calibration and are recorded in the instrument’s calibration
Using expired calibration gas may also cause inaccurate
meter readings. Calibration gas will naturally degrade over
time; shelf life varies from months to years, depending on the
gas mixture. When a bottle of calibration gas has exceeded its
expiration date, the specific concentration of gas needed for
calibration no longer exists (photo 1).
Exposure to harmful substances can destroy sensors or at
least reduce their accuracy. For example, holding a gas monitor next to an idling apparatus exhaust pipe will clog, coat,
and corrode sensors (photo 2). Manufacturers generally provide a two-year
sensor warranty. The O2 sensor tends to
have the shortest lifespan because it is
constantly exposed to oxygen. CO and
H2S sensors usually last longer than two
years. The LEL sensor lifespan will vary
based on its exposure to high concentrations of flammable gases or poisons. LEL
sensor poisons include silicone vapors
(found in caulking, moisture-removing
lubricants, and some vehicle appearance
care products), leaded gasoline vapors,
diesel exhaust, and refrigerant gases (freons). Simple preventive measures such as
storing the device in an interior compartment, not an exterior one (exposure to
cold reduces battery life), and not allowing the device to get immersed or even
soaking wet (water is bad for electronics) can avoid on-scene frustration. The
only way to ensure that your gas monitor and the numbers it
displays are accurate is through regular calibration and daily
There is more to using a gas monitor than knowing how to
turn it on and off. Firefighters need confidence in their ability
to accurately interpret what the gas monitor is telling them.
This confidence is developed through training and experience.
Gas monitors do not provide instantaneous results. It takes
between 20 seconds and two minutes for a gas sensor to
process a sample of gas and display a number, which is called
the “lag time.” If you are walking nonstop from room to room
noting gas readings, any numbers the monitor produces will
not accurately reflect the gas concentrations in your current
location. If you are using tubing or an extension probe, the
lag time will be longer, typically an additional two seconds
● Gas Detector accuracy
Table 1. False Positive Sources
Gases that produce Gases that produce
false CO readings
false H2S readings
Recharging batteries Nitrogen dioxide
Alcohol-based products Nitrogen dioxide
Body spray, cooking wine, Sulfur dioxide
home brewing
Diesel exhaust
Carbon monoxide
Dry cleaning chemical
Nitric oxide
per foot of tubing or probe. Check your monitor’s operating
manual to confirm the lag time of particular sensors (photo 3).
A gas sensor can sense only target gases that are in close
proximity to the sensor or directly in front of the inlet port of
attached tubing/probe. Gases can sink, rise, hang around, or
do any combination of movements depending on their vapor
density, natural or manmade air currents, temperature, and
weather conditions. Using the three-step low/middle/high
gas sampling technique will improve your chances of finding
target gases.
First, hold the gas monitor low below your knees for 20
seconds. Most gases are heavier than air, which is the reason we
start low. If during those 20 seconds the numbers start to change,
wait a full two minutes before reading the number (photo 4).
Second, move the monitor to waist height for 20 seconds.
If during that 20-second interval the numbers start to change,
wait a full two minutes before reading the number (photo 5).
Third, move the monitor above your head for 20 seconds. If
during those 20 seconds the numbers start to change, wait a
full two minutes before reading the number (photo 6).
Repeat this low/middle/high technique at any entrance to
a structure, room, or point of interest within the investigation
area. If more than one gas monitor is available, take advantage
of this resource. Extra monitors will reduce the amount of
time spent hunting for the source of a gas leak. They will also
provide redundancy and confirm any unusual readings.
If any sensor has been exposed to a concentration of gas
beyond its maximum detection limit, the sensor display may
read “OR” (over range), “+++” symbols, or “ERR” (error). Check
your operations manual to confirm how your sensors will react. Newer LEL sensors will also turn themselves off to prevent
deadly exposure to a high concentration of flammable gas.
Always check O2 levels when monitoring for flammable
gases. The LEL sensor requires a specific amount of O2 to
function properly. If less than the required amount of O2 is
present, the LEL sensor will produce an inaccurate number.
Check your operations manual to confirm the minimum
amount of O2 your LEL sensor requires. When less than the
minimum amount of O2 is present, a diffusion sampling technique is required so the LEL sensor will produce an accurate
reading. A confined space or hazardous materials team should
Internal combustion engines, pulp mills
Water and sewage treatment plants
Combustion of coal
or petroleum
Incomplete combustion
Metal etching, blasting
be equipped to conduct diffusion sampling. The CO and H2S
sensors do not require O2 when monitoring.
Some nontarget gases that are chemically similar to your
target gas may produce a “false positive” or a sensor reading
that is not the result of exposure to a target gas. CO and H2S
sensors routinely produce false positives. Some sources of CO
and H2S false positives are listed in Table 1.
Nontarget gases do not affect the LEL sensor. Humidity
(moisture in the air) and oxygen-displacing gases affect the
O2 sensor and will lower its readings. An O2 reading of 20.9
percent by volume is considered normal.
Exposure to any hazardous material can be considered
safe as long as the exposure is below a specific amount. The
authority having jurisdiction (AHJ) will mandate an exposure
standard for your fire department to follow. The intent of this
standard is to protect firefighters and civilians from exposure
to dangerous concentrations of hazardous materials. The AHJ
standard is usually adapted from the Occupational Safety and
Health Administration (OSHA), NIOSH, or the American Conference of Governmental Industrial Hygienists (ACGIH). The
“time weighted average” (TWA) is a term OSHA, NIOSH, and
the ACGIH use. The OSHA and the ACGIH TWA state that a
healthy worker can be safely exposed to a specific amount of
a specific hazardous material for eight hours a day, 40 hours
per week with no adverse health effects. The NIOSH TWA is
based on a weekly exposure rate of 10 hours a day, 40 hours
a week.
You can use the TWA as a conservative respiratory protection decision-making benchmark. For example, most gas
sensors have two alarms. Set the first alarm below the TWA
to warn firefighters that they must be prepared to evacuate
civilians and don respiratory protection. Set the second alarm
at the TWA. At this final alarm, firefighters must ensure that
all civilians have been evacuated and that all firefighters have
donned respiratory protection. The ACGIH TWA for CO is 25
parts per million (ppm); for H2S, it is 10 ppm.
In addition to respiratory protection requirements, firefighters must also know when conditions are untenable. Many
fire departments use 10 percent of LEL as benchmark for
Gas Detector accuracy ●
flammable gas incidents. A reading
of 0.0 percent of LEL is considered
safe. A reading between 1.0 and 10
percent of LEL is unsafe for civilians but safe for firefighters wearing
respiratory protection and personal
protective equipment (PPE). Readings greater than 10 percent of LEL
are considered dangerous. Evacuate
everyone until LEL readings are below 10 percent. O2 readings between
19.5 and 23 percent by volume are
generally considered safe. Readings
of less than 19.5 percent by volume
are unsafe for civilians but safe for
firefighters wearing respiratory protection. O2 readings above 23 percent
by volume are dangerous because
the ignition point of all materials has
been lowered. Exit the hazard area
immediately if the LEL or O2 sensors have been compromised
by high concentrations of gas, poisions, or low O2 levels.
It’s not practical or possible for a first-due company to
detect every gas or vapor that may be encountered. Remember, you can’t help anyone if you’re dead, and you can’t enjoy
retirement if you have cancer. Wear full PPE including respiratory protection during suspicious odor investigations and
a toxic, colorless, odorless, and tasteless gas. A product of combustion,
it is commonly found where fuelburning appliances are malfunctioning. Firefighters use positive-pressure
ventilation to clear structures of the
products of combustion. A positivepressure fan’s gasoline engine
will increase CO levels inside the
building when running outside the
entrance of a building.
H2S is a toxic, colorless gas that
smells like rotten eggs and is produced when organic materials decay.
The most significant sources of H2S
are petrochemical industries and
wastewater facilities. Ethyl mercaptan, an odorant that also smells like
rotten eggs, is added to propane and
natural gas so that people can detect
leaks by smell. If you can smell rotten eggs, you may not necessarily be dealing with H2S.
Finally, don’t compare your gas detection equipment to
that of the utility company. The utility company uses different
sensors that are more sensitive, designed to detect gas in very
minute concentrations. These sensors have higher maintenance requirements and are more sensitive to nontarget gases.
Flammable substances are the most frequently encountered
hazardous materials. The LEL sensor in conjunction with the
O2 sensor will be used to size up this invisible threat. As mentioned earlier, many fire departments deploy a single-sensor
gas monitor equipped with a CO sensor. A CO sensor is not
designed to detect any flammable gas or vapor. Don’t use a
CO sensor to detect propane, natural gas, gasoline or diesel
vapors, or anything else that will burn.
Corrosive substances are the second most commonly encountered hazardous materials. The standard multisensor package
(LEL, O2, CO, H2S), will not detect corrosive gases. Strong corrosive gases with high vapor pressure will destroy these sensors and burn firefighters through their bunker gear. By simply
attaching a small piece of inexpensive universal pH paper to the
top of your gas monitor, you can reduce the risk of exposure to
corrosive gases. The pH paper turns dark red if a strong acid is
present and dark blue if a strong base is present. In either situation, withdraw to a safe location and call a hazardous materials
team (photo 7).
Flammable gases will reach toxic levels long before they
are within their flammable range. CO and H2S sensors are not
designed to determine the toxicity of flammable gases. CO is
When a gas monitor fails to perform as expected, firefighters are quick to remark, “That gas monitor is a piece of junk.”
The truth is that neglect of routine maintenance and a lack
of knowledge are typically the sources of most gas detection
problems. Modern technology has not eliminated the need for
ongoing maintenance and training. Basing decisions on the
readings of poorly maintained equipment or on the interpretations of untrained personnel is a recipe for disaster. ●
● BRUCE LAKE is a hazardous materials technician and
chemical, biological, radiological, and nuclear specialist
with Halifax Regional Fire and Emergency in Nova Scotia,
Canada. He is a graduate of the Fire Service Management
Program at Dalhousie University. Prior to joining the fire
service, Lake served in the Canadian Army as a nuclear,
biological, and chemical defense officer.
Continuing Education
Firefighter Safety Depends on Gas Detector Accuracy
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1)Complacency towards maintenance of equipment often leads to a
“we’ll figure it out on-scene” mentality.
2)What is one recommendation the National Institute of Safety and
Health (NIOSH) recommended after a natural gas explosion in
Maryland on May 7, 2009 that injured eight firefighters?
a. Fire departments should purchase gas monitoring equipment for
all fire apparatus
b.Fire departments should rarely maintain gas monitoring equipment due to wear-and-tear on meters
c. Fire departments should ensure gas monitoring equipment is
adequately maintained and firefighters are routinely trained on
proper use
d.Fire departments should not monitor potentially hazardous
3)How can fire departments reduce the odds of repeating similar
accidents to the 2009 Maryland gas explosion?
a. Minimizing the accuracy of their metering equipment
b.Maximizing the accuracy of their metering equipment
c.Purchase more metering equipment
d.Calling for mutual-aid meters early on in the incident
4)Fire departments are commonly equipped with two popular gas
detection devices: the single-sensor gas monitor, or the:
a. Multi-sensor gas monitor
b.Carbon Dioxide meter
c. Sulfur Dioxide meter
d.Photo-ionization detector
5)Multi-sensor gas monitors are usually equipped with which of the
following sensors:
a. Lower explosive limit
c.Carbon monoxide
d.All of the above
6)A catalytic bead or electrochemical sensor converts the presence
of flammable or toxic gas into an electrically measurable signal on
most meters used by fire departments.
7)The fundamentals of gas detection involve consistent maintenance
c. Accurate interpretation of the calibration procedure
d.Accurate interpretation of the numbers
8)Firefighters can easily ensure accurate readings by _________the
gas monitor and avoiding activities that harm gas sensors
d.None of the above
9)Gas sensors start gaining accuracy soon after they are manufactured as a normal result of exposure to air and other gases.
10) The loss of accuracy of a gas meter is called:
a. Meter drift
b.Sensor drift
d.Sensor loss
11) _________ is one way to compensate for sensor drift.
a.Purchasing new sensors
b.Routine maintenance of meter housing
d.All of the above
12) M
ost manufacturers recommend that their gas sensors are
calibrated ___________ and after a significant exposure.
13) U
sing expired calibration gas may also cause inaccurate meter
Continuing Education
Firefighter Safety Depends on Gas Detector Accuracy
14) Exposure to __________ can destroy sensors or at least reduce
their accuracy.
a.Harmful substances
d.Toxic environments
18) Exposure limits are established by the authority having jurisdiction, and are usually adopted from OSHA, NIOSH, or the ACGIH.
19) _ __________ are the most frequently encountered hazardous
15) The time it takes the sensor to process the gas and display a
number is called:
a. Free time
b.Lag time
c. Sensor delay
d.Sensor interpretation
a. Sulfur dioxides
b.Aromatic hydrocarbons
c. Flammable substances
d.Toxic atmospheres
20) The standard multi-sensor package will not detect:
16) The lower explosive limit sensor requires a specific amount of
oxygen to function properly.
a.Harmful substances
b.Toxic gases
c.Corrosive gases
d.Flammable gases
17) _______ and ________ sensors routinely produce false positives.
a.CO and H2S
b.CO and O2
c.H2S and LEL
d.UEL and LEL
Continuing Education
Firefighter Safety Depends on Gas Detector Accuracy
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