This document describes lower explosive limit (LEL) gas sensors and their readings. It discusses that most LEL sensors are calibrated to methane or pentane and read measurements up to the LEL level. Beyond the LEL there is a fire hazard. It also describes the advantages and disadvantages of different sensor types, including wheatstone bridge sensors, catalytic bead sensors, and metal oxide sensors. Understanding how LEL sensors work and interpreting their readings is important for safety in identifying flammable gas risks.
2. Objectives
Identify the methodology behind flammable gas
sensors
Describe impact of lower explosive limit (LEL) levels
for a variety of gases and vapors
Describe advantages and disadvantages of basic
flammable gas sensors
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3. LEL Sensor Readings
Most sensors are calibrated to read methane or
pentane (most common)
Calibrated to read up to LEL
Some LEL sensors are available to read above the LEL by
reading in percent of volume.
Remember once the LEL is exceeded there is a FIRE
hazard
Will detect all flammable gases and vapors
Readings are only accurate for calibrated gases ( most
often methane or pentane)
Readings can be corrected using correction factors
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4. LEL Sensor Readings
Sensors calibrated to read up to LEL
Some new units will shut sensors off when atmospheres
exceeds the LEL
Sensor will deteriorate faster at higher levels
Some monitors allow you to change to read LEL of
percent by volume
In a methane environment
A reading of 100 percent for a sensor calibrated to
methane means there is 5 percent methane in the air by
volume
The LEL for methane is 5% volume in air
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5. LEL Sensor Readings
In a methane environment
A reading of 50 percent for a sensor calibrated to methane
means that there is 2.5 percent methane in the air by volume
An LEL sensor reading indicates presence of a
flammable gas or vapor
Low readings do not indicate an actual threat of fire
May indicate presence of flammable materials
One percent reading on LEL sensor indicates presence
of a flammable gas or vapor (use caution)
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6. Basic Principle of LEL Sensors
Target gas passes through sensor
Heated elements try and burn the gas
If a gas or vapor is flammable the senor will detect its
presence
Readings are only accurate for calibration gas
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7. Sensor Types
Wheatstone bridge sensor
Is a coiled piece of platinum wire in a heated sensor housing
New style has two separate wire coils in the middle of
the sensor
Wheatstone bridge sensor
When a flammable gas enters the sensor housing
Bridge attempts to burn the gas off
Burning creates a increase in heat and electrical
resistance in the sensor
This can be read on the sensor
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8. Problems with Wheatstone
Bridge Sensors
Don’t function in low oxygen environments less than
16 percent
Affected by lead vapors, corrosive vapors, and silicone
compounds can corrode filaments
Chronic exposures through high levels may saturate
the sensor
This makes it useless until it purges or recalibrated
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9. Problems with Wheatstone
Bridge Sensors
Most do not indicate when
they go above the LEL.
The sensor will hit 100 %
for a short time than
bounce back to 0% and
never rise again.
The bridge has burned out
and will not function
The Wheatstone bridge
reads on a scale of 0%100%
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10. Catalytic Bead Sensors
Also known as a pellistor sensor
Most common LEL sensor used today
Uses a bowl shaped string of metal with a bead in the
middle
Bead is coated with catalytic materials that aids in
burning process
Sensor has two beads
One exposed to the sample gas
The other is a reference bead
Wheatstone bridge and catalytic bead sensor are linear
sensors
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11. Catalytic Bead Sensors
The dual beads compare
Normally last up to 4-5
their electrical activity to
provide the meter
reading
The dual sensors can
compensate for temp.
humidity, and
atmosphere pressure
Catalytic Bead are more
precise than a
Wheatstone Bridge
years less likely to break
Sensor reads 0-100 %
Some monitors sensors
shut off at 100% LEL to
prevent saturation
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13. Metal Oxide Sensor
Tin-oxide element (heater) can burn gas
Metal oxide coating
Reads lower levels of flammable gases
Sensitivity allows it to read atmospheric gases or
materials
Readings can be falsely interpreted
Reality: sensor is reading gases
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14. Metal Oxide Sensor
MOS sensor is valuable as it can find small
amounts of contaminants in the air
Picks up dirt, dust and other particulates
Moister in air: Flammable gases, and even low
levels of vapors from a combustible liquid.
If the chemical has sufficient vapor pressure to get
into the air MOS typically can detect it.
Not a linear sensor (drawback)
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15. Metal Oxide Sensor
MOS LEL sensor do not
provide a readout but they
provide an audible warning
MOS reacts to tiny amounts
which is an outstanding
feature.
Most LEL monitors are not
that sensitive
MOS detectors are very useful
in determining whether
something is their and pin
pointing to where it is
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16. Summary
Ability to identify flammable risk is important
Understanding how the variety of LEL sensors
functions is critical for safety
Knowing LEL sensor readings is important for
making safe decisions
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