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.

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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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12. Catalytic Bead Sensors
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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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