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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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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 FIREhazard

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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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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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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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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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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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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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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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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Catalytic Bead Sensors Normally last up to 4-5

years less likely to break

Sensor reads 0-100 %

Some monitors sensors shut off at 100% LEL to prevent saturation

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The dual beads compare 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

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Catalytic Bead Sensors

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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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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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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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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