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Module 3 flammable gas detection, american fork fire rescue

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  • 1. 1
  • 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 2
  • 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 3
  • 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 4
  • 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) 5
  • 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 6
  • 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 7
  • 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 8
  • 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% 9
  • 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 10
  • 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 11
  • 12. Catalytic Bead Sensors 12
  • 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 13
  • 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) 14
  • 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 15
  • 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 16