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Combustible Gas Monitors

Combustible Gas Basics
A combustible gas is one that will burn when mixed with air (or oxygen) and ignited. Combustible gas-air mixtures can be burned over a wide range of concentrations. The actual minimum concentration varies from about 0.5% to about 15% by volume in air for most common CHCs. This concentration is always referred to as 100% LEL or LFL for that gas.

(measured in % of volume by air)

  Compounds   LFL/LEL UFL/UEL
Alcohol Methanol CH3OH 6.7 36
  Ethanol CH3CH2OH 3.3 19
  Propanol CH3CH2CH2OH 2.2 14
Esters Ethyl Acetate CH3COOC2H5 2.2 11
Ethers Ethyl Ether CH3CH2OCH2CH3 1.9 36
  Methyl Ether CH3OCH3 3.4 27
Hydrocarbons Methane CH4 5.0 15
  Propane CH3CH2CH3 2.1 9.5
  Butane CH3CH2CH2CH3 1.8 8.4
  Hexane CH3CH2CH2CH2CH2CH3 1.2 7.4
  Heptane CH3CH2CH2CH2CH2CH2CH3 1.05 6.7
  Acetylene HC=CH 2.5 100
  Gasoline (unleaded)   1.2 7.1
  Jet Fuel (JP-4)   1.3 8
  Naphtha   1.7 4.8
  Turpentine   0.7  
Cyclic Compounds Toluene C6H5CH3 1.2 7.1
  Xylene C6H4(CH3)2 1.1 6.4
  Ethylene Oxide CH2OCH2 3.6 100
Ketones Acetone CH3COCH3 2.6 13
  Methyl Ethyl Ketone CH3COC2H5 1.4 10
Inorganic Gases Carbon Monoxide CO 12.5 74
  Hydrogen Sulfide H2S 4.0 44
  Ammonia NH3 15 28
  Hydrogen H2 4 75

Combustible Gas Detector Setpoints
Combustible gas-air mixtures can be burned over a wide range of concentrations. For each there is a specific minimum concentration above which an ignition source will cause an explosion or flame front propagation. This is called the Lower Explosive Limit (LEL) of that gas or vapor. It is sometimes called the Lower Flammable Limit (LFL). The LEL is different for every gas. For most CHC’s, this minimum concentration varies from about 0.5% to about 15% by volume in air. This concentration represents 100% on the Delphian system. Some combustible gases are lighter than air, some are heavier. Be sure the characteristics of the gas are known before attempting to monitor it or enter an area containing the gas. Typical alarm settings are: Low alarm = 20% LEL, High alarm = 40% LEL, High-High alarm = 60% LEL. Only the user can judge whether these settings are appropriate.

Choosing between infrared and catalytic bead technology:

Advantages of Infrared Gas Detectors
• immune to poisons
• consumables (source & detector) tend to outlast catalytic sensors
• continuous self-monitoring means less calibration
• more fail safe because the system is actively checking itself
• can often perform more reliably in high or varying flow conditions
• fewer interfering gases
• even if flooded, will not lose the signal it can detect

Advantages of Catalytic Gas Detectors
• can perform more reliably in dusty/dirty atmospheres because they are not as sensitive as optics to the buildup of industrial contaminants
• can perform more reliably in high temperature applications
• not significantly affected by changes in pressure
• catalytic bead sensors with a constant temperature circuit are not affected by changes in humidity. All catalytic sensors are less sensitive to humidity and condensation
• can detect most combustible hydrocarbons
• can detect hydrogen which is invisible to infrared
• smaller footprint than infrared

Calilbration Gas Selection for Combustible Gas Detectors
It is not always desirable or necessary to calibrate with the specific gas which is most likely to be present.  The standard calibration gases selected by Delphian give excellent results for most applications.  Delphian's calibration kit can be used to calibrate for ANY combustible gas.  Delphian will recommend which calibration gas to use and provide calibration instructions.  See Transfer Factors

Glossary of gas detection terms  |   Part Numbers   |   How to specify a Delphian system

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