Gas detection 101

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Gas detection 101

  1. 1. Gas Detection 101 Paul Tarter South Central Regional Manager RAE Systems
  2. 2. Agenda Wheatstone Bridge LEL Sensor Electrochemical Photo-ionization
  3. 3. Wheatstone Bridge Catalytic Bead LEL Sensors
  4. 4. Wheatstone bridge catalytic bead LEL sensors Catalytic “Hot Bead” combustible sensor  Detect combustion gas by catalytic oxidation  When exposed to gas oxidation reaction causesbead to heat  Requires oxygen to detect gas Developed by Dr. Oliver Johnson 1926-1927 of Standard Oil Co. of CA ( now Chevron) Virtually EVERY combustible gas monitor today is derived form this design Variously Called “Wheatstone Bridge” or “Catalytic Bead” sensors
  5. 5. Wheatstone bridge catalytic bead sensor is like an electric stove•One element has a catalyst and onedoesn’t•Both are turned on low•The element with the catalyst“burns” gas at a lower level andheats up•As this is a combustion process aminimum of 12-16% oxygen isrequired•The hotter element has moreresistance and the WheatstoneBridge measures the difference inresistance between the twoelements
  6. 6. How a LEL Sensor Works
  7. 7. How a LEL Sensor Works
  8. 8. How a LEL Sensor Works
  9. 9. Catalytic LEL Sensor Response When a LEL monitor is calibrated to a gas (i.e. methane) it always thinks it is seeing methane. Like a truck that is designed to use a specific size of tire That size tire is calibrated to the speedometer.
  10. 10. Catalytic LEL Sensor Response If your monitor calibrated for methane is exposed to gasoline It would be like putting a different size tire on that truck and not adjusting the speedometer. It will still show a speed, but it will not be accurate
  11. 11. Catalytic LEL Response LEL sensors are typically calibrated for methane gas. The LEL of methane is 5%. When the meter reads 100% in a methane environment, there is 5% methane by volume in the room.  Propane = 1.6  Hexane, n- = 1.1  Turpentine = 2.9  Acetone = 2.2  Ammonia = 0.8  Phosphine = 0.3
  12. 12. Electrochemical (EC) Sensors EC sensors are available in a variety of gases.  Oxygen  Carbon monoxide  Hydrogen sulfide  Chlorine  Ammonia  Sulfur dioxide  Hydrogen chloride  Hydrogen cyanide  Nitrogen dioxide  Many others
  13. 13. Electrochemical (EC) Sensors Most are electrochemical sensors with electrodes (two or more) and chemical mixture sealed in a sensor housing. The gases pass over the sensor causing a chemical reaction within the sensor. Electrical charge is created which causes a readout to be displayed.
  14. 14. How an EC Sensor Works
  15. 15. How an EC Sensor Works
  16. 16. Photoionization Detectors (PID) Can detect a wide variety of gases in small amounts Will not indicate what materials are present Can identify potential areas of concern and possible leaks or contamination
  17. 17. PID Technology Technology uses an ultraviolet (UV) lamp to ionize any contaminants in the air. When contaminant particles become ionized, they carry an electrical charge which can be read. Gas that is sampled must have ionization potential (IP).
  18. 18. How a PID Works -Gas enters the instrument It is now “ionized” + Gas “Reforms” Charged gas ions and exits the It passes by instrument intact flow to charged the UV lamp plates in the sensor and current is produced
  19. 19. What Does a PID Measure? Carbon monoxide 14.01 Oxygen 12.1 Carbon tetrachloride 11.41 Methylene Chloride 11.32 Acetic acid 10.66 Ethylene 10.5 IPA 10.1 Vinyl Chloride 9.99 MEK 9.54 Benzene 9.24 Styrene 8.4 0 10.6
  20. 20. What Does a PID Measure? Organics:  Sulfur compounds  Mercaptans  Aromatics  Carbon disulfide  Benzene  Unsaturated hydrocarbons  Ethyl benzene  Butadiene  Toluene  Isobutylene  Xylene  Alcohols  Ketones & aldehydes  Ethanol  Acetone  Saturated hydrocarbons  MEK  Butane  Acetaldehyde  Octane  Amines & amides  Diethyl amine  Inorganics  Chlorinated hydrocarbons  Ammonia  Trichloroethylene (TCE)  Arsine
  21. 21. What PIDs Do Not Measure Radiation  Natural gas Air  Methane  Nitrogen  Ethane  Oxygen  Acids  Carbon monoxide  Hydrochloric acid  Water vapor  Hydrofluoric acid Toxics  Nitric acid  Carbon monoxide  Others  Hydrogen cyanide  Freons  Sulfur dioxide  Ozone
  22. 22. Questions??

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