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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 <ul><li>Wheatstone Bridge LEL Sensor </li></ul><ul><li>Electrochemical </li></ul><ul><li>Photo-ionization </li></ul>
  3. 3. Wheatstone Bridge Catalytic Bead LEL Sensors
  4. 4. Wheatstone bridge catalytic bead LEL sensors <ul><li>Catalytic “Hot Bead” combustible sensor </li></ul><ul><ul><li>Detect combustion gas by catalytic oxidation </li></ul></ul><ul><ul><li>When exposed to gas oxidation reaction causesbead to heat </li></ul></ul><ul><ul><li>Requires oxygen to detect gas </li></ul></ul><ul><li>Developed by Dr. Oliver Johnson 1926-1927 of Standard Oil Co. of CA ( now Chevron) </li></ul><ul><li>Virtually EVERY combustible gas monitor today is derived form this design </li></ul><ul><li>Variously Called “Wheatstone Bridge” or “Catalytic Bead” sensors </li></ul>
  5. 5. Wheatstone bridge catalytic bead sensor is like an electric stove <ul><li>One element has a catalyst and one doesn’t </li></ul><ul><li>Both are turned on low </li></ul><ul><li>The element with the catalyst “burns” gas at a lower level and heats up </li></ul><ul><li>As this is a combustion process a minimum of 12-16% oxygen is required </li></ul><ul><li>The hotter element has more resistance and the Wheatstone Bridge measures the difference in resistance between the two elements </li></ul>
  6. 6. Catalytic LEL Sensor Response <ul><li>When a LEL monitor is calibrated to a gas (i.e. methane) it always thinks it is seeing methane. </li></ul><ul><li>Like a truck that is designed to use a specific size of tire </li></ul><ul><li>That size tire is calibrated to the speedometer. </li></ul>
  7. 7. Catalytic LEL Sensor Response <ul><li>If your monitor calibrated for methane is exposed to gasoline </li></ul><ul><li>It would be like putting a different size tire on that truck and not adjusting the speedometer. </li></ul><ul><li>It will still show a speed, but it will not be accurate </li></ul>
  8. 8. Catalytic LEL Response <ul><li>LEL sensors are typically calibrated for methane gas. </li></ul><ul><li>The LEL of methane is 5%. </li></ul><ul><li>When the meter reads 100% in a methane environment, there is 5% methane by volume in the room. </li></ul><ul><ul><li>Propane = 1.82 </li></ul></ul><ul><ul><li>Hexane = 2 </li></ul></ul><ul><ul><li>Turpentine = 2.9 </li></ul></ul><ul><ul><li>Acetone = 2.2 </li></ul></ul><ul><ul><li>Ammonia = 0.8 </li></ul></ul><ul><ul><li>Phosphine = 0.26 </li></ul></ul>
  9. 9. Electrochemical (EC) Sensors <ul><li>EC sensors are available in a variety of gases. </li></ul><ul><ul><li>Oxygen </li></ul></ul><ul><ul><li>Carbon monoxide </li></ul></ul><ul><ul><li>Hydrogen sulfide </li></ul></ul><ul><ul><li>Chlorine </li></ul></ul><ul><ul><li>Ammonia </li></ul></ul><ul><ul><li>Sulfur dioxide </li></ul></ul><ul><ul><li>Hydrogen chloride </li></ul></ul><ul><ul><li>Hydrogen cyanide </li></ul></ul><ul><ul><li>Nitrogen dioxide </li></ul></ul><ul><ul><li>Many others </li></ul></ul>
  10. 10. Electrochemical (EC) Sensors <ul><li>Most are electrochemical sensors with electrodes (two or more) and chemical mixture sealed in a sensor housing. </li></ul><ul><li>The gases pass over the sensor causing a chemical reaction within the sensor. </li></ul><ul><li>Electrical charge is created which causes a readout to be displayed. </li></ul>
  11. 11. Photoionization Detectors (PID) <ul><li>Can detect a wide variety of gases in small amounts </li></ul><ul><li>Will not indicate what materials are present </li></ul><ul><li>Can identify potential areas of concern and possible leaks or contamination </li></ul>
  12. 12. PID Technology <ul><li>Technology uses an ultraviolet (UV) lamp to ionize any contaminants in the air. </li></ul><ul><li>When contaminant particles become ionized, they carry an electrical charge which can be read. </li></ul><ul><li>Gas that is sampled must have ionization potential (IP). </li></ul>
  13. 13. What Does a PID Measure?
  14. 14. What Does a PID Measure? <ul><li>Organics: </li></ul><ul><ul><li>Aromatics </li></ul></ul><ul><ul><ul><li>Benzene </li></ul></ul></ul><ul><ul><ul><li>Ethyl benzene </li></ul></ul></ul><ul><ul><ul><li>Toluene </li></ul></ul></ul><ul><ul><ul><li>Xylene </li></ul></ul></ul><ul><ul><li>Ketones & aldehydes </li></ul></ul><ul><ul><ul><li>Acetone </li></ul></ul></ul><ul><ul><ul><li>MEK </li></ul></ul></ul><ul><ul><ul><li>Acetaldehyde </li></ul></ul></ul><ul><ul><li>Amines & amides </li></ul></ul><ul><ul><ul><li>Diethyl amine </li></ul></ul></ul><ul><ul><li>Chlorinated hydrocarbons </li></ul></ul><ul><ul><ul><li>Trichloroethylene (TCE) </li></ul></ul></ul><ul><ul><li>Sulfur compounds </li></ul></ul><ul><ul><ul><li>Mercaptans </li></ul></ul></ul><ul><ul><ul><li>Carbon disulfide </li></ul></ul></ul><ul><ul><li>Unsaturated hydrocarbons </li></ul></ul><ul><ul><ul><li>B utadiene </li></ul></ul></ul><ul><ul><ul><li>Isobutylene </li></ul></ul></ul><ul><ul><li>Alcohols </li></ul></ul><ul><ul><ul><li>Ethanol </li></ul></ul></ul><ul><ul><li>Saturated hydrocarbons </li></ul></ul><ul><ul><ul><li>Butane </li></ul></ul></ul><ul><ul><ul><li>Octane </li></ul></ul></ul><ul><li>Inorganics </li></ul><ul><ul><li>Ammonia </li></ul></ul><ul><ul><li>Arsine </li></ul></ul>
  15. 15. What PIDs Do Not Measure <ul><li>Radiation </li></ul><ul><li>Air </li></ul><ul><ul><li>Nitrogen </li></ul></ul><ul><ul><li>Oxygen </li></ul></ul><ul><ul><li>Carbon monoxide </li></ul></ul><ul><ul><li>Water vapor </li></ul></ul><ul><li>Toxics </li></ul><ul><ul><li>Carbon monoxide </li></ul></ul><ul><ul><li>Hydrogen cyanide </li></ul></ul><ul><ul><li>Sulfur dioxide </li></ul></ul><ul><li>Natural gas </li></ul><ul><ul><li>Methane </li></ul></ul><ul><ul><li>Ethane </li></ul></ul><ul><li>Acids </li></ul><ul><ul><li>Hydrochloric acid </li></ul></ul><ul><ul><li>Hydrofluoric acid </li></ul></ul><ul><ul><li>Nitric acid </li></ul></ul><ul><li>Others </li></ul><ul><ul><li>Freons </li></ul></ul><ul><ul><li>Ozone </li></ul></ul>
  16. 16. Questions??