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
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
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.
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
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
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
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.
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
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).
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