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Vi verner liv, miljø og verdier... 1 Gas Detection Systems Sensor Operating Theory.

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Presentation on theme: "Vi verner liv, miljø og verdier... 1 Gas Detection Systems Sensor Operating Theory."— Presentation transcript:

1 Vi verner liv, miljø og verdier... 1 Gas Detection Systems Sensor Operating Theory

2 Vi verner liv, miljø og verdier... 2 Gas/Vapor Hazards- Flammable/Combustible Toxic Oxygen-deficiency Oxygen-enrichment

3 Vi verner liv, miljø og verdier... 3 Gas Detection Objectives Measure gas before hazardous concentration is present Provide outputs for Emergency Action Suitable for extreme environments (arctic, tropic, etc) Complement to Flame Detection equipment Cost-effective protection

4 Vi verner liv, miljø og verdier... 4 Gas Sensing Technologies Combustibles:Catalytic and Infrared Hydrogen Sulfide:Electrochemical & MOS Oxygen:Electrochemical Carbon Monoxide:Electrochemical Chlorine:Electrochemical Sulfur Dioxide:Electrochemical Nitrogen Dioxide:Electrochemical

5 Vi verner liv, miljø og verdier... 5 Key Gas Detection Definitions Measurement range:gas concentration at full scale sensor output Unit of Measurement:%LEL, PPM, or %volume Flashpoint:minimum temperature of liquid where enough vapor is given off to sustain a fire or explosion Vapor Density:tendency of a pure gas of vapor to rise or sink in clean air Calibration:process of matching transmitter signal to sensor measurement

6 Vi verner liv, miljø og verdier... 6 Combustible Gases-Lower & Upper Limits* Too lean for ignition Too rich for ignition Clean Air Combustibles Lower Explosive Limit (LEL) Upper Explosive Limit (UEL) Typical Combustible Gas Detector Range of Measurement 50% *No differentiation between the terms “Explosive” and “Flammable” as applied to the lower and upper limits of flammability

7 Vi verner liv, miljø og verdier... 7 Flammable Limits & Volumetric Equivalents Methane Gas:100% LEL=5% by volume in air 75%= 3.75“ 50%= 2.5%“ 25%= 1.25%“ 10%=.50%“

8 Vi verner liv, miljø og verdier... 8 Catalytic Sensor Operation Contact-based Destructive measurement Traditional method Only option for hydrogen gas detection Used w/ transmitter module to provide 4-20 ma signal output Typical life of 3-6 years 2 year mfgr warranty

9 Vi verner liv, miljø og verdier... 9 Active Element - Hot wire resistor coated with catalytic material Reference Element - Same as active but without catalyst Catalytic Oxidation of flammable gas generates heat & changes resistance of Active Element Explosion is prevented by Flame Arrestor Constant U or I power source required Must calibrate on start-up & routinely for accuracy Flammable gas Catalytic Sensor Operation

10 Vi verner liv, miljø og verdier... 10 Catalytic Sensor Operation

11 Vi verner liv, miljø og verdier... Catalytic Sensor Limitations FAIL TO DANGER Poisoning of Catalyst Plugging of Flame Arrestor Read Low @ High Gas Levels Frequent Calibration Service Life - Variable (Months)

12 Vi verner liv, miljø og verdier... Infrared Combustible Detectors Principal of Operation Gases are Transparent to Visible Light BUT... Gases are Strongly Opaque to Infrared Light at Some Wavelengths Methane Propane active absorption band reference absorption band

13 Vi verner liv, miljø og verdier... Point Infrared Detector Principle of Operation Flammable hydrocarbon gas

14 Vi verner liv, miljø og verdier... Fail-Safe Operation

15 Vi verner liv, miljø og verdier... Open Path IR Gas Detector Instrumental Layout IR Source Microprocessor and Electronics Gas Concentration (LEL) Reference Signal Detector Measurement Signal Detector Optical Beamsplitter Optical Filters 10 - 60 m Flammable hydrocarbon gas

16 Vi verner liv, miljø og verdier... Drivers for Optical Technology Safety Driven Design Requirements –FAIL SAFE No Unrevealed Sources of Failure –HIGH AVAILABILITY Long Service Life Low and Easy Maintenance

17 Vi verner liv, miljø og verdier... 17 Electrochemical Toxic Sensor Theory Specific to toxic gas type (H2S, CO, SO2, NO2, CL2 ) Consists of sensing(working), reference, and counter electrodes Vapor diffusion into capillary results in oxidation/ion reduction The signal (current) from the working electrode is converted to a voltage by operational amplifier Capillary Entrance O-ring Membrane Electrolytic fluid Sensing Electrode Reference Electrode Counter Electrode

18 Vi verner liv, miljø og verdier... 18 Electrochemical Toxic Sensor Operation All provide 4-20 ma signal output 0-100 ppm range typical Carbon monoxide ranges: 0-100, 0-500, or 0-1000 ppm- must specify Nitrogen dioxide range: 0-20 ppm Hydrophobic filter provides humidity protection All designed to meet X-proof req. except CL2, SO2 sensors (I.S. barrier required) All designed for calibration by a transmitter, controller or external control system.

19 Vi verner liv, miljø og verdier... 19 Traditional Sensor Calibration Matching transmitter output to raw sensor signal in clean air (zero) & mid-scale (span) conditions Manual or Automatic depending upon transmitter type Adjustments necessary if drifting has occurred Requires compressed, accurate calibration gases Always required @ start-up & after sensor replacement Every 30-90 days thereafter typical

20 Vi verner liv, miljø og verdier... 20 Gas Detector Calibration Requirements Catalytic Sensors:Field Calibration required for accuracy DEC IR Detectors:Routine calibration not required;Factory calibrated for methane only Electrochemical Sensors: Field Calibration required for accuracy DEC Open Path IR System:Routine calibration not required;Factory calibrated for methane only


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