Sensor Operating Theory Gas Detection Systems Sensor Operating Theory

Gas/Vapor Hazards- Flammable/Combustible Toxic Oxygen-deficiency Oxygen-enrichment

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

Gas Sensing Technologies Combustibles: Catalytic and Infrared Hydrogen Sulfide: Electrochemical & MOS Oxygen: Electrochemical Carbon Monoxide: Electrochemical Chlorine: Electrochemical Sulfur Dioxide: Electrochemical Nitrogen Dioxide: Electrochemical

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

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

Flammable Limits & Volumetric Equivalents Methane Gas: 100% LEL= 5% by volume in air 75% = 3.75 “ 50% = 2.5% “ 25% = 1.25% “ 10% = .50% “

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

Catalytic Sensor Operation 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

Catalytic Sensor Limitations FAIL TO DANGER Poisoning of Catalyst Plugging of Flame Arrestor Read Low @ High Gas Levels Frequent Calibration Service Life - Variable (Months)

Infrared Combustible Detectors reference absorption band active absorption band Principal of Operation Gases are Transparent to Visible Light BUT... Gases are Strongly Opaque to Infrared Light at Some Wavelengths Methane Propane

Point Infrared Detector Principle of Operation Flammable hydrocarbon gas

Fail-Safe Operation

Open Path IR Gas Detector Instrumental Layout 10 - 60 m Flammable hydrocarbon gas Optical Beamsplitter Measurement Signal Detector IR Source Optical Filters Microprocessor and Electronics Reference Signal Detector Gas Concentration (LEL)

Drivers for Optical Technology Safety Driven Design Requirements FAIL SAFE No Unrevealed Sources of Failure HIGH AVAILABILITY Long Service Life Low and Easy Maintenance

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

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.

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

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