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Gas Detection SystemsGas Detection Systems

Sensor Operating Theory

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Gas/Vapor Hazards-Gas/Vapor Hazards-

• Flammable/Combustible• Toxic• Oxygen-deficiency• Oxygen-enrichment

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Gas Detection Objectives 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

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Gas Sensing TechnologiesGas Sensing Technologies

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

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Key Gas Detection DefinitionsKey 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

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Combustible Gases-Lower & Upper Limits*

Combustible Gases-Lower & Upper Limits*

Too lean for ignition Too rich for ignition

Clean Air

Combustibles

LowerExplosiveLimit (LEL)

UpperExplosiveLimit (UEL)

Typical CombustibleGas Detector Range

of Measurement

50%

*No differentiation between the terms “Explosive” and “Flammable” as applied to the lower and upper limits of flammability

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Flammable Limits & Volumetric Equivalents

Flammable Limits & Volumetric Equivalents

Gas Type L.E.L. U.E.L.Methane 5% 15%Propane 2.1% 9.5%Hydrogen 4.0% 75%Hexane 1.1% 7.5%

Methane Gas: 100% LEL= 5% by volume in air

75% = 3.75 “50% = 2.5% “

25% = 1.25% “ 10% = .50% “

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Catalytic Sensor OperationCatalytic 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

DET-TRONICS

ALARM

INCREASE SETUP DECREASE

ACCEPT

CAL/RESET

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• 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

Flammablegas

Catalytic Sensor OperationCatalytic Sensor Operation

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Catalytic Sensor OperationCatalytic Sensor Operation

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Catalytic Sensor LimitationsCatalytic Sensor Limitations

FAIL TO DANGER•Poisoning of Catalyst•Plugging of Flame Arrestor•Read Low @ High Gas Levels

•Frequent Calibration•Service Life - Variable (Months)

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Infrared Combustible DetectorsInfrared 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

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Point Infrared DetectorPoint Infrared Detector

• Principle of Operation

Microprocessorand

Electronics

GasConcentration

(LEL)

IR Source Perm eable GasCell

ReferenceSignal Detector

M easurem entSignal Detector

O ptica l B eam splitte r

O ptica l F ilte rs

IR T ransparent W indows

Flammablehydrocarbon

gas

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Fail-Safe OperationFail-Safe Operation

DirtyOpticsandCleanAir

A ctive R eference

IR S igna l Leve l

HydrocarbonGas

A ctive R eference

IR S igna l Leve l

CleanAir

A ctive R eference

IR S igna l Leve l

A ctive R eference

IR S igna l Leve l

FailedSensor

A ctive R eference

IR S igna l Leve l

FailedIR-Source

Failed M icroprocessor = 0 mA

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Open Path IR Gas DetectorOpen Path IR Gas Detector

• Instrumental Layout

IR Source

Microprocessorand

Electronics

GasConcentration

(LEL)

ReferenceSignal Detector

MeasurementSignal Detector

Optical Beamsplitter

Optical Filters

10 - 60 m

Flammablehydrocarbon

gas

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Drivers for Optical TechnologyDrivers for Optical Technology

• Safety Driven Design Requirements– FAIL SAFE

• No Unrevealed Sources of Failure– HIGH AVAILABILITY

• Long Service Life• Low and Easy Maintenance

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Electrochemical Toxic Sensor Theory

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

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Electrochemical Toxic Sensor Operation

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.

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Traditional Sensor CalibrationTraditional 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

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Gas Detector Calibration Requirements

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