Gas Detection Systems

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Gas Detection Systems

Sensor Operating Theory


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Gas/Vapor Hazards-

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


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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 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 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*

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

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 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

ACCEPTCAL/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



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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

• 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


Point Infrared Detector

• Principle of Operation

Microprocessorand

Electronics

GasConcentration

(LEL)

IR Source Perm eable GasCell

ReferenceSignal Detector

Measurem entSignal Detector

O ptica l Beam splitte r

O ptica l F ilters

IR T ransparent W indow s

Flammablehydrocarbon

gas


Fail-Safe Operation

DirtyOpticsandCleanAir

Active R eference

IR S ignal Leve l

HydrocarbonGas

Active R eference

IR S ignal Leve l

CleanAir

Active R eference

IR S ignal Leve l

A ctive R eference

IR S igna l Level

FailedSensor

A ctive R eference

IR S igna l Level

FailedIR-Source

Failed M icroprocessor = 0 m A


Open 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


Drivers 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

• 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

• 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 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

• 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

Date post:	23-Mar-2016
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Gas Detection Systems

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