Gas Detection 101Gas Detection 101

Paul TarterPaul Tarter

South Central Regional ManagerSouth Central Regional Manager

RAE SystemsRAE Systems

AgendaAgenda

Wheatstone Bridge LEL SensorWheatstone Bridge LEL Sensor

ElectrochemicalElectrochemical

Photo-ionizationPhoto-ionization

Wheatstone Bridge Catalytic Bead Wheatstone Bridge Catalytic Bead LEL SensorsLEL Sensors

Wheatstone bridge catalytic bead Wheatstone bridge catalytic bead LEL sensorsLEL sensors

Catalytic “Hot Bead” combustible sensorCatalytic “Hot Bead” combustible sensor Detect combustion gas by catalytic oxidationDetect combustion gas by catalytic oxidation When exposed to gas oxidation reaction When exposed to gas oxidation reaction

causesbead to heatcausesbead to heat Requires oxygen to detect gasRequires oxygen to detect gas

Developed by Dr. Oliver Johnson 1926-1927 of Developed by Dr. Oliver Johnson 1926-1927 of Standard Oil Co. of CA ( now Chevron)Standard Oil Co. of CA ( now Chevron)

Virtually EVERY combustible gas monitor today is Virtually EVERY combustible gas monitor today is derived form this designderived form this design

Variously Called “Wheatstone Bridge” or “Catalytic Variously Called “Wheatstone Bridge” or “Catalytic Bead” sensorsBead” sensors

Wheatstone bridge catalytic bead Wheatstone bridge catalytic bead sensor is like an electric stovesensor is like an electric stove

•One element has a catalyst and one doesn’t•Both are turned on low•The element with the catalyst “burns” gas at a lower level and heats up•As this is a combustion process a minimum of 12-16% oxygen is required•The hotter element has more resistance and the Wheatstone Bridge measures the difference in resistance between the two elements

How a LEL Sensor WorksHow a LEL Sensor Works

How a LEL Sensor WorksHow a LEL Sensor Works

How a LEL Sensor WorksHow a LEL Sensor Works

Catalytic LEL Sensor ResponseCatalytic LEL Sensor Response

When a LEL monitor is When a LEL monitor is calibrated to a gas (i.e. calibrated to a gas (i.e. methane) it always methane) it always thinks it is seeing thinks it is seeing methane.methane.

Like a truck that is Like a truck that is designed to use a designed to use a specific size of tirespecific size of tire

That size tire is That size tire is calibrated to the calibrated to the speedometer.speedometer.

Catalytic LEL Sensor ResponseCatalytic LEL Sensor Response

If your monitor If your monitor calibrated for methane calibrated for methane is exposed to gasolineis exposed to gasoline

It would be like putting It would be like putting a different size tire on a different size tire on that truck and not that truck and not adjusting the adjusting the speedometer.speedometer.

It will still show a It will still show a speed, but it will not be speed, but it will not be accurateaccurate

Catalytic LEL ResponseCatalytic LEL Response

LEL sensors are typically calibrated for methane gas.LEL sensors are typically calibrated for methane gas. The LEL of methane is 5%.The LEL of methane is 5%. When the meter reads 100% in a methane environment, When the meter reads 100% in a methane environment,

there is 5% methane by volume in the room.there is 5% methane by volume in the room.

Propane = 1.6Propane = 1.6 Hexane, n- = 1.1Hexane, n- = 1.1 Turpentine = 2.9Turpentine = 2.9 Acetone = 2.2Acetone = 2.2 Ammonia = 0.8Ammonia = 0.8 Phosphine = 0.3Phosphine = 0.3

Electrochemical (EC) SensorsElectrochemical (EC) Sensors

EC sensors are available in a variety of gases.EC sensors are available in a variety of gases. OxygenOxygen Carbon monoxideCarbon monoxide Hydrogen sulfideHydrogen sulfide ChlorineChlorine AmmoniaAmmonia Sulfur dioxideSulfur dioxide Hydrogen chlorideHydrogen chloride Hydrogen cyanideHydrogen cyanide Nitrogen dioxideNitrogen dioxide Many othersMany others

Electrochemical (EC) SensorsElectrochemical (EC) Sensors

Most are electrochemical sensors with Most are electrochemical sensors with electrodes (two or more) and chemical electrodes (two or more) and chemical mixture sealed in a sensor housing.mixture sealed in a sensor housing.

The gases pass over the sensor The gases pass over the sensor causing a chemical reaction within the causing a chemical reaction within the sensor.sensor.

Electrical charge is created which Electrical charge is created which causes a readout to be displayed.causes a readout to be displayed.

How an EC Sensor WorksHow an EC Sensor Works

How an EC Sensor WorksHow an EC Sensor Works

Photoionization Detectors (PID)Photoionization Detectors (PID)

Can detect a wide variety of gases in small Can detect a wide variety of gases in small amountsamounts

Will not indicate what materials are Will not indicate what materials are presentpresent

Can identify potential areas of concern Can identify potential areas of concern and possible leaks or contaminationand possible leaks or contamination

PID TechnologyPID Technology

Technology uses an ultraviolet (UV) Technology uses an ultraviolet (UV) lamp to ionize any contaminants in the lamp to ionize any contaminants in the air.air.

When contaminant particles become When contaminant particles become ionized, they carry an electrical charge ionized, they carry an electrical charge which can be read.which can be read.

Gas that is sampled must have Gas that is sampled must have ionization potential (IP).ionization potential (IP).

How a PID WorksHow a PID Works

Gas enters the instrument

It passes bythe UV lamp

It is now “ionized” +

-

Charged gas ions flow to charged plates in the sensor andcurrent is produced

Gas “Reforms” and exits theinstrument intact

What Does a PID Measure?What Does a PID Measure?

8.4

9.24

9.54

9.99

10.1

10.5

10.66

11.32

11.41

12.1

14.01

0 10.6

Styrene

Benzene

MEK

Vinyl Chloride

IPA

Ethylene

Acetic acid

Methylene Chloride

Carbon tetrachloride

Oxygen

Carbon monoxide

What Does a PID Measure?What Does a PID Measure?

Organics:Organics: AromaticsAromatics

BenzeneBenzene Ethyl benzeneEthyl benzene TolueneToluene Xylene Xylene

Ketones & aldehydesKetones & aldehydes AcetoneAcetone MEKMEK AcetaldehydeAcetaldehyde

Amines & amidesAmines & amides Diethyl amine Diethyl amine

Chlorinated hydrocarbons Chlorinated hydrocarbons Trichloroethylene (TCE)Trichloroethylene (TCE)

Sulfur compoundsSulfur compounds MercaptansMercaptans Carbon disulfideCarbon disulfide

Unsaturated hydrocarbonsUnsaturated hydrocarbons BButadieneutadiene Isobutylene Isobutylene

AlcoholsAlcohols Ethanol Ethanol

Saturated hydrocarbonsSaturated hydrocarbons ButaneButane OctaneOctane

InorganicsInorganics Ammonia Ammonia Arsine Arsine

What PIDs Do Not MeasureWhat PIDs Do Not Measure

RadiationRadiation AirAir

NitrogenNitrogen OxygenOxygen Carbon monoxideCarbon monoxide Water vaporWater vapor

ToxicsToxics Carbon monoxideCarbon monoxide Hydrogen cyanideHydrogen cyanide Sulfur dioxideSulfur dioxide

Natural gasNatural gas Methane Methane Ethane Ethane

AcidsAcids Hydrochloric acidHydrochloric acid Hydrofluoric acidHydrofluoric acid Nitric acidNitric acid

OthersOthers FreonsFreons Ozone Ozone

Questions??Questions??