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Direct-Reading Gas Detection Instruments. Presented by. Michael D. Shaw. “DIRECT-READING”. Means that instrument provides display in engineering units (usually ppm or percent). Implication (not always true) that unit is reading in real time. GASES OF INTEREST. Combustible .

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Direct-Reading

Gas Detection Instruments

Presented by

Michael D. Shaw


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“DIRECT-READING”

Means that instrument provides display

in engineering units (usually ppm or percent)

Implication (not always true) that unit is reading in real time


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GASES OF INTEREST

Combustible

Oxygen (deficiency and enrichment)

Toxic


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MOST COMMON TECHNOLOGIES EMPLOYED

IN INDUSTRIAL HYGIENE APPLICATIONS


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CATALYTIC SENSORS

(also known as Pellistors)

Theory - Target gas is oxidized on catalytic element

The change in temperature causes

a change in resistance that is measured by the meter

Application – combustible gases


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CATALYTIC SENSORS

Advantage

Long life

Disadvantages

Different responses for each combustible gas

Can be poisoned

Most require at least 10% oxygen to work properly

Limited to percent-level detection


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METAL OXIDE SEMICONDUCTOR

(Also known as solid state, Figaro, or Taguchi sensors)

Theory – Target gas reacts with MOS (SnO2 ) and

changes its resistance – as measured by the meter

Application - Nearly all oxidizable gases


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METAL OXIDE SEMICONDUCTOR

Advantage

Inexpensive

Disadvantages

Not selective, and this is often misrepresented

Affected by humidity

Not truly analytical—better as “go/no-go” detectors


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NON-DISPERSIVE INFRARED (NDIR)

Theory – Target gas absorbs infrared light at a particular wavelength

Via Beer’s Law

calculations —concentration

can be determined

Application – Hundreds of compounds, especially organics


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BEER’S LAW ( FOR EXTRA CREDIT )

"The deeper the glass,The darker the brew,The less of the incidentLight that gets through"

I = I0ekP

I = intensity of light striking the detector

I0 = measured signal at zero concentration of target gas

k = system dependent constant

P = concentration of target gas

e = base for natural logarithms


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NON-DISPERSIVE INFRARED (NDIR)

Advantage

Extremely versatile

Disadvantages

Expensive

Can be fragile and complicated to service


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PHOTOIONIZATION DETECTOR (PID)

Theory - Ultraviolet light ionizes the target gas

Current produced is proportional to concentration

Application – Volatile Organic Compounds (VOC’s)


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PHOTOIONIZATION DETECTOR (PID)

Advantage

Detects a wide range of volatile organic compounds

Quite effective when used in conjunction

with chromatographic column → Portable GC’s

Disadvantages

Nonselective among organic vapors below

ionization potential of lamp

Affected by high humidity

UV lamps are expensive


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ELECTROCHEMICAL SENSORS

Theory – Target gas is adsorbed on an electrocatalytic sensing electrode, after passing through a diffusion medium, and is electrochemically reacted.

The current produced

by this reaction is

directly proportional

to the gas concentration.

Applications

Br2, CO, Cl2,

ClO2, C2H4,

ethylene oxide

HCHO, H2, hydrazine, HBr, HCl, HCN, H2S, NO, NO2, O3,

propylene oxide, SO2, and oxygen-- among others


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ELECTROCHEMICAL SENSORS

Advantages

Inexpensive

Linear output

Can be miniaturized

Disadvantages

Prone to interferences in unskilled hands

Affected by temperature— readily correctable


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WHAT IS THE MOST IMPORTANT THING

IN GAS DETECTION WHEN USING

DIRECT-READING INSTRUMENTS?

PROPER CALIBRATION!!

Without a clean zero gas and an accurate verified calibration standard—there is no point in doing any gas detection




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CONTINUOUS MONITORING SYSTEM

SINGLE POINT APPLICATION


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CONTINUOUS MONITORING SYSTEM

TWO POINT APPLICATION


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CONTINUOUS MONITORING SYSTEM

MULTI-POINT (THREE OR MORE) APPLICATION


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ARC-MAX®

DATA ACQUISITION, ARCHIVING,

AND REPORTING


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ARC-MAX® MAIN SCREEN


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ARC-MAX® SHIFT REPORT


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ARC-MAX® TRENDING


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ARC-MAX® ALARM LOG


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For Survey Applications,

We Recommend Our Nomad Data Logger

12-bit resolution

Models available for

0-100mV, 0-2.5V

and 4-20mA inputs

Self-contained units

also available for

temperature, humidity

and pH

Sampling rate adjustable—1 second to 10 hours in 1 second intervals

Storage capacity—16336 readings

Excellent software included



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Time history graph detail screen history graph

Zoomed in on particular portion of the curve



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CONCLUDING REMARKS history graph

We've looked at the most widely used operating

principles in direct-reading gas detection instruments

We've touched upon the importance of calibration

We've examined two approaches to data acquisition


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SPECIAL THANKS TO history graph

PROFESSOR GEORGE BYRNS


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