direct reading instruments

7/30/2019 Direct Reading Instruments

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Direct-ReadingGas 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

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

(also known as Pellistors)


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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)

TheoryTarget 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

AdvantageInexpensive

Disadvantages

Not selective, and this is often misrepresented

Affected by humidity

Not truly analyticalbetter as go/no-go detectors


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NON-DISPERSIVE INFRARED (NDIR)

TheoryTarget gas absorbs infrared light at a particular wavelength

Via Beers Law

calculations

concentration

can be determined

Application Hundreds of compounds, especially organics


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BEERS 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

ApplicationVolatile Organic Compounds (VOCs)


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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 GCs

Disadvantages

Nonselective among organic vapors below

ionization potential of lamp

Affected by high humidity

UV lamps are expensive


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ELECTROCHEMICAL SENSORS

TheoryTarget gas is adsorbed on an electrocatalytic sensingelectrode, after passing through a diffusion medium, and is

electrochemically reacted.

The current producedby this reaction isdirectly proportional

to the gasconcentration.

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 temperaturereadily correctable


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WHAT IS THE MOST IMPORTANT THINGIN GAS DETECTION WHEN USINGDIRECT-READING INSTRUMENTS?

PROPER CALIBRATION!!

Without a clean zero gas and an accurate verified

calibration standardthere is no point in doing any gas

detection


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MONITORING PRODUCTS

OFFERED BY


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PORTABLE ANALYZER


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CONTINUOUS MONITORING SYSTEMSINGLE POINT APPLICATION


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CONTINUOUS MONITORING SYSTEMTWO POINT APPLICATION


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CONTINUOUS MONITORING SYSTEMMULTI-POINT (THREE OR MORE) APPLICATION


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ARC-MAX

DATA ACQUISITION, ARCHIVING,

AND REPORTING


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ARC-MAXMAIN SCREEN


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ARC-MAXSHIFT REPORT


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ARC-MAXTRENDING


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ARC-MAX ALARM LOG


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For Survey Applications,

We Recommend OurNomad 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 adjustable1 second to 10 hours in 1 second intervals

Storage capacity16336 readings

Excellent software included


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Visualize your measurement session with an auto-scaling time history graph


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Time history graph detail screen

Zoomed in on particular portion of the curve


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Ability To View All Individual Data Points


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CONCLUDING REMARKS

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

PROFESSOR GEORGE BYRNS

direct reading instruments

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