Direct-Reading Gas Detection Instruments Presented by Michael D. Shaw.

Direct-ReadingGas Detection Instruments

Presented by

Michael D. Shaw

“DIRECT-READING”

Means that instrument provides displayin engineering units (usually ppm or percent)

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

GASES OF INTEREST

Combustible

Oxygen (deficiency and enrichment)

Toxic

MOST COMMON TECHNOLOGIES EMPLOYEDIN INDUSTRIAL HYGIENE APPLICATIONS

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)

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

METAL OXIDE SEMICONDUCTOR

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

Theory – Target gas reacts with MOS (SnO2 ) andchanges its resistance – as measured by the meter

Application - Nearly all oxidizable gases

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

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

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

NON-DISPERSIVE INFRARED (NDIR)

Advantage

Extremely versatile

Disadvantages

Expensive

Can be fragile and complicated to service

PHOTOIONIZATION DETECTOR (PID)Theory - Ultraviolet light ionizes the target gas

Current produced is proportional to concentration

Application – Volatile Organic Compounds (VOC’s)

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 belowionization potential of lamp

Affected by high humidity

UV lamps are expensive

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 isdirectly proportionalto 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

ELECTROCHEMICAL SENSORS

Advantages

Inexpensive

Linear output

Can be miniaturized

Disadvantages

Prone to interferences in unskilled hands

Affected by temperature— readily correctable

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

MONITORING PRODUCTSOFFERED BY

PORTABLE ANALYZER

CONTINUOUS MONITORING SYSTEMSINGLE POINT APPLICATION

CONTINUOUS MONITORING SYSTEMTWO POINT APPLICATION

CONTINUOUS MONITORING SYSTEMMULTI-POINT (THREE OR MORE) APPLICATION

ARC-MAX® DATA ACQUISITION, ARCHIVING,

AND REPORTING

ARC-MAX® MAIN SCREEN

ARC-MAX® SHIFT REPORT

ARC-MAX® TRENDING

ARC-MAX® ALARM LOG

For Survey Applications,We Recommend Our Nomad Data Logger

12-bit resolution

Models available for 0-100mV, 0-2.5Vand 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

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

Time history graph detail screen

Zoomed in on particular portion of the curve

Ability To View All Individual Data Points

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

SPECIAL THANKS TO

PROFESSOR GEORGE BYRNS