Direct-Reading Gas Detection Instruments

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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 analyticalbetter as go/no-go
detectors
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NON-DISPERSIVE INFRARED (NDIR)
Theory Target 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
Application Volatile 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
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 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 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 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