Gas Detection Instruments 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 Oxygen (deficiency and enrichment) Toxic

MOST COMMON TECHNOLOGIES EMPLOYED IN INDUSTRIAL HYGIENE APPLICATIONS

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

CATALYTIC SENSORS Advantage Disadvantages Long life 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 ) and changes 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 incident Light 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 below ionization 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 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

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 PRODUCTS OFFERED BY

PORTABLE ANALYZER

SINGLE POINT APPLICATION CONTINUOUS MONITORING SYSTEM SINGLE POINT APPLICATION

CONTINUOUS MONITORING SYSTEM TWO POINT APPLICATION

MULTI-POINT (THREE OR MORE) APPLICATION CONTINUOUS MONITORING SYSTEM MULTI-POINT (THREE OR MORE) APPLICATION

DATA ACQUISITION, ARCHIVING, 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, 0-2.5V 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

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