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Direct-Reading Gas Detection Instruments

Direct-Reading Gas Detection Instruments

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Direct-Reading Gas Detection Instruments

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

  2. “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

  3. GASES OF INTEREST Combustible Oxygen (deficiency and enrichment) Toxic

  4. MOST COMMON TECHNOLOGIES EMPLOYED IN INDUSTRIAL HYGIENE APPLICATIONS

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. NON-DISPERSIVE INFRARED (NDIR) Advantage Extremely versatile Disadvantages Expensive Can be fragile and complicated to service

  12. PHOTOIONIZATION DETECTOR (PID) Theory - Ultraviolet light ionizes the target gas Current produced is proportional to concentration Application – Volatile Organic Compounds (VOC’s)

  13. 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

  14. 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

  15. ELECTROCHEMICAL SENSORS Advantages Inexpensive Linear output Can be miniaturized Disadvantages Prone to interferences in unskilled hands Affected by temperature— readily correctable

  16. 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

  17. MONITORING PRODUCTS OFFERED BY

  18. PORTABLE ANALYZER

  19. CONTINUOUS MONITORING SYSTEM SINGLE POINT APPLICATION

  20. CONTINUOUS MONITORING SYSTEM TWO POINT APPLICATION

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

  22. ARC-MAX® DATA ACQUISITION, ARCHIVING, AND REPORTING

  23. ARC-MAX® MAIN SCREEN

  24. ARC-MAX® SHIFT REPORT

  25. ARC-MAX® TRENDING

  26. ARC-MAX® ALARM LOG

  27. 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

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

  29. Time history graph detail screen Zoomed in on particular portion of the curve

  30. Ability To View All Individual Data Points

  31. 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

  32. SPECIAL THANKS TO PROFESSOR GEORGE BYRNS