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

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Presentation on theme: "Direct-Reading Gas Detection Instruments Presented by Michael D. Shaw."— Presentation transcript:

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 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)

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 (SnO 2 ) 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 analyticalbetter as go/no-go detectors

9 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

10 BEERS LAW ( FOR EXTRA CREDIT ) "The deeper the glass, The darker the brew, The less of the incident Light that gets through" I = I 0 e kP I = intensity of light striking the detector I 0 = 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 (VOCs)

13 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

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 Br 2, CO, Cl 2, ClO 2, C 2 H 4, ethylene oxide HCHO, H 2, hydrazine, HBr, HCl, HCN, H 2 S, NO, NO 2, O 3, propylene oxide, SO 2, 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 standardthere 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 adjustable1 second to 10 hours in 1 second intervals Storage capacity16336 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

32 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

33 SPECIAL THANKS TO PROFESSOR GEORGE BYRNS


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