DIRECT-READING INSTRUMENTS

1. DIRECT-READING INSTRUMENTS

2. Objectives • After this session, students should: • Know circumstances when direct-reading devices are used • Understand basic operating principles • Recognize limitations of these methods

3. Introduction • Direct reading instruments are commonly used in industrial hygiene and safety situations. • Instantaneous indication of a contaminant. • Confined space entry • “Hot work“ • Emergency response • Datalogging • Screening for potential overexposure • Etc. • Noise instrumentation will be addressed later in this course.

4. Sample Duration • Devices are available for short term "grab samples" or long duration monitoring. • Short term sampling is best for screening and for "safety" purposes. • Usually these instruments have a pump to draw air into the device to minimize response time. • Longer duration monitors often use passive diffusion. • Less need for a quick response • Battery drain is minimized • Most can operate for a full shift, for time-weighted average (TWA) exposure determination. • Most modern instruments feature datalogging • Download to a personal computer for spreadsheets & graphing.

5. Direct-reading Indicator Tubes • Direct-reading indicator tubes are usefull tools • Often called detector tubes, length of stain tubes or Dräger tubes • Inexpensive and easy to use. • First patented 1915 to detect CO in coal mines. • Contain chemical reagents sealed in glass • Tubes are broken open and the contents exposed to the atmosphere to be tested • Airborne contaminants are indicated by a color change caused by chemical reactions between the contaminant and tube contents.

6. Direct-reading Indicator Tubes • Hand-operated piston or bellows pump for short-duration samples • Fixed volume of air with each stroke • The number of strokes depends upon the range of the particular tube and the airborne concentration. • Sometimes the measurement range of a tube can be extended by taking additional pump strokes. • Many hand-operated pumps have end-of-stroke indicators, stoke counters and tube breaking features.

7. Direct-reading Indicator Tubes • Hand-operated bellows pump

8. Direct-reading Indicator Tubes • Two methods for long-duration sampling • Passive sampling direct-reading indicator tubes • These devices do not use pumps • One end of the tube is broken open and the tube is worn in the worker’s the breathing zone • Air diffusing into the open end reacts to form a color change. • TWA is found by dividing the length of stain indication by sampling duration. • Motorized pumps can be used for long-term sampling • Pumps are low-flow, like those for sorbent tube sampling • Fitted with long-duration tubes and worn in the worker’s breathing zone • TWA is calculated by dividing tube indication by sample duration.

9. Direct-reading Indicator Tubes • Passive sampling direct-reading indicator tubes

10. Direct-reading Indicator Tubes • Indicator tubes are inexpensive, easy to use, and can measure a variety of contaminants • Limitations • For specific contaminants & concentration ranges. • Short shelf life, usually 2 years. • Atmospheric conditions of air temperature, humidity, and density can affect the result • Tubes may react with other compounds, especially if they are of the same chemical class. • ±5% to ±25% accuracy • This is not usually a problem unless tubes are used for compliance monitoring AND the measurement is near the regulatory limit.

11. Color Badges • Badges are passive dosimeters that change color to indicate a chemical exposure. • The earliest badges used lead acetate treated paper for hydrogens sulfide exposure. • H2Sreacts with the indicator to form black PbS. • A variety of badges are available, usually for acutely toxic gases. • Color badges are simple and easy to use, but they are subject to many of the factors listed above for detector tubes.

12. Color Badges

13. Combustible Gas Indicators • Combustible gas indicators (CGIs) • Nonspecific detectors • Heat-of-combustion sensor detects combustible gases or vapors. • Responds to any gas or vapor that will burn in air • Explosimeters use a resistance bridge electrical circuit. • A heated catalytic wire forms one portion of the bridge. • Any increase in temperature (caused by combustible vapors in the air) will be shown when the electrical balance of the bridge changes. • Other CGI instruments usually use a catalytic element • These instruments display percent lower explosive limit (LEL). • 100 %LEL is the lowest concentration of a gas or vapor that will support combustion in air.

15. Combustible Gas Indicators • Combustible gas instruments require adequate oxygen level to work properly • Accidents have occurred when low LEL was indicted • Normally the instrument readings increase as gas or vapor concentrations build. • When gas or vapor concentrations get too high, however, the reading may decrease or go to "zero". • Most instruments are now designed to "latch" in an alarm mode to prevent this problem.

17. Combustible Gas Indicators • Affected by lead or silicon vapors. • Catalytic element can be damaged, resulting in failure to properly identify hazardous conditions. • Calibrated with reference atmospheres • May not be accurately for other gases or vapors. • Standard practice to regard any LEL >10 or 20% as extremely hazardous. • Many CGIs also include electrochemical detectors for oxygen or toxic gases. • For confined space entry, etc. • Usually at least three or more detectors • LEL, oxygen, and CO or H2S

18. Electrochemical Detectors • Electrochemical cells (sensors) detect specific gases • A chemical reaction creates an electrical current when the gas enters the cell. • Electrochemical detectors must be calibrated frequently, and the sensors must be replaced periodically • Sensor life is decreased by dry conditions, exposure to air, etc. • Electrochemical detectors are getting smaller and sensors are lasting longer as the technology improves. • Commonly measure CO, H2S and O2 • Often with Multiple sensors (including LEL)

20. Photoionization Detectors • Photoionization detectors (PIDs) utilize ultraviolet light (UV) • Compounds in air ionized by light from a UV lamp • PIDs are nonspecific • Any compound that is ionized by UV may be indicated. • PIDs are available with lamps of different energy to help differentiate between chemicals • Commonly used on hazardous waste sites

21. Photoionization Detectors • Calibration • Generally calibrated for benzene or isobutylene • The user can correct instrument readings for other compounds • PID instruments display readings in parts per million, but this is accurate only if they have been properly calibrated. • Some PID instruments allow the operator to select a specific compound to be measured. • These instruments use preprogramed response data to display ppm for that specific compound.

22. Photoionization Detectors

23. Other chemical monitoring instruments • Many other direct-reading instruments are available. • Infrared (IR) analyzers measure compounds that absorb IR. Sophisticated IR instruments are pre-programmed to measure a number of compounds, based on the absorption characteristics (intensity and frequency). • Flame ionization detectors (FID) are non-specific analyzers capable of measuring a wide variety of chemicals. Many inexpensive devices use solid-state detectors.

24. Other chemical monitoring instruments Infrared (IR) analyzers

25. Particulate Monitors • Particle counting devices use laser technology and light scattering principles to count individual particles • Particle size counters determine particle size by measuring the amount of reflected light. • The condensation nucleus counter detects smaller particles (0.02 um diameter) than other devices. • Alcohol vapor is condensed on the particles, causing them to become large enough to be detected • Laser Fiber Monitor uses electrical fields align fibers. • Identifies the particle as a fiber by measuring reflected light from two directions • Provides real-time measurement of airborne fiber levels.