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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
Screening for potential overexposure
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 Drger 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
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 workers 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 workers 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
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.
H2S reacts 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)
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
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.