Presentation on theme: "Limitations of Direct Reading Occupational Hygiene Instruments Reproduced with permission of : Russell Bond Robert Golec Aleks Todorovic."— Presentation transcript:
Limitations of Direct Reading Occupational Hygiene Instruments Reproduced with permission of : Russell Bond Robert Golec Aleks Todorovic
Introduction Occupational Hygienists are using direct reading instruments more and more as the technology becomes available. As instruments become more sophisticated, there is a growing perception or a seductive tendency to blindly believe the numbers on the display
Outline Sample Atmosphere Gas - Vapour Electronic Confined Space PIDDiffusive Detector Tubes Particulates Light Scattering Devices
Direct-Reading Aerosol Monitors Light Scattering (Aerosol Photometers) – laser, IR, broad wavelength Piezo-Electric Mass Sensors Tapered Element Oscillating Microbalance (TEOM) Fibrous Aerosol Monitors – special type of aerosol photometer
Light Scattering/Aerosol Photometers Most common type of aerosol monitor Based on Mie’s theory of light scattering by spherical particles (light intensity of scattered light is related to wavelength of incident light and the diameter of the particles)
Theory of Light Scattering by Spherical Particles - Mie Light scattering is a combination of diffraction, refraction and reflection Intensity of scattered light is related to wavelength of incident light ( l ), the angle of scatter ( Q ) the and the diameter of the particle (d). If d>>l then most of the scattering occurs in the forward direction (Mie’s Scattering) If d<
"name": "Theory of Light Scattering by Spherical Particles - Mie Light scattering is a combination of diffraction, refraction and reflection Intensity of scattered light is related to wavelength of incident light ( l ), the angle of scatter ( Q ) the and the diameter of the particle (d).",
"description": "If d>>l then most of the scattering occurs in the forward direction (Mie’s Scattering) If d<
TSI Dust Trak 90 o light scattering angle Laser light source 0.1mm – 10 mm PM1, PM2.5, PM10, respirable10mm nylon (dorr-oliver) cyclone Flowrate up to 1.7 LPM (new Dust Trak 1.4 – 3 LPM) to 150 mg/m 3 hand-held, personal?
Environmental Devices Haz-Dust near forward scattering Infrared light source Inhalable, thoracic and respirable size selective sampling attachments flowrate 1 – 3.3 LPM 0.1mm – 100 mm (?) mg/m 3 personal
Casella Micro-Dust Near forward light scattering Infrared source TSP, PM10, PM2.5 or respirable flowrate N/A – diffusion 0 to 2500 mg/m 3 in 3 ranges hand-held
Calibration ISO , Al (Ultrafine) test dust (formerly called Arizona Road Dust). Particle size range 1um to 10 um % microns
Sources of Error Light scattering is an indirect measure of particulate mass concentration based on an assumed particle size distribution. Different types of dusts can have significantly different particle size distributions from the calibration dust which can lead to large deviation from the curve.
Sources of Error Aerosol particulate refractive index can have an effect on light scattering and therefore on the estimation of mass concentration when compared against a reference (ARD) aerosol curve.
Sources of Error Monitor calibration assumes that aerosol particle size distribution remains constant. Changes in the generation of the airborne aerosol or in the wind speed can change the particle diameter distribution and the instrument response. The ability to accurately measure the mass concentration of thoracic and inhalable dust fraction rely on the ratio of <10 micron (respirable) particles in the larger size range remaining constant.
Sources of Error Monitoring of high aerosol concentrations can lead to deposition on the instrument optics which can change the instrument’s response. At high humidity, water droplets can be detected by the photometer and cause a falsely high reading. Elongated aerosol particles (eg fibres) are poorly detected (unless fibres can be oriented in same direction).
Sources of Error Assuming that the composition of the aerosol is the same as the material from which it is being generated eg lead in soldering fume, silica in rock. Light scattering is ineffective for monitoring nanoparticles as mass concentration is very low. Number concentration is of more useful metric – Condensation Particle Counter
Overview of Limitations Light Scattering monitors are relatively good for measuring respirable aerosol concentration, but become tenuous when used for the thoracic sub-fraction and potentially misleading when used to measure the inhalable aerosol mass concentration – Maynard & Jensen
Minimising The Errors Consider the likely nature and particle size range of the aerosol of interest and the objectives of the monitoring. Verify the instrument’s response to the aerosol of interest by carrying out serial gravimetric sampling in parallel with the monitor and determine a correction (calibration) factor.
Minimising The Errors Use real-time light scattering aerosol measurements as a screening tool or to assess engineering controls but not as a decision making tool for health risk monitoring.
Monitoring for mercury Big issue in refineries and gas plants Associated with hydrocarbon formation Accumulation according to Hg properties Mostly elemental and sulphide forms Inhalation, skin and ingestion routes
AAS - How does it work? RF field excites Hg atoms yielding 253.7nm Doesn’t ‘see’ Hg compounds Sample air through cell (70-90L/hr) Absorbed radiation proportional to Hg conc
Gold Film resistance – How does it work? Sample gas passes gold film Hg affinity for gold Resistance change proportional to Hg captured H2S, SO2, - acid gases interfere Regeneration required start & end of monitoring and when film saturates Must balance sample and reference film resistance after regen
Gold Film resistance – How does it work?
Gas Detectors Single Gas Detectors Multi-Gas Detectors ◦Normally worn on the belt, used with chest harness or held by hand ◦Multitude of types to choose from ◦Vary in price ◦Vary in user interface
Gas Detectors Diffusion Monitors ◦Most commonly used ◦Utilises natural air currents to provide sample ◦Normal air is sufficiently energetic to bring sample to sensor ◦Only monitors atmosphere that immediately surrounds the monitor ◦Inability to sample at remote locations ◦May lead to a decision based on false information due to limited reach of user
Gas Detectors Sample Draw Monitors ◦Two types available Motorised sampling pump Hand operated squeeze bulb ◦Enables remote sampling from varying distances ◦Draws sample quicker to the sensors from distance ◦Liable for leakage – dilutes sample ◦Has time lag issues ◦Users need to be wary of adsorption of sample to sample line
Flammability & Toxicity Fire, explosion and toxicity are all important hazards requiring identification, assessment and control. Mines, confined spaces, refineries, gas plants etc...
Species Response Difference Gas/VaporLEL (%vol)Sensitivity (%) Acetone2.245 Diesel0.830 Gasoline1.445 Methane MEK Propane2.053 Toluene1.240 LEL Sensor sensitivity varies with chemical
Calibration typically to CH4
Low Oxygen Atmospheres O 2 required for combustion Active bead useless below ~10% O 2 Meter reads 0% LEL in 100% fuel vapour False security Reason for testing O 2 first, then LEL
LEL Sensor Poisons Common chemicals can degrade and destroy LEL sensor performance Acute Poisons act very quickly, these include compounds containing: ◦Silicone (firefighting foams, waxes) ◦Lead (old gasoline) ◦Phosphates and phosphorous ◦High concentrations of combustible gas
LEL Sensor Poisons Sensor Lifetime Sensor Output With an “Acute” LEL sensor poison the sensor is going to fail, but the time to failure is dosage dependant
LEL Sensor Poisons Chronic Poisons are often called “inhibitors” and act over time. Often exposure to clean air will allow the sensor to “burn-off” these compounds Examples include: ◦Sulfur compounds (H 2 S, CS 2 ) ◦Halogenated Hydrocarbons (Freons, trichloroethylene, methylene chloride) ◦Styrene
LEL Sensor Poisons With a “Chronic” LEL sensor poison the sensor recovers after an exposure, subsequent exposures will further degrade sensor output Sensor Lifetime Sensor Output
Measuring Flammability Techniques for high range combustible gas measurement ◦Dilution fittings ◦Thermal conductivity sensors ◦Calculation by means of oxygen displacement
Thermal Conductivity Each type of gas has a unique TC and thus a unique relative response The gas does not need to be combustible No oxygen is required for its operation
Thermal Conductivity Used frequently in: Petrochemical – blanketing Gas transmission – ensuring full supply Site remediation – remember City Of Casey Issues arise due to the fact that most TC sensors read in %VOL 1% VOL Methane = 20% LEL 1% VOL Propane = 47% LEL Make sure you’re reading in the right units!
How do toxic sensors work? Electrochemical (EC) substance specific sensors work by: ◦Gas diffusing into sensor reacts at surface of the sensing electrode ◦Sensing electrode made to catalyze a specific reaction ◦Use of selective external filters further limits cross sensitivity
Limitations of Electrochemical Sensors? Narrow temperature range Subject to several interfering gases such as hydrogen Lifetime will be shortened in very dry and very hot areas – must bump and calibrate more frequently to ensure accurate readings
Limitations of Electrochemical Sensors? Condensing Humidity will block the diffusion mechanism lowering readings Consistently high humidity can dilute electrolyte Lifetime will be shortened in very dry and very hot areas – must bump and calibrate more frequently to ensure accurate readings
Cross-sensitivity Data H 2 S r Note: High levels of polar organic compounds including alcohols, ketones, and amines give a negative response. *Estimated from similar sensors. Gas Conc.Response CO300 ppm<1.5 ppm SO 2 5 ppmabout 1 ppm NO35 ppm<0.7 ppm NO 2 5 ppmabout -1 ppm H2H2 100 ppm0 ppm HCN10 ppm0 ppm NH 3 50 ppm0 ppm PH 3 5 ppmabout 4 ppm CS ppm0 ppm Methyl sulfide100 ppm9 ppm Ethyl sulfide100 ppm10 ppm* Methyl mercaptan5 ppmabout 2 ppm Ethylene100 ppm< 0.2 ppm Isobutylene100 ppm0 ppm Toluene10000 ppm0 ppm* Turpentine3000 ppmabout 70 ppm*
Datalogging Most new CS monitors have sophisticated microprocessors that allow the continuous recording of data Data can quickly document worker exposure levels compared to sampling techniques Datalogging running continuously in the background provides valuable information when serious incidents happen
Datalogging Can be a TRAP – WATCH OUT! Datalogging is really a ‘snapshot’ of the event at that time The longer the datalogging interval the LESS resolution provided by the graph or tabular report If concentrations are expected to vary tighten your interval Some instruments log the ‘AVERAGE’ and some log ‘MAX’
Datalogging Can be a TRAP – WATCH OUT! Example: An instrument logs the highest value during the interval and the logging period is one hour 59 out of 60 minutes where at 1ppm 1 out of 60 minutes was at 10ppm The report would show the concentration for the entire logging period was 10ppm
Datalogging 8 Hour TWA calculation vs 12 Shift Example: employee has a personal gas monitor Employee works for 12 hours Gas monitor is programmed only to give TWA for 8 Hours Gas monitor is downloaded for data Results are produced What do you report as the result from the unit???
Traditional four-gas confined space entry monitors miss many common toxic gasses!
What is a PID? PID = Photo-Ionization Detector Detects VOCs (Volatile Organic Compounds) and Toxic gases from <10 ppb to as high as 15,000 ppm A PID is a very sensitive broad spectrum monitor, like a “low-level LEL”
Who uses PIDs? Anyone involved with the use of chemicals, gases and petroleum products Environmental Industrial Hygiene Safety Hazardous Materials Response (HazMat) Maintenance/Operations
A PID is like a Magnifying Glass A Magnifying glass lets a detective see fingerprints; a PID lets us “see” VOCs Benzene Ammonia Carbon Disulfide Styrene Xylene Jet FuelPERC
How does a PID work? An Ultraviolet lamp ionizes a sample gas which causes it to charge electrically The sensor detects the charge of the ionized gas and converts the signal into current The current is then amplified and displayed on the meter as “ppm”
100.0 ppm Gas enters the instrument It passes by the UV lamp It is now “ionized” Charged gas ions flow to charged plates in the sensor and current is produced Current is measured and concentration is displayed on the meter Gas “Reforms” and exits the instrument intact How does a PID work? An optical system using Ultraviolet lamp to breakdown vapors and gases for measurement
What does a PID Measure? Ionization Potential All gasses and vapors have an Ionization Potential (IP) IP determines if the PID can “see” the gas If the IP of the gas is less than the eV output of the lamp the PID can “see” it Ionization Potential (IP) does not correlate with the Correction Factor Ionization Potentials are found in RAE handouts (TN-106), NIOSH Pocket Guide and many chemical texts.
If the “wattage” of the gas or vapor is less than the “wattage” of the PID lamp then the PID can “see” the gas or vapor!
Some Ionization Potentials (IPs) for Common Chemicals Benzene MEK Vinyl Chloride IPA Ethylene Acetic Acid Methylene chloride Carbon Tet. Carbon Monoxide Styrene Oxygen Ionization Potential (eV) 11.7 eV Lamp 10.6 eV Lamp Not Ionizable What does a PID Measure? 9.8 eV Lamp
What does a PID Measure? Organics: Compounds Containing Carbon (C) ◦Aromatics - compounds containing a benzene ring BETX: benzene, ethyl benzene, toluene, xylene ◦Ketones & Aldehydes - compounds with a C=O bond acetone, MEK, acetaldehyde ◦Amines & Amides - Carbon compounds containing Nitrogen diethyl amine ◦Chlorinated hydrocarbons - trichloroethylene (TCE) ◦Sulfur compounds – mercaptans, carbon disulfide ◦Unsaturated hydrocarbons - C=C & C C compounds butadiene, isobutylene ◦Alcohol’s ethanol ◦Saturated hydrocarbons butane, octane Inorganics: Compounds without Carbon Ammonia Semiconductor gases: Arsine
What PIDs Do Not Measure Radiation Air ◦N 2 ◦O 2 ◦CO 2 ◦H 2 O Toxics ◦CO ◦HCN ◦SO 2 Natural gas ◦Methane CH 4 ◦Ethane C 2 H 6 Acids ◦HCl ◦HF ◦HNO 3 Others ◦Freons ◦Ozone O 3
Basic use of PID “Don’t worry, my PID will tell me what it is!” Will it?? Only if there is one substance and you know what it is!
Basic use of PID You won’t find the orange in the bunch of apples! All you’ll find is fruit!
Basic use of PID PID is very sensitive and accurate PID is not very selective
Basic use of PID PID is very sensitive and accurate PID is not very selective Ruler cannot differentiate between yellow and white paper
Basic use of PID PID is very sensitive and accurate PID is not very selective PID can’t differentiate between ammonia & xylene But both are toxic!
Basic use of PID Just because there is a Ionisation Energy listed doesn’t mean that the PID will respond.
Basic use of PID Basic rule of thumb is: The higher the boiling point the slower the response Compound should have a boiling point of less that 300 o C
PID Inherent Measurement Efficiency Observed PID response vs. concentration ◦Most commercial PIDs have a linear raw response in the ppb-ppm range ◦Begin to deviate slightly at ppm Electronics linearise the response at this time ◦At higher concentrations the response drops
PID Inherent Measurement Efficiency SAMPLE COLLECTION ◦Formation of other Photoproducts on the lamp PID lamps produce Ozone at ppb levels If the lamp is on and the pump off Ozone will accumulate ◦ Ozone may gradually damage internal rubber or plastic components ◦ At very low flows ozone may ‘scrub’ any organics present particularly in the low ppm range. ◦Try to always have a flow of air across the PID lamp
PID Measurement Parameters Factors that cause change in response ◦Lamp degradation ◦Coating of the PID lamp ◦Temperature ◦Pressure ◦Matrix gases ◦Humidity ◦Type of lamp ◦Manufacturers technology
PID Measurement Parameters Calibration Gas Selection IMPORTANT ◦Calibrating a PID to a specific gas DOES NOT make the instrument selective to that gas ◦A PID always responds to all the gases that the lamp can ionise ◦It gives a readout in equivalent units of the calibration gas
What is a Correction Factor? Correction Factors are the key to unlocking the power of a PID for Assessing Varying Mixtures and Unknown Environments
What is a Correction Factor? Correction Factor (CF) is a measure of the sensitivity of the PID to a specific gas CFs are scaling factors, they do not make a PID specific to a chemical, they only correct the scale to that chemical. Correction Factors allow calibration on cheap, non-toxic “surrogate” gas. Ref: RAE handout TN-106
CF Example: Toluene Toluene CF with 10.6eV lamp is 0.5 so PID is very sensitive to Toluene If PID reads 100 ppm of isobutylene units in a Toluene atmosphere Then the actual concentration is 50 ppm Toluene units 0.5 CF x 100 ppm iso = 50 ppm toluene
CF Example: Ammonia Ammonia CF with 10.6eV lamp is 9.7 so PID is less sensitive to Ammonia If PID reads 100 ppm of isobutylene units in an Ammonia atmosphere Then the actual concentration is 970 ppm Ammonia units 9.7 CF x 100 ppm iso = 970 ppm ammonia
PID Measurement Parameters Low CF = high PID sensitivity to a gas If the chemical is bad for you then the PID needs to be sensitive to it. In general, If Exposure limit is < 10 ppm, CF < 2 If the chemical isn’t too bad then the PID doesn’t need to be as sensitive to it If Exposure limit is > 10 ppm, CF < 10 Use PIDs for gross leak detectors when CF > 10
PID Measurement Parameters CAUTION ◦Only use the correction factor list provided by your instrument provider CompoundRAEBWIONBaselineIP (eV) Acetone Ammonia Butadiene JP Gasoline n-hexane
PID Measurement Parameters CAUTION ◦When calibrating a PID in mg/m3 units do not use CFs ◦The CF list only applies to ppmv to ppmv conversions ◦It is necessary to convert readings from IBE (isobutylene equivalents) back to ppmv before the CFs can be applied ◦Reconvert the ppmv value of the new compound to mg/m3
Factors effecting PID measurements Effects of Methane and other gases ◦No effect on PID reading of CO2, Ar, He, or H2 up to 5% volume ◦PIDs show a reduced response with > 1% volume methane
Factors effecting PID measurements Humidity Effects ◦Water vapour is ubiquitous in ambient air and reduce PID response ◦Condensation may also cause a false positive ‘leak‘ current ◦Compensation is possible – many different techniques available
Factors effecting PID measurements Humidity Effects ◦Using dessicant tubes is possible For non polar compounds such as TCE Heavy and polar compounds adsorb to the reagent causing a slower response Some amines absorb completely
Factors effecting PID measurements Effects of Sampling Equipment and Procedures. ◦Sampling from a distance using tubing causes delays in response and losses due to adsorption ◦Use only PTFE or metal tubing 3 metres of tygon will completely adsorb low volatility compounds – active sites on Tygon tubing act as sinks for organics and some inorganics eg, H2S, PH3
Conclusion Be careful Understand the limitations of the device Don’t be talked into buying an instrument. Check out its value and limitations