Limitations of Direct Reading Occupational Hygiene Instruments

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

Light Scattering Devices
Outline Sample Atmosphere Gas - Vapour Electronic Confined Space PID Diffusive Detector Tubes Particulates Light Scattering Devices

Aerosol Monitoring

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<<l then most of the scattering occurs in the back direction (Raleigh Scattering)

Light Scattering vs Particle Diameter
For aerosols >10 um and <0.1 um, Mie scattering intensity drops markedly. Most efficient at between 0.2 um and 1 um (best at about 0.3 um). Compare this with ISO respirable particle curve – next slide

Particle Diameters microns Grain Dust Wood dust Nanoparticles
Light Scattering Grain Dust Wood dust Nanoparticles Cement dust Fly Ash Flour Coal Dust ZnO fume Metal dust & fume Carbon Black Diesel Particulate microns

TSI Dust Trak 90o 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) 0.001 to 150 mg/m3 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/m3 personal

Casella Micro-Dust Near forward light scattering Infrared source
TSP, PM10, PM2.5 or respirable flowrate N/A – diffusion 0 to 2500 mg/m3 in 3 ranges hand-held

Particle size range 1um to 10 um
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. Calibration is only valid for the specific calibration aerosol and may differ by as much as 10 fold when used with an aerosol from a different source, composition or aerosol size.

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). For fibre monitoring, the fibres must be aligned by either electrostatic or magnetic fields in order to ensure that the light is scattered at the same angle. Generally, the use of light scattering is reasonable for cylindrical fibres eg amosite, ceramic fibres but not good for “curly” fibres such as chrysotile

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 Anecdote about the consultant who used a Dusttrak to assess lead exposure to operators soldering circuitry in an electronics workshop. Quartz is harder than rock, so when drilling into rock the quartz content of the respirable dust will be lower than that in the rock. Condensation Particle Counters use water or alcohol vapour condensation onto the nanoparticles to enlarge them so that they are detectable by light scattering

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 Aerosol Measurement in the Workplace ANDREW D. MAYNARD National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, Public Health Service, U.S. Department of Health and Human Services, Cincinnati, OH PAUL A. JENSEN Human Services, Morgantown, VA

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.

Future Trends Piezoelectric microbalance aerosol monitor
Two crystals vibrating at their resonance frequency. Dust is deposited on one of the crystals changing its vibration crystal w.r.t. the reference crystal. The change in frequency is proportional to the mass of particles.

Future Trends Tapered-Element Oscillating Microbalance (TEOM)
Similar to piezoelectric microbalance but has a single oscillating filter on which the dust is deposited. The change in vibration frequency before and after sampling is proportional to the mass of particles collected.

TEOM Miner’s helmet mounted coal dust monitor

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

Instrumental Detection Methods
Atomic absorption Gold film resistance Zeeman atomic absorption Resonant microbalance

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 High frequency electric field excites electrons of mercury atoms in the UV lamp to yield the unique emission spectra of elemental mercury – in particular the characteristic emission line at 253.7nm. This radiation can in turn be absorbed by mercury atoms in the sample air passing through the cell. The difference in 253.7nm radiation entering the cell and leaving the cell is proportional to the concentration of mercury atoms in the sampled air. The Manual says “The Mercury Tracker 3000 uses a high-frequency driven electrodeless Hg low pressure (EDL) lamp as UV source. It generates emission lines of an extremely narrow bandwidth which are congruent with the absorption lines of the Hg atoms. Cross-sensitivities are thus minimized.” This of course presumes that there are no other absorbing species in the samples air. On the face of it, with such a characteristic mercury emission line this would seem a reasonable assumption. However, “The relatively small number of atomic absorption lines (compared to atomic emission lines) and their narrow width (a few pm) make spectral overlap rare; there are only very few examples known that an absorption line from one element will overlap with another. Molecular absorption, in contrast, is much broader, so that it is more likely that some molecular absorption band will overlap with an atomic line. “ – From Wikipedia AAS page In other words: Hg might be the only substance to emit the 253.7nm line but it is not the only substance to absorb it!

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 After residual air is purged into the scrubber the bypass valve directs sample air over the sampling gold film. A reference gold film is not exposed to the sampled air and helps to compensate for the effects of temperature fluctuation. Differences in resistance between the sampling and reference gold films are detected using a sensitive electrical circuit called a Wheatstone bridge. These resistance changes are proportional to the quantity of mercury absorbed and can be directly related to mercury concentration in air and displayed to the user. The manufacturer quotes accuracy as +/-20% at 0.1mg/m3 Warm up time is nominally 1 minute to allow electronics time to thermally stabilise. Survey mode involves display of results at the end of each 3-second sample cycle. Because of this lag time the location of small sources may cause apparently inconsistent readings during “pinpointing” unless probe sweeps are slow relative to the 3-second cycle. Additional lag that may further add to this problem is caused by the length of the sample train often including a metal probe and 1 or 2 meters of plastic tube. Acid gases, especially when associated with water condensation, definitely can provide misleading readings - generally on the high side. Clearly there is some impact on the relationship between resistance and mercury concentration when these other compounds are involved. Unrefined petroleum products very often contain significant amounts of H2S and during purging and cleanout of hydrocarbons from vessels and pipes prior to maintenance operations the most common technique is to Due to the affinity mercury has for sulphur compounds it seems possible that the effect relates to chemical reaction with mercury already adsorbed onto the gold film. This may explain the apparent detection of mercury in places where mercury is not expected after a series of samples have been taken but which can appear to disappear after a regeneration cycle. This technology has developed a dubious reputation in the petroleum industry.

Gold Film resistance – How does it work?
After residual air is purged into the scrubber the bypass valve directs sample air over the sampling gold film. A reference gold film is not exposed to the sampled air and helps to compensate for the effects of temperature fluctuation. Differences in resistance between the sampling and reference gold films are detected using a sensitive electrical circuit called a Wheatstone bridge. These resistance changes are proportional to the quantity of mercury absorbed and can be directly related to mercury concentration in air and displayed to the user. The manufacturer quotes accuracy as +/-20% at 0.1mg/m3 Warm up time is nominally 1 minute to allow electronics time to thermally stabilise. Survey mode involves display of results at the end of each 3-second sample cycle. Because of this lag time the location of small sources may cause apparently inconsistent readings during “pinpointing” unless probe sweeps are slow relative to the 3-second cycle. Additional lag that may further add to this problem is caused by the length of the sample train often including a metal probe and 1 or 2 meters of plastic tube. Acid gases, especially when associated with water condensation, definitely can provide misleading readings - generally on the high side. Clearly there is some impact on the relationship between resistance and mercury concentration when these other compounds are involved. Unrefined petroleum products very often contain significant amounts of H2S and during purging and cleanout of hydrocarbons from vessels and pipes prior to maintenance operations the most common technique is to Due to the affinity mercury has for sulphur compounds it seems possible that the effect relates to chemical reaction with mercury already adsorbed onto the gold film. This may explain the apparent detection of mercury in places where mercury is not expected after a series of samples have been taken but which can appear to disappear after a regeneration cycle. This technology has developed a dubious reputation in the petroleum industry.

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...

Explosivity limits Too lean to burn = oxygen concentration too high.
Too rich to burn = fuel concentration too high. For obvious reasons we want to avoid concentrations in the combustible range between LEL and UEL Alarms generally trigger around 5 or 10% of LEL to ensure safety margin well below the 100%LEL Different fuel species have different concentrations at which combustion will be self sustaining – eg reactions that are easier to start and that yield higher energy for chain reaction tend to have lower LEL values

Species Response Difference
Gas/VaporLEL (%vol) Sensitivity (%) Acetone Diesel Gasoline Methane MEK Propane Toluene LEL Sensor sensitivity varies with chemical Why is the sensitivity different for different species? Smaller molecules tend to have lower heats of combustion but higher concentrations at 100%LEL. Larger molecules have the opposite tendencies. These effects tend to cancel each other out. As a result meter sensitivity depends largely on the diffusivity of fuel molecules through the sintered catalyst coating on the active bead. This explains why LEL meters are less sensitive to larger molecules. So what happens when we use a meter calibrated for methane to detect flammability of different atmospheres? eg When methane is at 100%LEL the methane calibrated meter reads 100%LEL When toluene is at 100%LEL the methane calibrated meter reads 40%LEL LEL meters may be calibrated to any flammable substance If calibration to methane then other gases will give different response Ideally calibration should be against the substance of interest Sensitivity will vary from sensor to sensor so correction factors should be used with suitable caution! Sensitivity will change over the life of the sensor What do we do if there is a mixture of vapours? What does the AS say? AS/NZS Explosive (flammable) atmospheric substance detectors A continuous-monitoring explosive (flammable) atmospheric substance detector should be fitted with latching, visible and audible alarms which activate at a concentration of airborne contaminant not greater than 10% of the LEL. NOTE: The LEL and UEL for flammable substances vary depending on the particular substance. AS/NZS gives data on upper and lower flammability limits for a number of flammable substances. Detectors used to measure LEL should be calibrated for the flammable substance under investigation. Where a mixture of flammable substances occur, the LEL of the mixture may not be known precisely and care is required to provide for the substance with the lowest LEL. Manufacturer’s information should be consulted to determine the sensitivity of the monitor to different flammable substances and any other factors that may impact on the ability of a particular monitor to measure the flammable substance in question. Where there is no exposure standard for a substance, expert guidance should be obtained.

Calibration typically to CH4
Different combustible gases produce different responses in the LEL meter The sensor heating response depends on 1) the species heat of combustion, 2) the concentration at the species LEL, 3) the diffusivity of the species. A gas to which the meter is more sensitive can “trick” the methane calibrated meter into thinking concentration is closer to the LEL than it actually is. This is not usually a problem except that a workplace may be evacuated before it is strictly required On the other hand, a gas with a lower heat of combustion may reach flammable concentrations before the methane-calibrated meter signals the alarm. By using an alarm level set very much below the LEL, say 10% the impact of the different sentitivities is not so critical

Low Oxygen Atmospheres
O2 required for combustion Active bead useless below ~10% O2 Meter reads 0% LEL in 100% fuel vapour False security Reason for testing O2 first, then LEL A rapidly increased LEL reading followed by declining or erratic reading may indicate low oxygen in the test space and should be treated with suspicion. The erratic reading may also result from exposure to sensor poisons that can inhibit its catalytic function The minimum oxygen level required for correct sensor operation is a function of design and may vary from one manufacturer to another

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 Output Sensor Lifetime
With an “Acute” LEL sensor poison the sensor is going to fail, but the time to failure is dosage dependant Sensor Output Sensor Lifetime

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 (H2S, CS2) Halogenated Hydrocarbons (Freons, trichloroethylene, methylene chloride) Styrene

LEL Sensor Poisons Sensor Output Sensor Lifetime
With a “Chronic” LEL sensor poison the sensor recovers after an exposure, subsequent exposures will further degrade sensor output Sensor Output Sensor Lifetime

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!

Toxic Gases and Vapors Detection techniques: Colorimetric Tubes
Electrochemical Sensors Non-dispersive infrared (NDIR) Photoionization detectors

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

EC Sensors Electrode contacts Metal housing
Capillary diffusion barrier Metal housing Reference electrode Counter electrode Electrolyte reservoir Electrode contacts Sensing electrode

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 H2S r
Note: High levels of polar organic compounds including alcohols, ketones, and amines give a negative response. *Estimated from similar sensors. Gas Conc. Response CO 300 ppm <1.5 ppm SO2 5 ppm about 1 ppm NO 35 ppm <0.7 ppm NO2 about -1 ppm H2 100 ppm 0 ppm HCN 10 ppm NH3 50 ppm PH3 about 4 ppm CS2 Methyl sulfide 9 ppm Ethyl sulfide 10 ppm* Methyl mercaptan about 2 ppm Ethylene < 0.2 ppm Isobutylene Toluene 10000 ppm 0 ppm* Turpentine 3000 ppm about 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 Ammonia Carbon Disulfide Benzene Styrene PERC Jet Fuel Xylene

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”

How does a PID work? - Ionization Detector Photo +
An optical system using Ultraviolet lamp to breakdown vapors and gases for measurement Current is measured and concentration is displayed on the meter. 100.0 ppm - Photo Ionization Detector + + + + - - - - + Gas “Reforms” and exits the instrument intact Gas enters the instrument It is now “ionized” Charged gas ions flow to charged plates in the sensor and current is produced It passes by the UV lamp

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!

What does a PID Measure? Some Ionization Potentials (IPs) for Common Chemicals 9.8 eV Lamp 10.6 eV Lamp 11.7 eV Lamp Not Ionizable 15 14.01 14 Ionization Potential (eV) 13 12.1 12 11.47 11.32 10.66 11 10.5 9.99 10.1 10 9.24 9.54 9 8.4 8 MEK IPA Styrene Carbon Monoxide Benzene Ethylene Methylene chloride Oxygen Acetic Acid Carbon Tet. Vinyl Chloride

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 N2 O2 CO2 H2O Toxics CO HCN SO2 Natural gas Methane CH4 Ethane C2H6 Acids HCl HF HNO3 Others Freons Ozone O3

“Don’t worry, my PID will tell me what it is!”
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

PID is not very selective Ruler cannot differentiate between yellow and white paper

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.

The higher the boiling point the slower the response
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 300oC

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

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

0.5CF x 100 ppmiso= 50 ppmtoluene
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 0.5CF x 100 ppmiso= 50 ppmtoluene

9.7CF x 100 ppmiso= 970 ppmammonia
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 9.7CF x 100 ppmiso= 970 ppmammonia

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

CAUTION Only use the correction factor list provided by your instrument provider Compound RAE BW ION Baseline IP (eV) Acetone 1.1 0.9 0.7 1.2 9.69 Ammonia 9.7 10.6 8.5 9.4 10.2 Butadiene 1 0.85 0.69 9.07 JP-8 0.6 0.51 0.48 Gasoline 0.73 n-hexane 4.3 4 3.3 4.5 10.18

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

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

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

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

Limitations of Direct Reading Occupational Hygiene Instruments

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