limitations of direct reading occupational hygiene instruments reproduced with permission of :...
TRANSCRIPT
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Limitations of Direct Reading Occupational Hygiene
Instruments
Reproduced with permission of :
Russell Bond Robert Golec
Aleks Todorovic
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IntroductionOccupational 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
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OutlineSample
Atmosphere
Gas - Vapour
Electronic
Confined Space PID
Diffusive
Detector Tubes
Particulates
Light Scattering Devices
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Aerosol Monitoring
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Direct-Reading Aerosol MonitorsLight Scattering (Aerosol Photometers) – laser, IR, broad wavelength
Piezo-Electric Mass SensorsTapered Element Oscillating Microbalance
(TEOM)Fibrous Aerosol Monitors – special type of
aerosol photometer
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Light Scattering/Aerosol PhotometersMost 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)
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Theory of Light Scattering by Spherical Particles - MieLight scattering is a combination of diffraction,
refraction and reflectionIntensity 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)
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Light Scattering vs Particle Diameter
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Particle Diameters
0.001 0.01 0.1 1 10 100 1000
microns
Grain Dust
Cement dust
Fly Ash
FlourCoal Dust
Metal dust & fume
Carbon Black
Diesel Particulate
ZnO fume
Light Scattering
Wood dustNanoparticles
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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?
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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 (?)• 0.01-200 mg/m3
• personal
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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
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CalibrationISO 12103-1, Al (Ultrafine) test dust
(formerly called Arizona Road Dust).
Particle size range 1um to 10 um
0 1 2 3 4 5 6 7 8 9 100
20
40
60
80
100
%
microns
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Sources of ErrorLight 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.
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Sources of ErrorAerosol 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.
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Sources of ErrorMonitor 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.
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Sources of ErrorMonitoring 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).
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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
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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
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Minimising The ErrorsConsider 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.
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Minimising The ErrorsUse 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.
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Future Trends
Piezoelectric microbalance aerosol monitor
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Future TrendsTapered-Element Oscillating Microbalance
(TEOM)
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TEOMMiner’s helmet mounted coal dust monitor
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Monitoring for mercuryBig issue in refineries and gas plantsAssociated with hydrocarbon formationAccumulation according to Hg propertiesMostly elemental and sulphide formsInhalation, skin and ingestion routes
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Instrumental Detection MethodsAtomic absorptionGold film resistanceZeeman atomic absorptionResonant microbalance
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AAS - How does it work?RF field excites Hg atoms yielding 253.7nmDoesn’t ‘see’ Hg compoundsSample air through cell (70-90L/hr)Absorbed radiation proportional to Hg conc
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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
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Gold Film resistance – How does it work?
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Gas Detectors
Single Gas DetectorsMulti-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
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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
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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
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Flammability & ToxicityFire, explosion and toxicity are all important
hazards requiring identification, assessment and control.
Mines, confined spaces, refineries, gas plants etc...
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Explosivity limits
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Species Response DifferenceGas/VaporLEL (%vol) Sensitivity (%)Acetone 2.2 45Diesel 0.8 30Gasoline 1.4 45Methane 5.0 100MEK 1.8 38Propane 2.0 53Toluene 1.2 40
LEL Sensor sensitivity varies with chemical
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Calibration typically to CH4
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Low Oxygen AtmospheresO2 required for combustionActive bead useless below ~10% O2
Meter reads 0% LEL in 100% fuel vapourFalse securityReason for testing O2 first, then LEL
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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
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LEL Sensor Poisons
Sensor Lifetime
Sen
sor
Ou
tpu
t
With an “Acute” LEL sensor poison the sensor is going to
fail, but the time to failure is dosage dependant
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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
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LEL Sensor Poisons
With a “Chronic” LEL sensor poison the sensor recovers after
an exposure, subsequent exposures will further degrade
sensor output
Sensor Lifetime
Sen
sor
Ou
tpu
t
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Measuring Flammability
Techniques for high range combustible gas measurement
◦Dilution fittings
◦Thermal conductivity sensors
◦Calculation by means of oxygen displacement
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Thermal ConductivityEach 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
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Thermal ConductivityUsed frequently in:
Petrochemical – blanketingGas transmission – ensuring full supplySite remediation – remember City Of Casey
Issues arise due to the fact that most TC sensors read in %VOL1% VOL Methane = 20% LEL1% VOL Propane = 47% LEL
Make sure you’re reading in the right units!
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Toxic Gases and Vapors
Detection techniques:◦Colorimetric Tubes◦Electrochemical Sensors◦Non-dispersive infrared (NDIR)◦ Photoionization detectors
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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
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EC Sensors
Capillary diffusion barrier
Metal housing
Reference electrode
Counter electrode
Electrolyte reservoir
Electrode contacts
Sensing electrode
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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
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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
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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.
GasConc. Response
CO 300 ppm <1.5 ppm
SO2 5 ppm about 1 ppm
NO 35 ppm <0.7 ppm
NO2 5 ppm about -1 ppm
H2 100 ppm 0 ppm
HCN 10 ppm 0 ppm
NH3 50 ppm 0 ppm
PH3 5 ppm about 4 ppm
CS2 100 ppm 0 ppm
Methyl sulfide 100 ppm 9 ppm
Ethyl sulfide 100 ppm 10 ppm*
Methyl mercaptan 5 ppm about 2 ppm
Ethylene 100 ppm < 0.2 ppm
Isobutylene 100 ppm 0 ppm
Toluene 10000 ppm 0 ppm*
Turpentine 3000 ppm about 70 ppm*
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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
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Datalogging
Can be a TRAP – WATCH OUT!Datalogging is really a ‘snapshot’ of the event
at that timeThe 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’
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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 1ppm1 out of 60 minutes was at 10ppmThe report would show the concentration for the
entire logging period was10ppm
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Datalogging
8 Hour TWA calculation vs 12 ShiftExample:
employee has a personal gas monitorEmployee works for 12 hoursGas monitor is programmed only to give TWA for
8 HoursGas monitor is downloaded for dataResults are producedWhat do you report as the result from the unit???
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Traditional four-gas confined space entry monitors miss many common toxic
gasses!
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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”
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Who uses PIDs?
Anyone involved with the use of chemicals, gases and
petroleum productsEnvironmentalIndustrial HygieneSafetyHazardous Materials Response (HazMat)Maintenance/Operations
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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
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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”
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100.0 ppm
Gas enters the instrument
It passes bythe UV lamp
It is now “ionized” Charged gas ions
flow to charged plates in the sensor andcurrent is produced
Current is measured and concentration is displayed on the meter.
+
-
+
-
+
-+
-
+
-Gas “Reforms” and exits theinstrument intact
How does a PID work?An optical system using Ultraviolet lamp to breakdown vapors and gases for measurement
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What does a PID Measure?
Ionization PotentialAll gasses and vapors have an Ionization
Potential (IP)IP determines if the PID can “see” the gasIf 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 FactorIonization Potentials are found in RAE
handouts (TN-106), NIOSH Pocket Guide and many chemical texts.
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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!
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8
9
10
11
12
13
14
15
8.4
9.24 9.549.99 10.1 10.5
10.6611.3211.47
12.1
14.01
Some Ionization Potentials (IPs) for Common Chemicals
Ben
zene
ME
K
Vinyl C
hloride
IPA
Ethylene
Acetic A
cid
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
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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
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What PIDs Do Not Measure
RadiationAir
◦N2
◦O2
◦CO2
◦H2OToxics
◦CO◦HCN◦SO2
Natural gas◦Methane CH4
◦Ethane C2H6
Acids◦HCl◦HF◦HNO3
Others◦Freons◦Ozone O3
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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!
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Basic use of PIDYou won’t find the orange in the bunch of apples!
All you’ll find is fruit!
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Basic use of PID
PID is very sensitive and accuratePID is not very selective
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Basic use of PIDPID is very sensitive and accuratePID is not very selective
Ruler cannot differentiate between yellow and white paper
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Basic use of PID
PID is very sensitive and accuratePID is not very selective
PID can’t differentiate between ammonia & xylene
But both are toxic!
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Basic use of PID
Just because there is a Ionisation Energy listed
doesn’t mean that the PID will respond.
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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
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PID Inherent Measurement EfficiencyObserved PID response vs. concentration
◦Most commercial PIDs have a linear raw response in the ppb-ppm range
◦Begin to deviate slightly at 500-1000 ppm Electronics linearise the response at this time
◦At higher concentrations the response drops
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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
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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
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PID Measurement Parameters
Calibration Gas SelectionIMPORTANT
◦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
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What is a Correction Factor?
Correction Factors are the key to unlocking the power of a PID for Assessing Varying
Mixtures and Unknown Environments
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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
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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
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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
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PID Measurement ParametersLow 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 < 2If 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
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PID Measurement ParametersCAUTION
◦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 0.9 0.85 0.69 9.07
JP-8 0.6 0.51 0.7 0.48
Gasoline 0.9 0.73 1.1 1.1
n-hexane 4.3 4 3.3 4.5 10.18
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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
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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
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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
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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
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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
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Conclusion
Be careful
Understand the limitations of the device
Don’t be talked into buying an instrument. Check out its value and limitations