Presentation on theme: "Limitations of Direct Reading Occupational Hygiene Instruments"— Presentation transcript:
1 Limitations of Direct Reading Occupational Hygiene Instruments Reproduced with permission of :Russell BondRobert GolecAleks Todorovic
2 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
5 Direct-Reading Aerosol Monitors Light Scattering(Aerosol Photometers) – laser, IR, broad wavelengthPiezo-Electric Mass SensorsTapered Element Oscillating Microbalance (TEOM)Fibrous Aerosol Monitors – special type of aerosol photometer
6 Light Scattering/Aerosol Photometers Most common type of aerosol monitorBased 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)
7 Theory of Light Scattering by Spherical Particles - Mie Light 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)
8 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
10 TSI Dust Trak 90o light scattering angle Laser light source 0.1mm – 10 mmPM1, PM2.5, PM10, respirable10mm nylon (dorr-oliver) cycloneFlowrate up to 1.7 LPM (new Dust Trak 1.4 – 3 LPM)0.001 to 150 mg/m3hand-held, personal?
11 Environmental Devices Haz-Dust near forward scatteringInfrared light sourceInhalable, thoracic and respirable size selective sampling attachmentsflowrate 1 – 3.3 LPM0.1mm – 100 mm (?)mg/m3personal
12 Casella Micro-Dust Near forward light scattering Infrared source TSP, PM10, PM2.5 or respirableflowrate N/A – diffusion0 to 2500 mg/m3 in 3 rangeshand-held
13 Particle size range 1um to 10 um CalibrationISO , Al (Ultrafine) test dust (formerly called Arizona Road Dust).Particle size range 1um to 10 um%microns
14 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.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.
15 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.
16 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.
17 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).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
18 Sources of ErrorAssuming 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 CounterAnecdote 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
19 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 & JensenAerosol Measurement in theWorkplaceANDREW D. MAYNARDNational Institute for Occupational Safety and Health, Centers for DiseaseControl and Prevention, Public Health Service, U.S. Department of Health andHuman Services, Cincinnati, OHPAUL A. JENSENHuman Services, Morgantown, VA
20 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.
21 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.
22 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.
23 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.
25 Monitoring for mercury Big issue in refineries and gas plantsAssociated with hydrocarbon formationAccumulation according to Hg propertiesMostly elemental and sulphide formsInhalation, skin and ingestion routes
27 AAS - How does it work? RF field excites Hg atoms yielding 253.7nm Doesn’t ‘see’ Hg compoundsSample air through cell (70-90L/hr)Absorbed radiation proportional to Hg concHigh 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 pageIn other words: Hg might be the only substance to emit the 253.7nm line but it is not the only substance to absorb it!
28 Gold Film resistance – How does it work? Sample gas passes gold filmHg affinity for goldResistance change proportional to Hg capturedH2S, SO2, - acid gases interfereRegeneration required start & end of monitoring and when film saturatesMust balance sample and reference film resistance after regenAfter 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/m3Warm 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.
29 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/m3Warm 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.
30 Gas Detectors Single Gas Detectors Multi-Gas Detectors Normally worn on the belt, used with chest harness or held by handMultitude of types to choose fromVary in priceVary in user interface
31 Gas Detectors Diffusion Monitors Most commonly used Utilises natural air currents to provide sampleNormal air is sufficiently energetic to bring sample to sensorOnly monitors atmosphere that immediately surrounds the monitorInability to sample at remote locationsMay lead to a decision based on false information due to limited reach of user
32 Gas Detectors Sample Draw Monitors Two types available Motorised sampling pumpHand operated squeeze bulbEnables remote sampling from varying distancesDraws sample quicker to the sensors from distanceLiable for leakage – dilutes sampleHas time lag issuesUsers need to be wary of adsorption of sample to sample line
33 Flammability & Toxicity Fire, explosion and toxicity are all important hazards requiring identification, assessment and control.Mines, confined spaces, refineries, gas plants etc...
34 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 UELAlarms generally trigger around 5 or 10% of LEL to ensure safety margin well below the 100%LELDifferent 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
35 Species Response Difference Gas/VaporLEL (%vol) Sensitivity (%)AcetoneDieselGasolineMethaneMEKPropaneTolueneLEL Sensor sensitivity varies with chemicalWhy 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%LELWhen toluene is at 100%LEL the methane calibrated meter reads 40%LELLEL meters may be calibrated to any flammable substanceIf calibration to methane then other gases will give different responseIdeally calibration should be against the substance of interestSensitivity will vary from sensor to sensor so correction factors should be used with suitable caution!Sensitivity will change over the life of the sensorWhat do we do if there is a mixture of vapours? What does the AS say?AS/NZSExplosive (flammable) atmospheric substance detectorsA continuous-monitoring explosive (flammable) atmospheric substance detector should befitted with latching, visible and audible alarms which activate at a concentration of airbornecontaminant 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 flammablesubstances.Detectors used to measure LEL should be calibrated for the flammable substance underinvestigation. Where a mixture of flammable substances occur, the LEL of the mixture maynot be known precisely and care is required to provide for the substance with the lowestLEL.Manufacturer’s information should be consulted to determine the sensitivity of the monitorto different flammable substances and any other factors that may impact on the ability of aparticular monitor to measure the flammable substance in question.Where there is no exposure standard for a substance, expert guidance should be obtained.
36 Calibration typically to CH4 Different combustible gases produce different responses in the LEL meterThe 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 requiredOn 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
37 Low Oxygen Atmospheres O2 required for combustionActive bead useless below ~10% O2Meter reads 0% LEL in 100% fuel vapourFalse securityReason for testing O2 first, then LELA 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 functionThe minimum oxygen level required for correct sensor operation is a function of design and may vary from one manufacturer to another
38 LEL Sensor PoisonsCommon chemicals can degrade and destroy LEL sensor performanceAcute Poisons act very quickly, these include compounds containing:Silicone (firefighting foams, waxes)Lead (old gasoline)Phosphates and phosphorousHigh concentrations of combustible gas
39 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 dependantSensor OutputSensor Lifetime
40 LEL Sensor PoisonsChronic Poisons are often called “inhibitors” and act over time. Often exposure to clean air will allow the sensor to “burn-off” these compoundsExamples include:Sulfur compounds (H2S, CS2)Halogenated Hydrocarbons (Freons, trichloroethylene, methylene chloride)Styrene
41 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 outputSensor OutputSensor Lifetime
42 Measuring Flammability Techniques for high range combustible gas measurementDilution fittingsThermal conductivity sensorsCalculation by means of oxygen displacement
43 Thermal ConductivityEach type of gas has a unique TC and thus a unique relative responseThe gas does not need to be combustibleNo oxygen is required for its operation
44 Thermal Conductivity Used frequently in: Petrochemical – blanketingGas transmission – ensuring full supplySite remediation – remember City Of CaseyIssues arise due to the fact that most TC sensors read in %VOL1% VOL Methane = 20% LEL1% VOL Propane = 47% LELMake sure you’re reading in the right units!
46 How do toxic sensors work? Electrochemical (EC) substance specific sensors work by:Gas diffusing into sensor reacts at surface of the sensing electrodeSensing electrode made to catalyze a specific reactionUse of selective external filters further limits cross sensitivity
48 Limitations of Electrochemical Sensors? Narrow temperature rangeSubject to several interfering gases such as hydrogenLifetime will be shortened in very dry and very hot areas – must bump and calibrate more frequently to ensure accurate readings
49 Limitations of Electrochemical Sensors? Condensing Humidity will block the diffusion mechanism lowering readingsConsistently high humidity can dilute electrolyteLifetime will be shortened in very dry and very hot areas – must bump and calibrate more frequently to ensure accurate readings
50 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.ResponseCO300 ppm<1.5 ppmSO25 ppmabout 1 ppmNO35 ppm<0.7 ppmNO2about -1 ppmH2100 ppm0 ppmHCN10 ppmNH350 ppmPH3about 4 ppmCS2Methyl sulfide9 ppmEthyl sulfide10 ppm*Methyl mercaptanabout 2 ppmEthylene< 0.2 ppmIsobutyleneToluene10000 ppm0 ppm*Turpentine3000 ppmabout 70 ppm*
51 DataloggingMost new CS monitors have sophisticated microprocessors that allow the continuous recording of dataData can quickly document worker exposure levels compared to sampling techniquesDatalogging running continuously in the background provides valuable information when serious incidents happen
52 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 reportIf concentrations are expected to vary tighten your intervalSome instruments log the ‘AVERAGE’ and some log ‘MAX’
53 Datalogging Can be a TRAP – WATCH OUT! Example: An instrument logs the highest value during the interval and the logging period is one hour59 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
54 Datalogging 8 Hour TWA calculation vs 12 Shift Example: 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???
55 Traditional four-gas confined space entry monitors miss many common toxic gasses!
56 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 ppmA PID is a very sensitive broad spectrum monitor, like a “low-level LEL”
57 Who uses PIDs?Anyone involved with the use of chemicals, gases and petroleum productsEnvironmentalIndustrial HygieneSafetyHazardous Materials Response (HazMat)Maintenance/Operations
58 A PID is like a Magnifying Glass A Magnifying glass lets a detective see fingerprints; a PID lets us “see” VOCsAmmoniaCarbon DisulfideBenzeneStyrenePERCJet FuelXylene
59 How does a PID work?An Ultraviolet lamp ionizes a sample gas which causes it to charge electricallyThe sensor detects the charge of the ionized gas and converts the signal into currentThe current is then amplified and displayed on the meter as “ppm”
60 How does a PID work? - Ionization Detector Photo + An optical system using Ultraviolet lamp to breakdown vapors and gases for measurementCurrent is measured and concentration is displayed on the meter.100.0 ppm-PhotoIonizationDetector++++----+Gas “Reforms”and exits theinstrument intactGas enters theinstrumentIt is now“ionized”Charged gas ionsflow to chargedplates in thesensor andcurrent is producedIt passes bythe UV lamp
61 What does a PID Measure? Ionization Potential All 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” itIonization Potential (IP) does not correlate with the Correction FactorIonization Potentials are found in RAE handouts (TN-106), NIOSH Pocket Guide and many chemical texts.
62 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!
63 What does a PID Measure?Some Ionization Potentials (IPs) for Common Chemicals9.8 eV Lamp10.6 eV Lamp11.7 eV LampNot Ionizable1514.0114Ionization Potential (eV)1312.11211.4711.3210.661110.59.9910.1109.249.5498.48MEKIPAStyreneCarbon MonoxideBenzeneEthyleneMethylene chlorideOxygenAcetic AcidCarbon Tet.Vinyl Chloride
64 What does a PID Measure? Organics: Compounds Containing Carbon (C) Aromatics - compounds containing a benzene ringBETX: benzene, ethyl benzene, toluene, xyleneKetones & Aldehydes - compounds with a C=O bondacetone, MEK, acetaldehydeAmines & Amides - Carbon compounds containing Nitrogendiethyl amineChlorinated hydrocarbons - trichloroethylene (TCE)Sulfur compounds – mercaptans, carbon disulfideUnsaturated hydrocarbons - C=C & C C compoundsbutadiene, isobutyleneAlcohol’sethanolSaturated hydrocarbonsbutane, octaneInorganics: Compounds without CarbonAmmoniaSemiconductor gases: Arsine
65 What PIDs Do Not Measure RadiationAirN2O2CO2H2OToxicsCOHCNSO2Natural gasMethane CH4Ethane C2H6AcidsHClHFHNO3OthersFreonsOzone O3
66 “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!
67 Basic use of PID You won’t find the orange in the bunch of apples! All you’ll find is fruit!
68 Basic use of PID PID is very sensitive and accurate PID is not very selective
69 Basic use of PID PID is very sensitive and accurate PID is not very selectiveRuler cannot differentiate between yellow and white paper
70 Basic use of PID PID is very sensitive and accurate PID is not very selectivePID can’t differentiate between ammonia & xyleneBut both are toxic!
71 Basic use of PIDJust because there is a Ionisation Energy listed doesn’t mean that the PID will respond.
72 The higher the boiling point the slower the response Basic use of PIDBasic rule of thumb is:The higher the boiling point the slower the responseCompound should have a boiling point of less that 300oC
73 PID Inherent Measurement Efficiency Observed PID response vs. concentrationMost commercial PIDs have a linear raw response in the ppb-ppm rangeBegin to deviate slightly at ppmElectronics linearise the response at this timeAt higher concentrations the response drops
74 PID Inherent Measurement Efficiency SAMPLE COLLECTIONFormation of other Photoproducts on the lampPID lamps produce Ozone at ppb levelsIf the lamp is on and the pump off Ozone will accumulateOzone may gradually damage internal rubber or plastic componentsAt 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
75 PID Measurement Parameters Factors that cause change in responseLamp degradationCoating of the PID lampTemperaturePressureMatrix gasesHumidityType of lampManufacturers technology
76 PID Measurement Parameters Calibration Gas SelectionIMPORTANTCalibrating a PID to a specific gas DOES NOT make the instrument selective to that gasA PID always responds to all the gases that the lamp can ioniseIt gives a readout in equivalent units of the calibration gas
77 What is a Correction Factor? Correction Factors are the key to unlocking the power of a PID for Assessing Varying Mixtures and Unknown Environments
78 What is a Correction Factor? Correction Factor (CF) is a measure of the sensitivity of the PID to a specific gasCFs 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
79 0.5CF x 100 ppmiso= 50 ppmtoluene CF Example: TolueneToluene CF with 10.6eV lamp is 0.5 so PID is very sensitive to TolueneIf PID reads 100 ppm of isobutylene units in a Toluene atmosphereThen the actual concentration is 50 ppm Toluene units0.5CF x 100 ppmiso= 50 ppmtoluene
80 9.7CF x 100 ppmiso= 970 ppmammonia CF Example: AmmoniaAmmonia CF with 10.6eV lamp is 9.7 so PID is less sensitive to AmmoniaIf PID reads 100 ppm of isobutylene units in an Ammonia atmosphereThen the actual concentration is 970 ppm Ammonia units9.7CF x 100 ppmiso= 970 ppmammonia
81 PID Measurement Parameters Low CF = high PID sensitivity to a gasIf 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 itIf Exposure limit is > 10 ppm, CF < 10Use PIDs for gross leak detectors when CF > 10
82 PID Measurement Parameters CAUTIONOnly use the correction factor list provided by your instrument providerCompoundRAEBWIONBaselineIP (eV)Acetone22.214.171.124.29.69Ammonia9.710.68.59.410.2Butadiene10.850.699.07JP-80.60.510.48Gasoline0.73n-hexane4.343.34.510.18
83 PID Measurement Parameters CAUTIONWhen calibrating a PID in mg/m3 units do not use CFsThe CF list only applies to ppmv to ppmv conversionsIt is necessary to convert readings from IBE (isobutylene equivalents) back to ppmv before the CFs can be appliedReconvert the ppmv value of the new compound to mg/m3
84 Factors effecting PID measurements Effects of Methane and other gasesNo effect on PID reading of CO2, Ar, He, or H2 up to 5% volumePIDs show a reduced response with > 1% volume methane
85 Factors effecting PID measurements Humidity EffectsWater vapour is ubiquitous in ambient air and reduce PID responseCondensation may also cause a false positive ‘leak‘ currentCompensation is possible – many different techniques available
86 Factors effecting PID measurements Humidity EffectsUsing dessicant tubes is possibleFor non polar compounds such as TCEHeavy and polar compounds adsorb to the reagent causing a slower responseSome amines absorb completely
87 Factors effecting PID measurements Effects of Sampling Equipment and Procedures.Sampling from a distance using tubing causes delays in response and losses due to adsorptionUse only PTFE or metal tubing3 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
88 Conclusion Be careful Understand the limitations of the device Don’t be talked into buying an instrument. Check out its value and limitations