Limitations of Direct Reading Occupational Hygiene

Instruments

Reproduced with permission of :

Russell Bond Robert Golec

Aleks Todorovic

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

OutlineSample

Atmosphere

Gas - Vapour

Electronic

Confined Space PID

Diffusive

Detector Tubes

Particulates

Light Scattering Devices

Aerosol Monitoring

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

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)

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)

Light Scattering vs Particle Diameter

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

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 (?)• 0.01-200 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

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

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.

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.

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.

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).

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 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.

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.

Future Trends

Piezoelectric microbalance aerosol monitor

Future TrendsTapered-Element Oscillating Microbalance

(TEOM)

TEOMMiner’s helmet mounted coal dust monitor

Monitoring for mercuryBig issue in refineries and gas plantsAssociated with hydrocarbon formationAccumulation according to Hg propertiesMostly elemental and sulphide formsInhalation, skin and ingestion routes

Instrumental Detection MethodsAtomic absorptionGold film resistanceZeeman atomic absorptionResonant microbalance

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

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 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

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 & ToxicityFire, explosion and toxicity are all important

hazards requiring identification, assessment and control.

Mines, confined spaces, refineries, gas plants etc...

Explosivity limits

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

Calibration typically to CH4

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

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

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

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

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

Measuring Flammability

Techniques for high range combustible gas measurement

◦Dilution fittings

◦Thermal conductivity sensors

◦Calculation by means of oxygen displacement

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

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!

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

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.

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*

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 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’

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

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???

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 productsEnvironmentalIndustrial HygieneSafetyHazardous 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 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

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.

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!

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

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

RadiationAir

◦N2

◦O2

◦CO2

◦H2OToxics

◦CO◦HCN◦SO2

Natural gas◦Methane CH4

◦Ethane C2H6

Acids◦HCl◦HF◦HNO3

Others◦Freons◦Ozone O3

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 PIDYou 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 accuratePID is not very selective

Basic use of PIDPID is very sensitive and accuratePID is not very selective

Ruler cannot differentiate between yellow and white paper

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!

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 300oC

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

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 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

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

0.5CF x 100 ppmiso= 50 ppmtoluene

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

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

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

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