Limitations of Direct Reading Occupational Hygiene Instrume - PowerPoint Presentation

Slide1

Limitations of Direct Reading Occupational Hygiene Instruments

Reproduced with permission of :

Russell Bond

Robert Golec

Aleks TodorovicSlide2

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 displaySlide3

OutlineSlide4

Aerosol MonitoringSlide5

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 photometerSlide6

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)Slide7

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)Slide8

Light Scattering vs Particle DiameterSlide9

Particle Diameters

microns

Grain Dust

Cement dust

Fly Ash

Flour

Coal Dust

Metal dust & fume

Carbon Black

Diesel Particulate

ZnO

fume

Light Scattering

Wood dust

NanoparticlesSlide10

TSI Dust

Trak

90

o

light scattering angle

Laser light source

0.1

m

m

–

10

m

m

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/m

3

hand-held, personal?Slide11

Environmental Devices Haz-Dust

near forward scattering

Infrared light source

Inhalable, thoracic and respirable size selective sampling attachments

flowrate

1 – 3.3 LPM

0.1

m

m

–

100

m

m (?)

0.01-200 mg/m

3

personalSlide12

Casella Micro-Dust

Near forward light scattering

Infrared source

TSP, PM10, PM2.5 or respirable

flowrate

N/A – diffusion

0 to 2500 mg/m

3

in 3 ranges

hand-held

Slide13

Calibration

ISO 12103-1, Al (Ultrafine) test dust (formerly called Arizona Road Dust).

Particle size range 1um to 10

u

m

%

micronsSlide14

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.Slide15

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.Slide16

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.Slide17

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).Slide18

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 CounterSlide19

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 & JensenSlide20

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.Slide21

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.Slide22

Future Trends

Piezoelectric microbalance aerosol monitorSlide23

Future Trends

Tapered-Element Oscillating Microbalance (TEOM)Slide24

TEOM

Miner’s helmet mounted coal dust monitorSlide25

Monitoring for mercuryBig issue in refineries and gas plants

Associated with hydrocarbon formation

Accumulation according to Hg properties

Mostly elemental and sulphide forms

Inhalation, skin and ingestion routesSlide26

Instrumental Detection MethodsAtomic absorption

Gold film resistance

Zeeman atomic absorption

Resonant microbalanceSlide27

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

concSlide28

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 regenSlide29

Gold Film resistance – How does it work?Slide30

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 interfaceSlide31

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 userSlide32

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 lineSlide33

Flammability & ToxicityFire, explosion and toxicity are all important hazards requiring identification, assessment and control.

Mines, confined spaces, refineries, gas plants etc...Slide34

Explosivity limits Slide35

Species Response Difference

Gas/

VaporLEL

(%

vol

) Sensitivity (%)

Acetone 2.2 45

Diesel 0.8 30

Gasoline 1.4 45

Methane 5.0 100

MEK

1.8 38

Propane 2.0 53 Toluene 1.2 40

LEL Sensor sensitivity varies with chemical Slide36

Calibration typically to CH4Slide37

Low Oxygen AtmospheresO2

required for combustion

Active bead useless below ~10% O

2

Meter reads 0% LEL in 100% fuel vapour

False security

Reason for testing O

2

first, then LELSlide38

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

Slide39

LEL Sensor Poisons

Sensor Lifetime

Sensor Output

With an “Acute” LEL sensor poison the sensor is going to fail, but the time to failure is dosage dependantSlide40

LEL Sensor Poisons

Chronic Poisons are often called “inhibitors” and act over time. Often exposure to clean air will allow the sensor to “burn-off” these compounds

Examples include:

Sulfur compounds (H

2

S, CS

2

)

Halogenated Hydrocarbons (Freons, trichloroethylene, methylene chloride)

StyreneSlide41

LEL Sensor Poisons

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

Sensor Lifetime

Sensor OutputSlide42

Measuring Flammability

Techniques for high range combustible gas measurement

Dilution fittings

Thermal conductivity sensors

Calculation by means of oxygen displacementSlide43

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

operationSlide44

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!Slide45

Toxic Gases and Vapors

Detection techniques:

Colorimetric Tubes

Electrochemical Sensors

Non-dispersive infrared (NDIR)

Photoionization detectorsSlide46

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

Slide47

EC Sensors

Capillary diffusion barrier

Metal housing

Reference electrode

Counter electrode

Electrolyte reservoir

Electrode contacts

Sensing electrodeSlide48

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

Slide49

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

Slide50

Cross-sensitivity Data H

2

S

r

Note

: High levels of polar organic compounds including alcohols, ketones, and amines give a negative response.

*Estimated from similar sensors.

Gas

Conc.

Response

CO

300 ppm

<

1.5 ppm

SO

2

5 ppm

about 1 ppm

NO

35 ppm

<0.7 ppm

NO

2

5 ppm

about -1 ppm

H

2

100 ppm

0

ppm

HCN

10

ppm

0 ppm

NH

3

50 ppm

0 ppm

PH

3

5

ppm

about 4 ppm

CS

2

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

*Slide51

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 happenSlide52

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’Slide53

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

10ppmSlide54

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???Slide55

Traditional four-gas confined space entry monitors miss many common toxic gasses!Slide56

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”Slide57

Who uses PIDs?

Anyone involved with the use of chemicals, gases and petroleum products

Environmental

Industrial Hygiene

Safety

Hazardous Materials Response (HazMat)

Maintenance/OperationsSlide58

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 Fuel

PERCSlide59

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”Slide60

100.0 ppm

Gas enters the

instrument

It passes by

the UV lamp

It is now

“ionized”

Charged gas ions

flow to charged

plates in the

sensor and

current is produced

Current is measured and concentration is displayed on the meter.

+

-

+

-

+

-

+

-

+

-

Gas “Reforms”

and exits the

instrument intact

How does a PID work?

An optical system using Ultraviolet lamp to breakdown vapors and gases for measurement

Photo

Ionization

DetectorSlide61

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.Slide62

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!Slide63

8

9

10

11

12

13

14

15

8.4

9.24

9.54

9.99

10.1

10.5

10.66

11.32

11.47

12.1

14.01

Some Ionization Potentials (IPs) for Common Chemicals

Benzene

MEK

Vinyl Chloride

IPA

Ethylene

Acetic

Acid

Methylene

chloride

Carbon

Tet.

Carbon Monoxide

Styrene

Oxygen

Ionization Potential (eV)

11.7 eV Lamp

10.6 eV Lamp

Not Ionizable

What does a PID Measure?

9.8 eV LampSlide64

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 Slide65

What PIDs Do Not Measure

Radiation

Air

N

2

O

2

CO

2

H

2

O

Toxics

COHCN

SO

2

Natural gas

Methane CH

4

Ethane C

2

H

6

Acids

HCl

HF

HNO3

OthersFreonsOzone O3Slide66

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!Slide67

Basic use of PID

You won’t find the orange in the bunch of apples!

All you’ll find is fruit!Slide68

Basic use of PID

PID is very sensitive and accurate

PID is not very selectiveSlide69

Basic use of PID

PID is very sensitive and accurate

PID is not very selective

Ruler cannot differentiate between yellow and white paperSlide70

Basic use of PID

PID is very sensitive and accurate

PID is not very selective

PID can’t differentiate between ammonia & xylene

But both are toxic!Slide71

Basic use of PID

Just because there is a

Ionisation

Energy listed doesn’t mean that the PID will respond.Slide72

Basic use of PID

Basic rule of thumb is:

The higher the boiling point the slower the response

Compound should have a boiling point of less that 300

o

CSlide73

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 500-1000 ppm

Electronics linearise the response at this time

At higher concentrations the response dropsSlide74

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 lampSlide75

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 technologySlide76

PID Measurement Parameters

Calibration Gas Selection

IMPORTANT

Calibrating a PID to a specific gas DOES NOT make the instrument selective to that gas

A PID always responds to all the gases that the lamp can ionise

It gives a readout in equivalent units of the calibration gasSlide77

What is a Correction Factor?

Correction Factors are the key to unlocking the power of a PID for Assessing Varying Mixtures and Unknown EnvironmentsSlide78

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-106Slide79

CF Example: Toluene

Toluene CF with 10.6eV lamp is 0.5 so PID is very sensitive to Toluene

If PID reads 100 ppm of isobutylene units in a Toluene atmosphere

Then the actual concentration is 50 ppm Toluene units

0.5

CF

x 100 ppm

iso

= 50 ppm

tolueneSlide80

CF Example: Ammonia

Ammonia CF with 10.6eV lamp is 9.7 so PID is less sensitive to Ammonia

If PID reads 100

ppm

of isobutylene units in an Ammonia atmosphere

Then the actual concentration is 970

ppm

Ammonia units

9.7

CF

x 100

ppm

iso

= 970

ppm

ammoniaSlide81

PID Measurement Parameters

Low CF = high PID sensitivity to a gas

If the chemical is bad for you then the PID needs to be sensitive to it. In general,

If Exposure limit is < 10

ppm

, CF

<

2

If the chemical isn’t too bad then the PID doesn’t need to be as sensitive to it

If Exposure limit is > 10

ppm

, CF

< 10Use

PIDs for gross leak detectors when CF > 10Slide82

PID Measurement Parameters

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

0.9

0.850.69

9.07JP-80.60.51

0.70.48

Gasoline0.90.73

1.11.1

n-hexane4.34

3.34.510.18Slide83

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/m3Slide84

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 methaneSlide85

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 availableSlide86

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 completelySlide87

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, PH3Slide88

ConclusionBe careful

Understand the limitations of the device

Don’t be talked into buying an instrument. Check out its value and limitations