Limitations of Direct
Reading Occupational
Hygiene Instruments
Reproduced with permission of :
Russell Bond
Robert Golec
Aleks Todorovic
Introduction
Occupational Hygienists are using direct
reading instruments more and more as
the technology becomes available.
 As instruments become more
sophisticated, there is a growing
perception or a seductive tendency to
blindly believe the numbers on the display

Outline
Sample
Atmosphere
Gas Vapour
Electronic
Confined
Space
Particulates
Diffusive
PID
Detector
Tubes
Light
Scattering
Devices
Aerosol Monitoring
Direct-Reading Aerosol Monitors
Light Scattering
(Aerosol Photometers) – laser, IR, broad
wavelength
 Piezo-Electric Mass Sensors
 Tapered Element Oscillating Microbalance
(TEOM)
 Fibrous Aerosol Monitors – special type of
aerosol photometer

Light Scattering/Aerosol
Photometers

Most common type of aerosol monitor

Based on Mie’s theory of light scattering
by spherical particles (light intensity of
scattered light is related to wavelength of
incident light and the diameter of the
particles)
Theory of Light Scattering
by Spherical Particles - Mie
Light scattering is a combination of diffraction,
refraction and reflection
 Intensity of scattered light is related to
wavelength of incident light (l), the angle of
scatter (Q) the and the diameter of the particle
(d).
If d>>l then most of the
scattering occurs in the forward
direction (Mie’s Scattering)
If d<<l then most of the
scattering occurs in the back
direction (Raleigh Scattering)
Light Scattering vs Particle Diameter
Particle Diameters
Light Scattering
Grain Dust
Wood dust
Nanoparticles
Cement
dust
Fly Ash
Flour
Coal Dust
ZnO fume
Metal dust & fume
Carbon Black
Diesel
Particulate
0.001
0.01
0.1
1
microns
10
100
1000
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
Calibration
ISO 12103-1, Al (Ultrafine) test dust
(formerly called Arizona Road Dust).
Particle size range 1um to 10 um
100
80
%
60
40
20
0
0 1 2 3 4 5 6 7 8 9 10
microns
Sources of Error
Light scattering is an indirect measure of
particulate mass concentration based on
an assumed particle size distribution.
 Different types of dusts can have
significantly different particle size
distributions from the calibration dust
which can lead to large deviation from the
curve.

Sources of Error

Aerosol particulate refractive index can
have an effect on light scattering and
therefore on the estimation of mass
concentration when compared against a
reference (ARD) aerosol curve.
Sources of Error

Monitor calibration assumes that aerosol particle
size distribution remains constant. Changes in
the generation of the airborne aerosol or in the
wind speed can change the particle diameter
distribution and the instrument response.

The ability to accurately measure the mass
concentration of thoracic and inhalable dust
fraction rely on the ratio of <10 micron
(respirable) particles in the larger size range
remaining constant.
Sources of Error
Monitoring of high aerosol concentrations
can lead to deposition on the instrument
optics which can change the instrument’s
response.
 At high humidity, water droplets can be
detected by the photometer and cause a
falsely high reading.
 Elongated aerosol particles (eg fibres) are
poorly detected (unless fibres can be
oriented in same direction).

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 Errors

Consider the likely nature and particle size
range of the aerosol of interest and the
objectives of the monitoring.

Verify the instrument’s response to the
aerosol of interest by carrying out serial
gravimetric sampling in parallel with the
monitor and determine a correction
(calibration) factor.
Minimising The Errors

Use real-time light scattering aerosol
measurements as a screening tool or to
assess engineering controls but not as a
decision making tool for health risk
monitoring.
Future Trends

Piezoelectric microbalance aerosol monitor
Future Trends

Tapered-Element Oscillating Microbalance
(TEOM)
TEOM

Miner’s helmet mounted coal dust monitor
Monitoring for mercury





Big issue in refineries and gas plants
Associated with hydrocarbon formation
Accumulation according to Hg properties
Mostly elemental and sulphide forms
Inhalation, skin and ingestion routes
Instrumental Detection Methods
Atomic absorption
 Gold film resistance
 Zeeman atomic absorption
 Resonant microbalance

AAS - How does it work?




RF field excites Hg atoms yielding 253.7nm
Doesn’t ‘see’ Hg compounds
Sample air through cell (70-90L/hr)
Absorbed radiation proportional to Hg conc
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 Detectors
 Multi-Gas Detectors

◦ Normally worn on the belt, used with chest
harness or held by hand
◦ Multitude of types to choose from
◦ Vary in price
◦ Vary in user interface
Gas Detectors

Diffusion Monitors
◦ Most commonly used
◦ Utilises natural air currents to provide sample
◦ Normal air is sufficiently energetic to bring
sample to sensor
◦ Only monitors atmosphere that immediately
surrounds the monitor
◦ Inability to sample at remote locations
◦ May lead to a decision based on false
information due to limited reach of user
Gas Detectors

Sample Draw Monitors
◦ Two types available
 Motorised sampling pump
 Hand operated squeeze bulb
◦ Enables remote sampling from varying
distances
◦ Draws sample quicker to the sensors from
distance
◦ Liable for leakage – dilutes sample
◦ Has time lag issues
◦ Users need to be wary of adsorption of sample
to sample line
Flammability & Toxicity
Fire, explosion and toxicity are all
important hazards requiring identification,
assessment and control.
Mines, confined spaces, refineries, gas
plants etc...
Explosivity limits
Species Response Difference
Gas/VaporLEL
Acetone
Diesel
Gasoline
Methane
MEK
Propane
Toluene
(%vol) Sensitivity (%)
2.2
0.8
1.4
5.0
1.8
2.0
1.2
45
30
45
100
38
53
40
LEL Sensor sensitivity varies with
chemical
Calibration typically to CH4
Low Oxygen Atmospheres





O2 required for combustion
Active bead useless below ~10% O2
Meter reads 0% LEL in 100% fuel vapour
False security
Reason for testing O2 first, then LEL
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
Sensor Output
LEL Sensor Poisons
With an “Acute” LEL sensor
poison the sensor is going to
fail, but the time to failure is
dosage dependant
Sensor Lifetime
LEL Sensor Poisons
Chronic Poisons are often called
“inhibitors” and act over time. Often
exposure to clean air will allow the sensor
to “burn-off” these compounds
 Examples include:

◦ Sulfur compounds (H2S, CS2)
◦ Halogenated Hydrocarbons (Freons,
trichloroethylene, methylene chloride)
◦ Styrene
Sensor Output
LEL Sensor Poisons
With a “Chronic” LEL sensor
poison the sensor recovers after
an exposure, subsequent
exposures will further degrade
sensor output
Sensor Lifetime
Measuring Flammability

Techniques for high range combustible gas
measurement
◦ Dilution fittings
◦ Thermal conductivity sensors
◦ Calculation by means of oxygen displacement
Thermal Conductivity

Each type of gas has a unique TC and
thus a unique relative response

The gas does not need to be combustible

No oxygen is required for its operation
Thermal Conductivity

Used frequently in:
 Petrochemical – blanketing
 Gas transmission – ensuring full supply
 Site remediation – remember City Of Casey

Issues arise due to the fact that most TC
sensors read in %VOL
 1% VOL Methane = 20% LEL
 1% VOL Propane = 47% LEL

Make sure you’re reading in the right units!
Toxic Gases and Vapors

Detection techniques:
◦
◦
◦
◦
Colorimetric Tubes
Electrochemical Sensors
Non-dispersive infrared (NDIR)
Photoionization detectors
How do toxic sensors work?

Electrochemical (EC) substance specific
sensors work by:
◦ Gas diffusing into sensor reacts at surface of
the sensing electrode
◦ Sensing electrode made to catalyze a specific
reaction
◦ Use of selective external filters further limits
cross sensitivity
EC Sensors
Metal
housing
Electrode
contacts
Capillary
diffusion
barrier
Sensing
electrode
Reference
electrode
Electrolyte
reservoir
Counter
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
Gas
CO
Conc.
r
Response
300 ppm
<1.5 ppm
SO2
5 ppm
about 1 ppm
NO
35 ppm
<0.7 ppm
NO2
5 ppm
about -1 ppm
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*
5 ppm
about 2 ppm
Ethylene
100 ppm
< 0.2 ppm
Isobutylene
100 ppm
0 ppm
10000 ppm
0 ppm*
3000 ppm
about 70 ppm*
H2
Methyl mercaptan
Toluene
Turpentine
Note: High
levels of
polar
organic
compounds
including
alcohols,
ketones,
and amines
give a
negative
response.
*Estimated
from similar
sensors.
Datalogging
Most new CS monitors have sophisticated
microprocessors that allow the
continuous recording of data
 Data can quickly document worker
exposure levels compared to sampling
techniques
 Datalogging running continuously in the
background provides valuable information
when serious incidents happen

Datalogging

Can be a TRAP – WATCH OUT!
 Datalogging is really a ‘snapshot’ of the event
at that time
 The longer the datalogging interval the LESS
resolution provided by the graph or tabular
report
 If concentrations are expected to vary tighten
your interval
 Some instruments log the ‘AVERAGE’ and
some log ‘MAX’
Datalogging

Can be a TRAP – WATCH OUT!
 Example:
 An instrument logs the highest value during the
interval and the logging period is one hour
 59 out of 60 minutes where at 1ppm
 1 out of 60 minutes was at 10ppm
 The report would show the concentration for the
entire logging period was
10ppm
Datalogging

8 Hour TWA calculation vs 12 Shift
 Example:
 employee has a personal gas monitor
 Employee works for 12 hours
 Gas monitor is programmed only to give TWA for
8 Hours
 Gas monitor is downloaded for data
 Results are produced
 What do you report as the result from the unit???
Traditional four-gas confined space entry
monitors miss many common toxic
gasses!
What is a PID?

PID = Photo-Ionization Detector

Detects VOCs (Volatile Organic
Compounds) and Toxic gases from
<10 ppb to as high as 15,000 ppm

A PID is a very sensitive broad
spectrum monitor, like a “low-level
LEL”
Who uses PIDs?
Anyone involved with the
use of chemicals, gases and
petroleum products





Environmental
Industrial Hygiene
Safety
Hazardous Materials Response (HazMat)
Maintenance/Operations
A PID is like a Magnifying Glass
A Magnifying glass lets a detective see
fingerprints; a PID lets us “see” VOCs
Ammonia
Carbon
Disulfide
Benzene
Styrene
PERC
Xylene
Jet Fuel
How does a PID work?

An Ultraviolet lamp ionizes a sample gas
which causes it to charge electrically

The sensor detects the charge of the
ionized gas and converts the signal into
current

The current is then amplified and
displayed on the meter as “ppm”
How does a PID work?
An optical system using Ultraviolet
lamp to breakdown vapors and
gases for measurement
+
Gas enters the
instrument
It is now
“ionized”
It passes by
the UV lamp
+
-
Current is measured and
concentration is
100.0 ppm displayed on the meter.
+
-
+
-
-
+
Charged gas ions
flow to charged
plates in the
sensor and
current is produced
Gas “Reforms”
and exits the
instrument intact
What does a PID Measure?





Ionization Potential
All gasses and vapors have an Ionization
Potential (IP)
IP determines if the PID can “see” the gas
If the IP of the gas is less than the eV
output of the lamp the PID can “see” it
Ionization Potential (IP) does not correlate
with the Correction Factor
Ionization Potentials are found in RAE
handouts (TN-106), NIOSH Pocket Guide
and many chemical texts.
If the “wattage” of the
gas or vapor is less
than the “wattage” of
the PID lamp then the
PID can “see” the gas
or vapor!
What does a PID Measure?
Some Ionization Potentials (IPs) for Common Chemicals
15
Ionization
Potential
(eV)
9.8 eV
Lamp
10.6 eV
Lamp
Not Ionizable
11.7 eV
Lamp
14.01
14
13
12.1
12
11
10.5
9.99 10.1
10
11.3211.47
10.66
9.24 9.54
9 8.4
Carbon
Monoxide
Oxygen
Carbon Tet.
Methylene
chloride
Acetic Acid
Ethylene
IPA
MEK
Benzene
Styrene
Vinyl Chloride
8
What does a PID Measure?

Organics: Compounds Containing Carbon (C)
◦ Aromatics - compounds containing a benzene ring
 BETX: benzene, ethyl benzene, toluene, xylene
◦ Ketones & Aldehydes - compounds with a C=O bond
 acetone, MEK, acetaldehyde
◦ Amines & Amides - Carbon compounds containing Nitrogen
 diethyl amine
◦ Chlorinated hydrocarbons - trichloroethylene (TCE)
◦ Sulfur compounds – mercaptans, carbon disulfide
◦ Unsaturated hydrocarbons - C=C & C C compounds
 butadiene, isobutylene
◦ Alcohol’s
 ethanol
◦ Saturated hydrocarbons
 butane, octane

Inorganics: Compounds without Carbon
 Ammonia
 Semiconductor gases: Arsine
What PIDs Do Not Measure
 Radiation
 Air
◦
◦
◦
◦
N2
O2
CO2
H 2O
 Toxics
◦ 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 PID
You won’t find the orange in the
bunch of apples!
All you’ll find is fruit!
Basic use of PID


PID is very sensitive and accurate
PID is not very selective
Basic use of PID


PID is very sensitive and accurate
PID is not very selective
Ruler cannot differentiate between yellow and
white paper
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!
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
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 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 Selection
 IMPORTANT

◦ Calibrating a PID to a specific gas DOES NOT
make the instrument selective to that gas
◦ A PID always responds to all the gases that the
lamp can ionise
◦ It gives a readout in equivalent units of the
calibration gas
What is a Correction Factor?
Correction Factors are
the key to unlocking the
power of a PID for
Assessing Varying
Mixtures and Unknown
Environments
What is a Correction Factor?
Correction Factor (CF) is a measure of
the sensitivity of the PID to a specific
gas
 CFs are scaling factors, they do not
make a PID specific to a chemical, they
only correct the scale to that chemical.
 Correction Factors allow calibration on
cheap, non-toxic “surrogate” gas.
 Ref: RAE handout TN-106

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 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 < 10

Use PIDs for gross leak detectors when CF
> 10
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
0.85
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