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Synergist® Solutions: Real-time Dust Monitoring
Advantages of Real-time Measurements in Industrial Hygiene
By Sreenath Avula and Greg Olson
Real-time monitoring in industrial hygiene has revolutionized the measurement of workplace hazards. Direct-reading
instruments have enabled industrial hygienists to be proactive, allowing them to measure physical and chemical hazards
as they are being generated. The time-consuming alternative is to wait for an 8-hour reference method sample to be
completed at the end of the work shift and then send it to a laboratory for subsequent analysis. Real-time monitoring
using direct-reading instruments also enables source identification and is a very useful tool for source apportionment
and modeling, evaluation and validation of engineering controls, and corrective actions during personal and ambient
work area monitoring activities. Immediate access to information supports real-time decision making and is the
industrial hygienist’s greatest advantage over reference sampling methods. This article will focus on advancements in
real-time monitoring for dust, also known as particulates or aerosols.
There are plenty of instruments with which industrial hygienists can monitor dust, and these instruments are primarily
based on two distinct technologies: photometers and optical particle counters. Both instruments rely strongly on the
optical properties of the particulates to indirectly estimate the mass concentration. The light scattered by particles can
be converted to mass concentration through calibration. Other technologies used to measure mass include beta
attenuation gauge and vibrating mass monitor, both of which are more suited for outdoor ambient monitoring and will
not be discussed in this article.
Photometers
Photometers are ideal for industrial hygiene applications and are highly
desirable for applications such as personal exposure monitoring,
ambient workplace monitoring, point source monitoring, emissions
monitoring, validation of engineering controls and trend analysis and
screening. They can be used to sample solid and liquid aerosol-like
dusts, mists, fumes, smoke, diesel exhaust, fog and condensates. They
are capable of handling a wide range of concentrations and have a
linear response to particulate concentration from very low (as low as
0.001 mg/m3) to extremely high (>100 mg/m3). Photometers are
calibrated to Arizona Road Dust, or ISO 12103-1 A1 test dust.
Figure 1. Schematic of a typical photometer.
A photometer’s principle of operation is based on measuring scattered
light from a cloud of particles. The light source is typically a laser diode
that produces a coherent beam of light, which is then focused onto the
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particles using focusing optics. The higher the concentration of particles, the higher the amount of light scattered from
the particles. The resulting scattered light from the particles is collected by the collecting optics—an assembly of lenses
or a mirror. The collecting optics transfer the collected light onto the detector, which produces a current that is
converted to a measurable voltage. The change in the voltage is directly proportional to the mass concentration. Figure
1 details the schematic of a typical photometer.
Photometers cannot discriminate between different particle sizes. Therefore, if the user is interested in sampling a
specific size fraction (for example, respirable fraction PM 2.5 or PM 10 ), a size-selective inlet must be used on the front end
of the photometer. Size-selective inlets are designed to operate at a specific flow rate. Photometers with size selective
inlets also come with flow control, where the desired flow rate is maintained under all operating conditions. The
DustTrak™ DRX Aerosol Monitor from TSI Incorporated is an exception and does not need any size-selective inlets.
The DustTrak DRX uses a patented technology that utilizes both the photometric and single particle-counting
technologies to enhance the performance of the real-time, direct-reading photometer. The DRX technology counts only
particles greater than 1.0 µm, unlike an optical particle counter (OPC), which counts all particles in the sensing chamber.
The OPC detection circuit in the DRX appends to the photometer measurement range that tends to drop off for particles
larger than around 4.0 µm. The optics and electronics are more complicated than those of a simple photometer (see
Figure 2), but the measurement is based on the speed of the signal processing and size-discriminating algorithms.
Figure 2. Schematics of the optics and electronics of the DustTrak DRX Aerosol Monitor.
Aerosol Measurement
Signal Acquisition and Processing
Aerosol Inlet
HEPA Filter
Optics
Light Trap Chamber
Orifice
Photometric DC
Voltage Offset
Analyzer
Sheath Air
Size Segregated
Mass Concentration
(eg. PM1, PM2.5, PM4 & PM10)
Mirror
Photo
Detector
Beam Shaping
Optics
Viewing
Volume
Gravimetric
Filter
Protection
Filter
Pump
Flowmeter
Pulse Height
Pulse Height
Offset
Laser Diode
Exhaust
Single Particle
Pulse Height
Analyzer
Dampening
Chamber
No Particles
(background)
Low Concentration
(Counting Pulses)
Photometric Signal
Laser
Beam
High Concentration
(Pulses + Photometry)
Single-particle counting allows particles to be classified into PM 1.0 , PM 2.5 , respirable and PM 10 fractions without the use
of size-selective inlets. The advantage of DRX over a traditional OPC is DRX’s ability to discriminate particle size even at
very high concentrations (up to 150 mg/m3).
Photometers have their limitations like all other instruments. They are sensitive to humidity and will tend to overestimate actual concentration if the humidity exceeds 70 percent. When sampling under high humidity conditions, some
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manufacturers provide the option to heat the inlet to remove humidity. Others provide humidity compensation by
determining the humidity in the sensing chamber and making a correction to the measured concentration.
Photometers’ responses are also sensitive to particle size distribution, density and refractive index of the sampled
aerosol. To overcome these sensitivities, all photometer manufacturers allow the user to program a custom calibration
factor. By performing a side-by-side comparison between a photometer and gravimetric analysis, the photometer
response can be adjusted to match gravimetric concentration. This correction factor, or calibration factor, is valid as long
as the dust being sampled does not change much in terms of size distribution or composition.
Optical Particle Counters (OPCs)
OPCs measure particle concentrations based on the optical properties of dust. However, unlike photometers, they are
designed to count and size particles being sampled one at a time. The particles are focused into a thin beam and made
to pass a thin sheath of laser light. As each particle passes though the laser light, it registers a pulse of light across the
detector, which is then converted to a voltage pulse. The height of the pulse is proportional to the particle size. The
pulse heights are related to particle size through calibration with spherical polystyrene latex particles (PSL). OPCs are
also sensitive to changes in dust composition (density and refractive index), but are not affected by changes in size
distribution.
OPCs cannot handle high dust concentrations and are more difficult to calibrate to the particulates being measured.
Their measurements are also significantly affected if the sampled flow rate changes. Therefore, flow control is very
critical to accurate concentration measurements.
The optics in an OPC are more complicated than in a photometer because the light scattered from individual particles
has to be collected and focused onto the photo detector. Since the light scattered from individual particles has a lot less
intensity than a cloud of particles, the collecting optics are designed to collect as much scattered light as possible using
an elliptical mirror with a large aperture.
Figure 3. Schematic of OPC optics.
See Figure 3 for a schematic of OPC
optics.
Since OPCs count particles one at a time,
they cannot handle high concentrations
(> 1.0 mg/m3). The upper concentration
limit depends on the particle size. At
high concentrations, the probability of
more than one particle in the sensing
chamber at a given time increases,
resulting in coincidence losses. OPCs are
therefore ideal for applications such as
semiconductor cleanroom monitoring,
pharmaceutical cleanroom monitoring,
indoor air quality studies and research
studies.
OPCs work well at low concentrations,
but at high concentrations they tend to
underestimate concentration due to
coincidence errors. The conversion from
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count concentration to mass concentration also depends on the assumptions made about the density and refractive
index of the sampled particles. This plays a larger role in an OPC than in a photometer because OPCs can be calibrated
only with PSL, which are perfectly spherical unit density particles.
Choosing the Appropriate Instrument
Industrial hygienists have plenty of direct-reading instruments to choose from in order to measure dust in real time. IHs
must consider the application and choose the appropriate instrument and technology for their sampling needs. Not all
instruments are ideal for all applications. Instrument choice is determined by units of measure needed (mass and/or size
distribution), flow rate, expected concentration, frequency of sampling and application. See Figure 4 for general
guidelines for practitioners for choosing the appropriate instrument for each specific application.
Figure 4. Guidelines to aid IHs in choosing appropriate instruments.
Photometer
Indoor Air Quality Conventional Studies
Indoor Air Quality UF Particle Studies
Industrial Workplace
Monitoring
Outdoor Environmental
Monitoring
Emissions Monitoring
Respirator Fit-testing
Filter Testing
Clean Room
Monitoring
Pharmaceutical &
Semiconductor
Clean Room
Research and
Development
Cost Comparison
Optical Particle
Counter
Single
Channel
Multichannel
Portable
Handheld
Good
Excellent
Poor
Good
Poor
Poor
N/A
N/A
N/A
N/A
N/A
Good
N/A
N/A
Good
N/A
N/A
Excellent
Excellent Excellent
Good
Excellent
Excellent Excellent
Good
Good
Good
Good
Poor
Poor
Excellent
Poor
Poor
Poor
Excellent
Poor
Fair
Good
Good
Fair
$
$$
$$
$
Sreenath Avula, PhD, is the product management specialist for TSI, Inc. He can be reached at (651) 490-3867 or
[email protected]. Greg Olson is senior industrial hygienist and global product specialist for TSI, Inc. He can be reached
at (651) 490-4002 or [email protected].
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