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Instructions,
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A Real Time Aerosol
Monitor and Mechanism of Operation (The Microdust Pro) OPERATION
Figure 1 Schematic representation of near forward light scatter. What is near forward light scatter? In
1908, the German physicist G. Mie derived a theory to explain the behaviour of
light when normal Rayleigh scatter no longer applies. Many people have tried to explain the principles of
Mie Theory (forward light scatter) in layman’s terms, but the science
is hard to follow. The Mie scattering calculations are so complex that large
computers are required, particularly if the complex refractive index must be
used, as for metallic scattering particles. For non-spherical particles even
more complex approaches are required, and special cases such as ellipsoids and
rod-shaped particles have been solved by workers such as R. Gans; even more
complex are mixtures of particles of different sizes. These solutions are
important in the investigation of colloids, aerosols, smokes, smogs, and so on,
where the particle sizes and shapes may be deduced from the light scattering
behavior. Figure 1 shows the principle of near forward
light scatter. The light source on
the left of the diagram represents an infrared beam of 880nm wavelength which is
passed through a number of lenses to collimate it.
In clean air conditions, the beam is stopped by the light stop
represented in black. As the beam
passes through the sensing volume, where particulate is introduced, the beam is
scattered by the particles. This is
picked up by the photo detector, which amplifies the signal and converts it to a
mass concentration. The narrow
angle of scatter is represented by the angle b
and is around 12 to 20° depending upon configuration. This concept is better explained in figure 2, which shows the actual layout of the Microdust pro probe. Under clean air conditions, with the metal slider pulled over the sensing volume, the light comes to rest on the light stop. However, when the metal slider covering the sensing volume is drawn back, introducing the particulate, the beam is scattered around the light stop and onto the photo detector behind it. The amount of scattered light is directly proportional to the particulate concentration. CLEAN AIR CONDITION
PARTICULATE INTRODUCED TO THE SENSING VOLUME Figure 2: Schematic showing the arrangement of the Microdust pro Probe FAQ’s: MEASURING
PARAMETERS What range of particle sizes will the pro
monitor? The pro will measure across a
wide range of particle sizes, the smallest being around 0.1 mm to the largest,
around 10 mm. However, as
with all optical instruments, the particle size influences the instrument
response. Optimum sensitivity is in
the respirable size range. Will it measure PM2.5 and PM10?
Figure 3: Poly Urethane Foam Filters (PUF) Work undertaken by the Health & Safety Laboratory (HSL) in Sheffield
tested the foam containing cassettes within the CIS sampler at a flow rate of
3.5 litres per minute. The
techniques used in the tests were previously developed to measure the size
selection characteristics of cyclones and impactors.
The cassette designs and foam plug dimensions were optimised by repeating
tests with different plugs until suitable dimensions had been identified.
Results indicated that with suitable preparation and analytical
procedures, the modified CIS foam samplers developed can be used for monitoring
of exposure to PM10 and PM2.5, thoracic and respirable
aerosol fractions (Kenny & Stancliffe, 1997). A gravimetric adapter is necessary for monitoring PM10 or PM2.5
with PUF’s in the Microdust. A
personal sampling pump with a capacity of 3.5 lmin-1 will also be
required (such as a Casella Vortex pump). If a size selective PUF is used, how can
the instrument then be calibrated? In the same way as the gravimetric comparison is
performed, all the material which is size selected on the basis of the Poly
Urethane Foam (PUF) filter passes through the sample volume of the Microdust
probe and is collected on a standard filter behind the probe.
We can then calibrate the instrument based upon the gravimetric filter
results. How often does the PUF need changing, and how will
that need manifest itself or influence readings? PUF’s are generally only designed for fairly short
sample durations, as they will suffer partial blockage over time.
This will affect the size selectivity characteristics.
Our advice to users is to change the PUF filter after every sample run,
up to a full continuous 24 hour period. Little
is presently known of how PUF loading affects the particle size selection
characteristics, although tests have shown that the D50 (50th
percentile) of the PUF will gradually decrease as the particulate loading
increases. We therefore recommend
that the PUF foams are not reused and are transported carefully. Is
the pro a particle counter? FAQ’s:
CALIBRATION & CLEANING
How is the pro calibrated? There are several methods for calibrating the pro: 1.
The instrument is calibrated to a known reference dust standard before
leaving the factory. This is
covered below. 2.
The instrument can be returned to this “factory” calibration setting
at any time during its life by the use of a calibration insert and the
appropriate software within the instrument.
This is covered below. 3.
It is also possible to “set” a calibration on the pro using a special technique of gravimetric calibration.
This is covered in detail below. How can the instrument be calibrated to a
known dust type? When faced with varying dust types, the response of the unit will differ slightly due to variations in particulate sizes, refractive indices and colour. The relationship between these is described below. In order to correct for this difference, it is necessary to alter the response of the pro. This involves the collection of a gravimetric (filtered) sample of the dust after it has passed through the probe optics. In this way, one collects two averages over the exposure period. One is from the filter, whilst the other is provided by the averaging function within the instrument. It is then possible to derive the difference in these two figures and correct accordingly (figure 4).
Figure 4:
Comparing real time & gravimetric averages
For example, the instrument is run for 8 hours,
yielding the following average concentrations: Real
time component:
=
35 mgm-3 Gravimetric
component
=
40 mgm-3 Therefore the real time component is under-reading by
12.5% - we need to increase the gain of the photometer by this amount.
The optical calibration insert is marked as “30”, so we turn up the
gain (span) until the display on the instrument is showing: 30
multiplied by 1.125 (12.5%)
=
33.75 (34) Now the real time component equals the gravimetric
component and the instrument is calibrated to that particular dust type.
This is an extreme example to show how the procedure is performed.
What is meant by an iso-kinetic calibration system traceable
to national standards?
What is the optical calibration insert and how can it
not change or be degraded with time? The calibration process provides a “reference”
calibration. Once the instrument is
correctly adjusted, a small insert is made, which consists of a piece of optical
glass with particular physical characteristics.
When this is inserted into the sampling volume, the infrared light is
scattered and a value is displayed on the instrument.
As the optical glass will not degrade with time (the only way to degrade
it is to destroy it), the reference standard will always provide a calibration
back to factory settings. Are commercial standards of filtered samples
available for calibration? What is the purge for optical cleaning? The
purge refers to the nozzle or nipple on the lower end of the probe.
This is provided as a cleaning mechanism and is connected to two separate
tubes which pass up through the probe and terminate as air blowers at 90°
across both the emitter and receiver lenses. How often is cleaning required? FAQ’s: ACCURACYHow accurate is the pro? Any scientific measuring instrument is as accurate as
the methodology employed by not only the instrument itself, but also the
operator. The Microdust pro
has been shown in independent tests to be a highly versatile particulate monitor
with highly repeatable results and excellent linearity (see below). Unlike many other hand-held particulate devices, the pro
incorporates a unique post-sampling calibration facility which allows the user
to calibrate the instrument to a specific dust type.
This is explained in more detail in the calibration
section. Is the pro
sensitive to different particle sizes? All
optical sensors are suited to different sizes of particles, depending upon their
source wavelength, focal distances and other optical parameters.
By using an 880nm wavelength source, the instrument exhibits the highest
response in the “respirable” dust size range.
This has a D50 cut of around 3-4mm according to the ISO sampling convention
curves. However, the response shown to differing dust sizes can be shown in figure 6 in which the unit has been tested against various sizes and compositions. One important point to note is that all the responses are straight lines, so adjusting the gain of the photometer (gravimetric calibration – see calibration section) will ensure the correct response.
Figure 6: Photometer response to different particulate sizes How sensitive is the instrument to
variations in particulate colour and refractive index? The Microdust uses a very narrow angle of scatter to
deduce the particulate concentration (in the order of 12-20°).
This reduces the amount of light scattered in the reflected component and
instead concentrates on the diffracted and refracted components (see figure 7
below). The most important
component is the diffracted light, which is the light “bending” around the
object. Diffraction generally does
not depend on particulate colour or refractive index. The use of a narrow angle eliminates most of the uncertainty involved with differences in colour or refractive index. However, there is likely to be some deviation, but generally this is small. A good indication of this is when users gravimetrically calibrate instruments for two completely different types of dust (in terms of colour). These may involve a recalibration factor of around 0.95 to 1.05
Figure 7:
Simplified illustration of narrow angle forward light scatter What is the linear response range if only zero and
one calibration standard is used? The linear response of the Microdust has been tested
to concentrations up to around 30mgm-3 within our wind tunnel.
The response is linear within this range (R2=0.9933
for 0 to 25mgm-3 – see figure 8 below). Concentrations in excess of this amount are difficult
to generate within a wind tunnel, so a separate high concentration dust
“box” is used, in which we can generate dusts in concentrations up to around
2200mgm-3. Linearity has been tested up to this level, producing
excellent results (R2=0.9986)
– see figure 9 below.
Figure 9: Microdust response to Arizona Road Dust (ARD) for low concentrations
Figure 10: Microdust response to ARD for high concentrations When are gravimetric samples required for
calibration? Why are adapters required to ensure accurate
measurements when used in static monitoring applications? Optional adapters are required simply to yield a
smooth and consistent flow of sample air through the instrument probe.
Two general types are available. The first is an aspirated
adapter which uses a small fan (aspirator) to pull air through the
probe at a given flow. This unit
fits over the probe and is supplied with its own power supply. The second type is a gravimetric adapter which generally uses a small personal sampling pump as the “pulling” device, with the added advantage of having a filter assembly located behind the sample volume (figure 10). Essentially, this is the same principle as the iso-kinetic calibration, where the sample is drawn through the probe at a known flow rate and captured on a filter. The weighed filter gives us a mass (in mg) and we can deduce the amount of air which has been drawn through (in m3). We can then compare the instrument “average” concentration over the sample period with the gravimetric component and adjust accordingly.
Figure 10: Gravimetric adapter for static monitoring & calibration FAQ’s:
SOFTWARE & DATALOGGING Can I use the “WinDust pro” PC software
with Windows NT Operating Systems? Yes,
the software supplied is 32 bit, which runs on Windows 95, 98 and NT. What are the major features of the
software? The WinDust
pro software offers the user the ability to perform the following
functions: ·
Create a
“profile” for the instrument. This
specifies which parameters are to be logged, the logging interval, and an
“identifier” or name for the instrument.
This is particularly useful when more than one instrument is owned. ·
Transfer
this control profile from the PC to the instrument. ·
Recover
data from the instrument ·
Accumulation
and processing of real time data for historical analysis and report generation ·
Production
of line graphs based upon historical data ·
Tabular
and report presentation of data files ·
Real time
presentation of prevailing conditions via the RS232 connection What is the maximum logging duration? The
internal logger within the Microdust pro
has a non-volatile 64Kb memory. This
has the potential to store 32 separate runs.
It is possible to set the logging interval from 2 seconds to 10 minutes.
For example if the interval is set at 2 seconds, it is possible to record
8.75 hours of data. If the interval
is every 5 minutes, this equates to a total logging time of 50 days.
Up to 15,700 data records may be stored and downloaded. What parameters are logged? Recorded values are as follows: ·
Average
concentration over the logging interval (mgm-3) ·
Maximum
concentration over the logging interval (mgm-3) ·
Date and
time stamp How do I transfer data from the instrument
to the PC? FAQ’s:
PRACTICAL CONSIDERATIONS Which “concentration range” instrument
do I need for my application? Previous versions of the Microdust were sold as
products with different ranges (eg 0-25 mgm-3 or 0-250 mgm-3).
The Microdust pro has the advantage that there is only one model which covers the
entire measurement range (0 – 2500 mgm-3).
It is possible to “fix” the range of the instrument, or have it as an
“auto-ranging” device. How are the dust concentrations presented
on the instrument? The
pro incorporates a 128 x 64 pixel
graphical backlit display. Dust
concentrations are presented in two ways: ·
Numerical values – instantaneous concentrations are displayed on
the screen in large numerals, as well as a value for the Time Weighted Average (TWA)
which is calculated from the moment the instrument is switched on.
This value can be reset at any time by a “one button” reset.
Maximum concentrations are also displayed in this way.
Figure 11: Real time scrolling graphical representation of dust levels Will the probe get contaminated in a dirty
environment? The optics within the probe are susceptible to the
build up of dust over time. For
this reason, it is necessary to clean the optics at regular intervals.
In normal operating environments, this is achieved by using the clean air
“purge” facility. For environments subject to large concentrations of dust (eg
process control), it may be necessary to connect a constant air supply to the
purge to eliminate any dust from settling on the lenses. Both of these
situations are described in the calibration section
of this document. Can I connect the output of the pro
into other systems? Yes – the pro
has two possible outputs, an RS232 serial digital interface (up to 56k baud) or
an analogue output (0 – 2.5Vdc full scale deflection).
Both of these outputs have been used in different situations, for example
monitoring processes, laboratory tests or controlling dust generation systems in
the pharmaceutical industry. Can I use the pro outside? It is possible to use the Microdust pro
outdoors, but this is generally restricted to short term monitoring,
particularly in site boundary monitoring. However,
the instrument is not designed to be used long term.
For applications which require longer sampling periods, Casella CEL
manufacture other specialist products such as the Environmental Enclosure or the
APM950 (Ambient Particulate Monitor). Does real time monitoring replace sampling
with personal sampling pumps? Real time dust monitoring is a very useful tool in
many situations, but will not replace personal sampling pumps in occupational
hygiene monitoring. This is because
the methods used in gravimetric sampling are prescribed by the Health and Safety
Executive in document MDHS 14/3 and must be adhered to if an employee’s health
is in question. However, real time
monitoring is a useful tool in investigating the general concentrations of dust
to see whether further sampling is needed.
The high cost of equipment and analysis services for gravimetric sampling
often makes the Microdust pro a cost
effective survey tool. For how long will the pro operate on its own batteries? Two battery alternatives are offered with the
Microdust pro: ·
Alkaline cells (4 x AA/ MN1500 cells)
– these will provide around 2700mAh which equates to around 20 hours operation
(typically) ·
Rechargeable Nickel Cadmium cells – these provide around 960mAh which equates
to around 10 hours operation (typically). The advantage of using rechargeable cells is that
they can be recharged in
the instrument, from the internal fast charging circuit.
However, it is very important that alkaline cells are not charged, as
this will seriously damage the instrument and is potentially dangerous. Can I use the pro to check concentrations within a “clean room”? No, legislation states that clean rooms are checked
with instrumentation capable of yielding an output of numbers of particles per
cubic metre, rather than mass
per cubic metre. This can only be
achieved using a particle counter. Can I use the probe in a hot exhaust stack? Unfortunately
the Microdust pro’s probe is not
designed to monitor particulate within hot exhaust stacks directly.
However, it is possible to obtain a real time concentration by passing
the hot sample through a number of cooling traps.
The maximum temperature we would recommend the probe is subjected to is
around 40°C. What problems have been identified
with this type of instrument? It is hard to find fault with the Microdust, but some
comments we have received are as follows. It
is interesting to note that most of these comments have arisen as a result of
operator error / abuse: ·
Contamination of the lenses. All
optical instruments will suffer contamination of components exposed to dusts,
fumes or gases. However, an
effective and regular cleaning cycle will eliminate this problem. ·
Optic misalignment. The
lenses within the probe can be moved if the probe is dropped or hit hard on a
surface. This requires a strip down
and rebuild of the probe. This
problem has been greatly reduced by the introduction of “screw-in” lenses,
as opposed to “glued in”. ·
Battery abuse. The Microdust pro
can use Nickel Cadmium batteries which can be seriously abused by poor charge
management regimes. Instruments
which are overcharged or left in a state of discharge for long periods of time
will be subject to these problems. How long does the infrared lamp last? This is a difficult question to answer as the
infrared source is a solid state photo diode which provides incredible
performance. To try and put a value
on the lifetime, we first started supplying the AMS (predecessor to the
Microdust) in 1989, which used the same diode and none of these have ever been
returned due to lamp failure. We
would estimate an MTBF of 15-20 years. Obviously
the source strength of the diode will deteriorate over time, although this is
corrected by periodic zero and span calibration, which effectively corrects the
offset and multiplier values. What are the terms of warranty, service,
etc.?
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