Dustviewer
General
Description
Sampling efficiency
The Dustviewer
is a simple and cheap method to measure the actual coarse dust concentration.
It is a rotary impactor: the dust is sampled on a rotating strip by
inertial impaction. There are several rotary impactors commercially
available. A commonly used one is the Rotorod sampler. The advantage
of the Dustviewer is the practical use at field locations without
any facilities. up
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A strip of 5 mm width and 210 mm length is mounted in a holder and rotated radially by a battery-driven drill. The strip is coated by a silicone oil to stick the dust particles. The holder contains a graduated scale from 1 (at a radius of 0.08 m) to 10 (at a radius of 0.008 m). The rotating speed is controlled by a bicycle computer. The computer shows the mean velocity (AVG in km/h), the maximum velocity (MAX in km/h), the sampling time (STP in hh:mm:ss) and the covered distance (TRP in km). |
It is adjusted
in such way that velocity and distance apply for the strip at a radius
r of 0.08 m. The mean velocity at this radius is about 40 km/h. The
volume V of sampled air is calculated using TRP and the width b of
the strip (0.005 m). Dust is sampled at both sides of the strip, so: |
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The dust concentration C is calculated by dividing the collected dust
mass m by the sampled air volume:
The dust concentration can be estimated immediately after the measurement
by considering the value x at the graduated scale of the holder where
the dust is just visible. As the sampled volume of air is proportional
tot the radius, the some goes for the amount of collected dust. So, collected
dust is best visble at the tips of the strip and gradually disappears
to the axis. The visibility treshold for coal dust on a white surface
is 0.1 g/m2. With the value x on the holder where dust is just visible,
the dust concentration is:
This is valid for good contrast between strip and dust.
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The sampling efficiency depends on the particle size, width of the strip
and the sampling velocity. The sampling velocity is proportional to the
radius. So, the sampling efficiency changes with the radius. This shift
in sampling efficiency can be used to assess the aerodynamic particle
size of the dust particles.
