A new instrument to detect tiny polluting particles that vehicles emit
A GROUP of American and British scientists claims to have developed an instrutment to measure the levels of fine air-borne particles from vehicle exhausts, which cause respiratory disorders in
thousands of people each year.
The vibrating tube impactor (VTI),
developed by scientists led by David
Booker Of AEA Technology, along with
an Anglo-American instrument company Graseby Anderson, can measure the
concentration of these particles - each
smaller than 10 micrometres across -
upto 4 size ranges simultaneously (New
Scientist, Vol 146, No 1975).
Other instruments, says Booker, can
only measure the total mass of the particles, rather than splitting them into size fractions. Determining the amount of
each fraction is important because different sized particles penetrate people's
lungs at different levels and, therefore,
cause characteristic health problems.
The VTI consists of 4 glass u-shaped
tubes connected in series. Each successive tube has a smaller bore so that dustladen air accelerates as it passes from
one tube to another. The tubes can also
be set up to have successively sharper
bends.
When the air enters the first tube,
larger and heavier particles fail to negotiate the bend and stick to the tube wall. Progressively smaller particles are
deposited in the successive u-tubes.
Each tube gets heavier as it collects the
particles. To measure this increase in
weight, each tube is vibrated at fixed frequency. The heavier each tube becomes,
the greater energy is required to maintain the vibration.
AEA Technology has tested the VTI
alongside the best particle monitor
previously available - called the
tapered element oscillating microbalance (REOM). Because TEOM captures
particles in a filter, it can measure
only I size range of particles at a
time. Besides, "the VTI'S u-tubes are
more robust and cheaper to produce
than TOEM'S tapered elements," say researchers.
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