Page 215 - Advanced Mine Ventilation

Respirable Dust Sampling and Measurement 195

whenever the dust concentration was high. The instrument was also considerably heav-
ier and more expensive than the personal samplers.

12.2.4 The Microorifice Uniform Deposit Impactor
Hering [5] describes a multistage impactor (Model 298) where the size separation was
done by impaction. The incoming air was made to pass through a number of orifices
decreasing in size, thus imparting higher velocities to the airstream. Larger particles
were collected on the first few stages, and the finer particles were collected on succes-
sively lower stages. The deposits were made on substrates that were preweighed. The
increase in weight was a measure of the dust deposited. It was an excellent research
tool but could not be used to measure compliance with legal standards. A further
refinement of this instrument was called “microorifice uniform deposit impactor” or
MOUDI. It was also a good research tool but impractical for in-mine use to determine
compliance. These instruments yielded a mass distribution of airborne dust from which
the respirable fraction could be calculated.

12.3 Dust Concentration Measurement by Light-
Scattering Instruments

In late 1970s, the US government made a proposal to monitor dust concentration at a
mining machine continuously (in real time) and calculate the time when the limit of
3 3
16 mg/m -h (2 mg/m 8 h exposure) dust exposure was reached. At this time, the
power to the mining machine would be cut, and a new crew of workers will be called
in to restart mining.
In the early 1980s, three instruments based on the principle of scattered
light measurement came into the market. They are briefly described in the following
article.

12.3.1 The British SIMSLIN Dust Monitor
The Research Division of NCB first developed the SIMSLIN II (safety in mines scat-
tered light instrument) monitor. The mine air passed through an elutriator to conform
to the BMRC sampling curve and next passed through an open chamber where it scat-
tered a laser beam, produced by a gallium arsenide diode in the near-infrared (IR)
range. With a mean wavelength of 0.9 mm, the scattered radiation over 12e20 degrees
in forward direction was measured by photometers, and dust concentration was given
3
in mg/m . The instrument could give both instantaneous and cumulative readings. It
measured the surface area of dust particles. It had two ranges for dust concentration,
3
0e20 and 0e200 mg/m .