Page 210 - Advanced Mine Ventilation

190 Advanced Mine Ventilation

do. These size-selection curves are already discussed in Chapter 7. Most health stan-
dards are based on these gravimetric instruments. However, these sampling instru-
ments still suffered from some design defects and newer instruments, such as the
tapered element oscillating microbalance (TEOM) instruments, were developed.
This chapter will provide a brief introduction to prominent instruments of each era
with their merits and demerits.
The second issue with sampling and measurement of a random variable, such as the
dust concentration in air, is the statistical reliability. Like the measurement of any other
random variable, a minimum number of samples are required to derive an average that
has 95% confidence of being within 20% of the true mean. A case study for silica
measurement will be discussed.

12.1 Early Dust Measuring Instruments

Before 1960, there were three main instruments that collected and counted the number
of particles in air. They are as follows:
• The Konimeter
• Midget Impinger
• Thermal Precipitator
In the light of current knowledge, they provided a poor measurement of true risk.

12.1.1 The Konimeter

This instrument was developed in 1916 by Kotze but is obsolete now. A spring-loaded
piston pump draws 5 cc of dusty air and impinges it on a grease-coated glass plate. The
glass plate can be rotated to collect up to 30 samples. The glass plate is put under a
microscope. Each sample appears as a circle. Typically, only a 30 section is used

to count the number of particles, and it is multiplied by 12 to get the total number
of particles in 5 cc of air. The dust concentration was expressed as “dust index” that
is equal to the number of dust particles per cc of air. It should be noted that 100
ppcc equals 2.83 million particles per cubic feet. Konimeter has a low collection effi-
ciency for particles below one micron. Large aggregates of dust can be shattered by
impact and give a much higher count. This instrument was mostly used in Australia,
India, Canada, Germany, and Poland.

12.1.2 The Midget Impinger

This instrument was mainly used in the United States. The mine air sample was drawn
3
through a 1 mm nozzle at a fixed rate for a fixed time. Typically, 0.5 ft (14,163 cc) of
air was collected in a flask containing about 30 cc of water. The dust particles are
further collected in a 1 mm deep hemocytometer cell after letting the dust settle
down for 30 min. The cells were put under a microscope and dust particles, smaller
than 10 microns, were counted. There are 4000 fields per cc of water, but only two