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Respirable Dust Sampling and Measurement 191

to three fields were counted to get an average. An average count of 50 per field was
equal to:

6
58:8 10 particles per cubic foot ¼ 2; 079 pp cc.
The instrument suffered from the same defects as the Konimeter and was eventually
abandoned.

12.1.3 Thermal Precipitator

In this device, the sampled air is drawn vertically downward through a narrow channel
between two microscope cover glasses as shown in Fig. 12.1A. An electrically heated
wire is stretched horizontally across the center of the unit. If a cold body (the micro-
scope slide) is inserted near the hot wire, airborne dust particles are deposited on the
slide in the form of a strip because of the thermal gradient, hence the name thermal
precipitator. After sampling, the cover glasses are evaluated microscopically, usually
with a 2 mm oil immersion objective. The volume of air sampled is metered so that the
dust concentration can be expressed as “number of particles per cc.”
The sampling rate of the thermal precipitator is 7 cc/min. The instrument is pro-
vided with a water aspirator and powered with a miner’s cap lamp battery, which
makes it safe for gassy mines. Sampling time is from a few minutes to 24 h, depending
on the dust concentration.
If the sample contains siliceous dust, the slides may be acid treated, as in a konim-
eter, to remove carbonaceous and acid-soluble particles.
The outstanding features of the thermal precipitator are the following:

1. It collects all particles less than 10 mm size with 100% efficiency (thermal deposition be-
comes more effective the smaller the particle size).
2. It avoids further breakage of aggregates.
Although the instrument provides a dust sample of high accuracy, the final result is
still subject to error, as only a portion of the total sample is counted. A variant of this

Figure 12.1A Standard thermal precipitator dust sampler.
Source: By Hartmann, John Wiley, 1982.