collected. These devices may also be single-staged or multistage (for example, consist
of two or more devices connected in tandem).
(2) Filtration. In many cases, drawing contaminated air through a filter
medium and collecting the particulate contaminant is the preferred method. Filtration
devices allowing a range of sampling rates (high volume or low volume) are available.
A variety of filter materials are used, including polyvinylchloride (PVC), cellulose ester
(CE), and glass fiber (GF). The filter medium chosen will depend to some extent on the
size of the individual particles of contaminant (see Figures 2-5 and 2-9).
Figure 2-9. Filters.
(3) Respirable dust sampling. Some standards are based upon the dust
being of respirable size, meaning the dust will not be filtered out before reaching the air
sacs in the lungs. In this case, a cyclone is utilized before the filter, which slows the
dust down, to enable the filter to capture it (see Figure 2-5).
(4) Precipitators. Electrostatic precipitators are frequently used to collect
samples of contaminants occurring in the form of fumes. They are very efficient when
collecting particles smaller than 1 micron in diameter. Electrostatic or electric
precipitation differs from particle collection mechanisms in that the forces acting to
separate the particles from the gas in which they are suspended are electric rather than
inertial or thermal. Charged particles passing through the precipitator are attracted to
oppositely charged plates or tubes. Electrostatic precipitators can be extremely
hazardous in potentially explosive environments.
d. The Sampling Train. Discussions thus far have borne on various devices
used in sampling the air in the worker's environment. In actual practice, these devices
are interconnected in a certain order to obtain the desired sample; this interconnection
of instruments and devices is known as a sampling train (see Figure 2-10). A sampling
train for particulate would consist of these critical elements, in this order: an inlet orifice
(opening); particulate separator (filter, precipitator, impinge, and so forth); air flow meter