Environmental Engineering Reference
In-Depth Information
Figure 9.1
Real-time electronic CO
monitor.
2
Real-time sampling/analysis is conducted with electronic direct-read
instruments or by gas sampling tubes. A variety of electronic direct-read
instruments are commercially available for determining concentrations of
gases, vapors, or particulate matter. They are usually pump-driven devices
which draw air at a known, low, constant rate into a small chamber where
sensors measure specific chemical or physical properties of contaminants
under test. Commonly used principles for gas/vapor substances include
electrochemistry, photometry, infrared absorption, and chemiluminescence.
Commonly used principles for airborne particles include optical techniques
that measure light scattering or absorption and piezoelectric resonance. Por-
table, direct-read, real-time instruments are widely used to measure CO
,
CO, and RSP in indoor air. Electronic instruments have a continuous output
of concentration readings, and as a consequence, concentration values can
be continuously recorded. An electronic direct-read sampling instrument for
CO
2
is illustrated in
Figure 9.1
.
Gas sampling tube systems are used to measure a variety of contami-
nants in industrial workplaces and, less commonly, IAQ/IE investigations.
They consist of a gas syringe or bellows into which a gas sampling tube
designed to detect and quantify a specific gas or gases is inserted. The
syringe/bellows is designed to draw a minimum sample volume of 0.1 L.
Larger volumes can be utilized by employing multiple pumping actions.
Gas sampling tubes are hermetically sealed prior to use. They contain a
granular sorbent such as silica gel, alumina, or pumice, impregnated with
one or more chemicals that react on contact with a specific contaminant or
contaminant group, producing a colored or stained substrate medium. The
length of the stain or colored sorbent is proportional to the concentration of
contaminants in the air volume sampled. The concentration is typically read
from a calibrated scale printed on the side of the tube. A gas sampling tube
system is illustrated in
Figure 9.2
.
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