Environmental Engineering Reference
In-Depth Information
Figure 9.2
Gas sampling tube system.
Gas sampling tubes are commercially available for a large number (100+)
of gases and concentration ranges. Their accuracy and precision varies. They
are designed in most cases to provide an accuracy within ±25% of the true
value with similar precision. Gas sampling tube systems are attractive for
conducting sampling in indoor and other environments. They are relatively
inexpensive, simple to use, and provide quasi-real-time sampling results.
Unfortunately, they have significant limitations. These include, in many
cases, relatively poor accuracy and precision, lack of specificity, high LODs,
and limited shelf-life (1+ year). In addition the length of the color-stained
substrate tends to be indeterminate (no sharp demarcation) and concentra-
tions may be difficult to read. These limitations vary for the contaminants
being sampled. Despite these limitations, gas sampling tube use is common
in IAQ investigations. Best results have been reported for measurements of
CO and CO
.
2
Passive sampling techniques are widely used in
measuring indoor contaminants. They are particularly useful for determining
concentrations integrated or averaged over a period of hours, a week, or even
months. In most cases, they are based on the principle of collecting contami-
nants by diffusion onto or into a sorbent medium. In passive samplers used
for gas-phase substances, the sampling rate, and therefore volume of air that
comes into contact with the collecting medium, can be determined from the
cross-sectional area of the sampling face, the length/depth through the sampler
that gases must travel to be collected on absorbing surfaces, and the diffusion
constant of the gas being collected. The Palmes diffusion tube (
Figure 9.3
)
was
the first passive device to be developed and used. Palmes tubes are still com-
monly used to measure NO
b.
Passive sampling.
levels in buildings. They have a sampling rate of
55 ml/hr, and concentrations are usually integrated over a period of 7 days.
Since the development of passive samplers in the 1970s, a number of
new devices using the same sampling principles have been developed. These
2
