Environmental Engineering FOURTH EDITION

380 ENVIRONMENTAL ENGINEERING

Chopper

Reference cell

a

Infrared source

0 6 Sample in

Sample cell

+Sample out

0 0

00 00

Diaphragm

U

Recorder

Figure 19-6. Nondispersive infrared spectrophotometer for CO measurement.

energy by the CO causes the gas in the detector cells to expand, but the detector under
the reference cell will receive more energy (since the reference gas did not absorb)
and expand, moving the diaphragm separating the two detector cells. The diaphragm
movement is detected electronically, and the continuously transmitted signal is read
out on a recorder.

REFERENCE METHODS

Many methods are available for measuring air pollutant concentration, and new, more
rapid, and more accurate and precise methods are always being developed. The U.S.
Environmental Protection Agency (EPA) has designated a series of reference methods
with which results from all other methods can be compared. Although the EPA standard
reference methods may not always be the most sensitive, they have been standardized
and have a history of independently duplicated results. Some reference methods change
with each annual edition of the Code of Federal Regulations. The reader is encouraged
to access the EPA webpage at http://www.epa.gov/ttn/emc/ for the most recent standard
measuring and monitoring methods. The existence of reference methods is especially