Page 461 - Fundamentals of Air Pollution 3E

I. Subjective Standards 409

Fig. 25-1. Ringelmann smoke chart.

practice prohibits the emission of smoke whose density is greater than
a specified Ringelmann number. Over recent decades, there has been a
progressive decrease in the Ringelmann number thus specified, culminating
in the specification of number 1 for large new steam power plants in the
United States. In addition to subjective observation of smoke density, con-
tinuous emission monitoring systems (CEMs) have been developed to mea-
sure objectively, by means of a photocell, the decrease in intensity of a
beam of light projected through the plume prior to its emission
(Fig. 25-2). Such systems are discussed in Chapter 32.
Subjective evaluation of odor emission is made difficult by the phenome-
non of odor fatigue, which means that after persons have been initially
subjected to an odor, they lose the ability to perceive the continued presence
of low concentrations of that odor. Therefore, all systems of subjective
odor evaluation rely on preventing olfactory fatigue by letting the observer
breathe odor-free air for a sufficient time prior to breathing the odorous
air and evaluating its odor content. Usually an activated charcoal bed is