Based on the range of velocity heads, the environmental engineer selects a probe with a properly sized nozzle to maintain isokinetic sampling of particulate matter. As shown in Figure 5.11.7, a converging stream develops at the nozzle face if the sampling velocity is too high. Under this subisokinetic sampling condition, an excessive amount of lighter particles enters the probe. Because of the inertia effect, the heavier particles, especially those in the range of 3p, or greater, travel around the edge of the nozzle and are not collected. The result is a sample indicating an excessively high concentration of lighter particles, and the weight of the solid sample is in error on the low side.

Conversely, portions of the gas stream approaching at a higher velocity are deflected if the sampling velocity is below that of the flowing gas stream. Under this su-perisokinetic sampling condition, the lighter particles follow the deflected stream and are not collected, while the heavier particles, because of their inertia, continue into the probe. The result is a sample indicating a high concentration of heavier particles, and the weight of the solid sample is in error on the high side.

Isokinetic sampling requires precisely adjusting the sampling rate with the aid of the pitot tube manometer readings and nomographs such as APTD-0576 (Rom). If the pressure drop across the filter in the sampling unit becomes too high, making isokinetic sampling difficult to maintain, the filter can be replaced in the midst of a sample run.

Measuring the concentration of particulate matter requires a sampling time for each run of at least 60 min and

FIG. 5.11.7 Particle collection and sampling velocity.

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