FIGURE 4-3 Schematic illustrating the essential elements associated with measurement of emissions from agricultural sources that can be characterized as ground-level area sources such as dairies, cattle feed yards, field operations, and agricultural burning.

Point Sources

In theory, measurement of the emission rates of gaseous substances from a mechanically ventilated animal facility requires only the concentration of the substance being emitted and the ventilation rate, but accurate measurement of these two factors is difficult in practice. Ventilation rate is affected by many factors including the length of time the fans operate, fan design, fan speed, fan maintenance, motor startup time, static pressure, outside wind speed, wind direction, and infiltration. In practice, measurement of the concentration of an emitted substance is often difficult because of frequently changing ventilation rates. For example, in negative-pressure ventilated facilities, fans do not usually operate continuously, but rather cycle on and off for short periods of time. These short bursts of ventilation (followed by little or no ventilation) are necessary to create sufficient negative pressure to bring in air through the inlets at a proper speed to promote air mixing. If fans operated continuously, animals might become chilled or excess fuel would have to be expended to warm the buildings. Concentration measurement often becomes more difficult at high ventilation rates because substances may be diluted and be present only in very low concentrations. Since ventilation rates can be very high during warm weather and/or with large animals that give off large quantities of heat, even small absolute errors in the measurement of the concentration of an emitted substance can result in significant errors

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