ABSTRACT
In many applications in air sampling, it is important to maintain a constant flow rate for a period of time. This chapter discusses the operation of air movers and several types of flow control devices, such as critical orifices, electronic velocity or mass flow sensors, and differential pressure flow controllers. Each orifice has a unique and nonadjustable critical flow rate. The air sampling personnel must select a large-enough diameter orifice to provide the approximate flow rate desired. Differential pressure flow controllers allow the desired flow rate to be set by the operator by means of an adjustable valve in the flow path, usually downstream of the sample collector. Clearly, a thorough understanding of the sampling objectives, regulatory guidelines, and the head losses and required flow rates of the sampling apparatus is prerequisite to the selection of an air mover. The basic categories from which to choose are: positive displacement pumps, centrifugal pumps, evacuated canisters, and liquid displacement flasks.
