Air-flow tracking provides quick response to changes in exhaust air-flow rate resulting from fume hood sash movement. Air-flow tracking can provide more stable operations under certain conditions such as momentary door opening or transient readings by the pressure sensor, as described above. However, air-flow tracking control does not maintain specific differential pressure and therefore cannot guarantee that a laboratory's air pressure is always negative with respect to adjacent areas. The differential flow is determined using a rule of thumb and is established during the testing and balancing procedure because it cannot be determined beforehand. Consequently, the construction integrity of the laboratory directly affects the chances of attaining the specified differential flow. In addition, because the actual air-flow velocity desired (usually 50 to 100 fpm) is not known, the rule-of-thumb differential flow value (typically 200 cfm) may be excessive and waste energy. [Coogan, 1996] [DeLuga, 1995]
