Many Fume Hood VAV Systems also have a significant time lag. VAV dampers adjust and regulate the exhaust volumes based on the sash opening, which determine the volume of incoming air. As the sash opening increases, the damper allows a larger volume of air to flow through it. However, commonly used VAV systems have a lag time before the damper opens to the required position. If the air flow rate is not re-established within 2 to 3 seconds after the sash is opened, fugitive contaminants may escape from the fume hood, leading to a loss of containment. VAV Systems that deliver near real-time responses are relatively more expensive, and are hence not commonly used.
Given the dynamic nature of both systems, susceptibility of sensors and the constant shifts in differential pressure, the labs can very quickly move out of balance leading to a loss of containment from the fume hoods.