During these unscheduled inspections, we measure average face velocity and conduct a smoke test to determine if air flow appears adequate.
Fume hoods protect workers by capturing, containing and exhausting potentially hazardous vapors.
A face velocity below 60 fpm is unsatisfactory since the hood may not have enough draw to pull contaminants away from the worker. The face velocity for a constant volume fume hood is dependent on the sash height. At the maximum sash opening, the face velocity is the lowest. In contrast, the VAV maintains a constant average face velocity independent of the sash height. However, above a certain sash height, the exhaust fan may not be capable of drawing enough air to maintain a constant velocity.