The basic concept of this system is as same as volume based, the only difference is: in Volume based the control is done by monitoring & measuring the volume flow of exhaust air(Schneider) whereas in Velocity based it is done by the face velocity of exhaust air(TEL).
The VAV controller AFA1000/E placed at each fume hood senses the face velocity of the fume hood with respect to sash position and actuates the actuator damper accordingly. The fresh air bleed controller AFA 1000/E/BLD measures the negative duct pressure values and actuates the bleed damper accordingly to maintain the capacity of constant speed exhaust blower. The room controller AFA 1000/E/RM measures the room differential pressure and varies the supply air flow to maintain the negative pressure inside the lab. The supply air is independently controlled by measuring differential room pressure only.