The combined laboratory pressure sensing and air-flow tracking control system continuously measures both total room exhaust and absolute pressure differential. A general laboratory exhaust damper and a supply damper modulate to maintain the required minimum air flow and pressure differential. The supply damper begins in a minimum position that provides cooling and heating. Variations in the fume hood sash location affect total laboratory exhaust. When all fume hoods are in their minimum position, the general exhaust is in its maximum open position. When the exhaust volume increases because a fume hood sash is raised, the general exhaust damper begins to close to maintain the required negative pressure differential. At some point, depending on the size of the laboratory and the number of hoods, the general exhaust will reach a fully closed position, and the supply damper will begin to open to maintain the required negative pressure differential. Temperature control comes from the operation of the heating/cooling coil's control valve.
During the cooling season, the laboratory heat gain may require more supply air to be provided than is needed just to make up the exhaust air. Accordingly, the pressure control system will open the supply damper to change the amount of supply air for cooling purposes. The pressure differential is maintained by opening the general exhaust air damper. There is a corresponding increase in total laboratory exhaust and, consequently, energy use. [Coogan, 1996] [Laboratory Control and Safety Solutions Application Guide ,1994]