DeLuga (1995) explains that:
The most direct method of room static pressure control is to sense the static pressure between the laboratory and the adjacent area, and control the ventilation airflow of the two areas to create the desired static pressure difference. The laboratory room controller measures the static pressure of the laboratory with respect to the adjacent area by means of the differential pressure sensor located in the wall between the laboratory and the adjacent area. The room controller modulates the supply air damper to maintain the static pressure at setpoint. Aside from maintaining control the static pressure of the room, the room controller must also ensure that the laboratory is always being ventilated at a minimum number of air changes per hour (ACH). This requires that the room controller always maintain control over the total room exhaust cfm. If the laboratory has variable air volume (VAV), the individual fume hood controllers continue to vary the total amount of air leaving their respective fume hood to maintain required fume hood face velocity. These variations in the fume hood exhausts will affect the total room exhaust cfm. As a result, the room controller must constantly monitor the total room exhaust cfm and modulate the room general exhaust damper to ensure that the required minimum air change per hour rate is maintained.
Each individual fume hood controller provides the room controller with its respective fumehood exhaust cfm value. The room controller adds all of the fume hood exhaust cfm values together and also adds the room general exhaust airflow rate to calculate the total room-exhaust cfm. As fume hoods begin exhausting more air, thus increasing the total room exhaust, the room controller must close off the room general exhaust air damper to reduce the amount of air taken from the room by the general exhaust. As the fume hoods exhaust less air, the room controller must modulate the room general exhaust air damper open to increase the amount of air taken from the room by the general exhaust.
A specific amount of supply air in relation to the total room exhaust cfm is simultaneously required to makeup for the total room-exhaust and also provide control over the room's static pressure. As the fume hood sashes are opened, increasing the total room exhaust cfm beyond the amount of air needed to maintain the minimum ACH, the room controller must increase the amount of supply air to makeup for the increased total room exhaust cfm. There may be times, particularly during the cooling season, when the heat gain of the laboratory will require more conditioned supply air than that needed to just provide the makeup air. In these instances, the room controller will increase the amount of supply air and maintain control over the room static pressure by also opening the room general exhaust damper as necessary.
Note that at any time someone in the room may reposition a fume hood sash and thus cause a change in the total room exhaust cfm. As this happens, the room controller will modulate the room general exhaust damper and the supply air damper to compensate for the increase or decrease in the fume hood exhaust. This action is intended to maintain the minimum required room ventilation rate and the room's static pressure level.
