The pressure in a space can be maintained negative (or positive) with respect to a reference pressure by adjusting the volume of air supplied to and returned from the space. A differential-pressure sensor determines the pressure difference between two remote sensors, one in the pressurized zone and the other at the zone of reference. On a signal from the differential indicator, the outside-air and return-air dampers are adjusted until the pressure set-point is achieved.
If the pressure must be increased, more air is supplied to the space than is returned, by a signal to the damper motors. In the event a sudden pressure change sends a false signal, such as from a door opening, then a snubber is used in the pneumatic tube line from the static-pressure sensor. This is a sort of shock absorber that keeps the system from reacting drastically to every modulating short-term influence.
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Excerpt from Energy Conservation Projects.