ÆP and pseudo-ÆP control schemes have certain characteristics which the designer/owner needs to be aware of. A reasonable pressure differential to maintain using normal construction techniques is approximately 0.01” water gauge (w.g.) To put this into perspective, 0.01” w.g. = 0.00036 PSI. This is an extremely small pressure differential (signal) to measure and providing adequate calibration for the instrument is also difficult. The fluctuations (noise) in this signal, which are caused by the opening and closing of doors, people traffic, elevators, stack effects and atmospheric disturbances like wind, are on the order of 0.1” w.g. This represents a signal to noise ratio of approximately 1:10. Imagine trying to determine the level of a lake to within an inch when the waves are a foot high. To do so, it would be necessary to average out the wave crests and troughs. It can be done, but it takes time. If you want great accuracy you have to average over a long period of time. If you need to respond quickly to the signal then you can’t be as accurate. Accuracy and speed or response are in direct conflict. For true stability in a ÆP system, the response time is usually measured in minutes. Therefore, many of these systems and instruments sacrifice stability for speed and can oscillate about the setpoint for quite some time before settling down to stable control. Unfortunately, this settling down period is often greater than the frequency of upsets and the controlled device may oscillate all day long until everyone goes home. pseudo-ÆP systems which measure the air velocity are somewhat faster and more stable because the velocity signals and noise are proportional to the square root of the differential pressure. This improves the signal to noise ratio to approximately 1:3. This simple change in the measured variable improves the system performance by a factor Another undesirable characteristic of both of these pressure measuring devices is that the measured variables (pressure or velocity) totally disappear when the laboratory door is opened. Some controllers have the ability to freeze the output for a predetermined time delay to compensate for this. However, if the door is left open long enough, the pressure control system will start to shut down the supply volume in order to bring the space back to a negative setpoint. When this occurs, the air from the hallway flows into the open laboratory to replace the exhaust air and the hallway pressure may drop. Other lab pressure controls which use this hallway as a pressure reference may also start to close down the supply air to their labs thereby creating a cascade effect. As more air is drawn into the affected labs from the hallway, the pressure will continue to drop even more. As you can imagine, this can cause serious building pressure problems. However, for facilities not requiring critical room pressure control and where the effects of settling time and stability are not an issue from a hazard assessment standpoint and where the HVAC system and architectural designs can minimize the cascade failure effect mentioned above, this control system may be used with some success. In comparison, the signals measured using airflow stations by ÆV systems are on the order of 1000 feet per minute and the noise is on the order of 100 feet per minute resulting in a signal to noise ratio of approximately 10:1. This allows much more accurate, stable, and rapid response to changing inputs characteristic of a VAV