We have two Data Centers at our facility. One has the CRAC fan's controlled by the temperature sensor that controls the chilled water fan and the other center has the fans controlled by differential pressure between the under floor and the rack space. We are adding a third data center being designed by an outside A/E. They want to modulate the fan speed to maintain a zero differential airflow between the rack space and the ceiling plenum. I hear their explanation of it but do not fully understand it to the degree of understanding any potential disadvantages. Could you shed any light on the philosophy behind this mode of controlling the CRAC fans, any draw backs, and what do you see as the best method of controlling the CRAC fans.