Yeah, that's right - I'm "That Guy". You know you love these threads.... I have an application where I need to be relatively sure of the responsiveness of an HVAC system to a control input. In a lab space that has an air change rate of 6 ACH when unoccupied and 8 ACH when occupied, the HVAC system must react quickly to an occupancy sensor noting someone entering the lab to avoid the possibility that the lab worker could walk in the door and open a container of chemicals and not have appropriate ventilation. 1 minute is a typical requirement for going from unoccupied to occupied mode. Personally, I think the whole exercise is a little silly, but that's neither here nor there.... So what is the actual responsiveness? When you walk in a door, an infrared or ultrasonic occupancy sensor will notice your presence in a fraction of a second and relay it to a computer. From there - on this system - a solenoid valve opens to send a pneumatic signal (a change from 10 to 20 psi of air) to each of 100 airflow control devices, up to 300 feet away(1). After the airflow control devices receive this signal, they change their valve position and thus their airflow in less than 1 second (by manufacturer specification). Then this new airflow travels down the duct, up to 100 feet and out a diffuser (2). The typical duct airflow velocity is about 500 fpm. The air then it enters a room at an average of 200 fpm, dropping to 50 fpm by the time it reaches a lab bencth 8' below (3). So: how long does it take from the time the person walks into the room until the new airflow is "felt" at the lab bench? The primary point of contention is #2, but lets go over all three of the questionable ones: 1. How long does it take for a pneumatic signal to propagate 300' down a tube? Since the air is pressurized, you're basically "filling" the system with air and the change in pressure will take a little while to propagate. Maybe 10 seconds, but it is really tough to know. 2. Since air flowing through a duct at low pressure (2" water gauge here) does not change its density significantly (a tiny fraction of a percent), it can therefore be considered incompressible. As a result, conservation of mass demands that the entire mass of air in the duct change its velocity at the same time. Since this isn't quite true, what actually happens is that the velocity change propagates as a longitudinal pressure wave through the duct, just like a water hammer effect. Longitudinal pressure waves have another name: "sound waves". So the changer in airflow traverses the 100' to the diffuser at the speed of sound, taking just under a second to change the airflow in the entire duct. 3. Once it leaves the duct, the air disperses and no longer propates changes at the speed of sound. Starting at 200fpm and ending at 50fpm, the average velocity will be around 125 fpm, traversing the distance in 3.8 seconds.....unless velocity drops as a square function of distance, in which case the average speed is a little higher and the time a little lower. Thoughts?