Volumetric airflow indicator and control device

An automated system for controlling the volumetric rate of air delivered to a clean room, comprising:

• an air duct for supplying air to the clean room, the air duct having an inlet opening, and wherein the air duct includes a convergent-divergent portion with a throat region that defines a minimum cross-section area across the convergent-divergent portion, wherein air flow in a path from the inlet opening through the throat region;
• an L-shaped tube sensor positioned at a location substantially at the throat region and having an orifice facing the inlet opening, at least part of the tube sensor and the orifice intruding into the airflow path the tube sensor being operable to generate a first signal indicating total air pressure at that location;
• a pressure tap sensor positioned at a location substantially at the throat region, the pressure tap sensor being operable to generate a second signal indicating static air pressure at that location;
• a variable frequency drive connected to a fan operably positioned for driving air through the air duct, wherein the variable frequency drive controls the fan speed, thereby adjusting the air rate through the air duct;
• a programmable device connected to the tube sensor for receiving the first signal and to the pressure tap sensor for receiving the second signal, the programmable device being operably connected to the variable frequency drive such that the progranunable device is capable of adjusting the fan speed, and further the programmable device being operable to create a selected air rate through the air duct which generally corresponds to a set-point pressure differential between the first and second signals.