Abstract: Gas flow control units in cabinets are connected to distribute process gas, on demand, to a plurality of utilization locations known as "tool" locations in a semi-conductor manufacturing plant. Each gas flow control cabinet has an exhaust duct which connects to an exhaust duct in the plant through which waste gases from the cabinet are exhausted by a fan to the atmosphere. Low flow alarm signals are generated if the exhaust flow is below a desired level. A programmable controller is provided for controlling the alarm function according to the size of the exhaust ducting from the plant so as to facilitate installation and operation of each cabinet in a variety of locations with different duct sizes.