Flow sensing apparatus

A flow sensing apparatus having increased sensitivity is disclosed for sensing the flow of gas, particularly through a confined passage such as the ducts in heating, ventilating and air conditioning systems. The flow sensing apparatus comprises a tube divided by a wall into an upstream chamber and a downstream chamber. The upstream chamber can have a constant orifice but preferably the walls of the tube in the upstream end are tapered to form a tapered chamber section. A total pressure sensing probe is located in the center of the tube and has its opening in the upstream chamber of the tube. The total sensing probe has its opening parallel to the flow of gas and facing upstream, the opening of the total pressure sensing probe preferably being in the wall dividing the tube into the upstream and downstream chambers. A static pressure sensing probe is located in the downstream chamber. The static pressure probe may be located in the walls of the tube and have its opening perpendicular to the flow of gas. Alternatively the static pressure probe may be located in the wall dividing the tube into chambers with its opening parallel to the flow of gas and facing downstream. The sensing probes may be connected to a measuring device that provides a readout of the sensed pressures or pressure differential or to a control unit that is responsive to the sensed pressures or pressure differential.