Fluid flow sensing and control method and apparatus

Abstract

A gas mass flow sensor or probe ( ) of an air flow sensing and control system ( ) based upon hot-wire type devices where flow sensing is achieved by controlling the difference between two temperature sensing elements ( and ) suspended in the flow stream ( ). The first element ( ) is used to measure the ambient temperature of the fluid flow and the second element ( ) is maintained at a programmed temperature ( ) above ambient by a current-fed heater ( ). The mass flow rate density is determined from heater current ( ) required to maintain the temperature difference and the total flow in the duct is inferred. To measure the flow in which the elements ( ) are submerged, electronic circuitry monitors the two temperature elements ( ) and controls the amount of current through the heater ( ) such that there will always be a predetermined difference between the ambient and heated temperatures. The sensor provides an output signal ( ) for use by other devices ( ). The output in this case is a current signal proportional to the mass flow and scaled to the input range of a utilization device. The sensor circuit ( ) shuts off power to the heater ( ) to prevent overheating in the case of exposure to operating temperatures in excess of a predetermined maximum. These functions are all performed at a location which is closely adjacent to a duct ( ) in which the fluid flows.