Thermal fluid flow sensing method and apparatus for sensing flow over a wide range of flow rates

In a fluid flow sensor:

• heater means disposed in heat exchanging relation with the fluid in a channel for heating fluid flowing in the channel;
• first and second thermal sensor means disposed in heat exchanging relation with said heater means and with the flow of fluid in the channel;
• said first thermal sensor means being disposed upstream of said second thermal sensor means so that flow of fluid in said channel will cool said first thermal sensor means more than said second thermal sensor means;
• third thermal sensor means disposed in heat exchanging relation with the fluid in said channel and being substantially out of heat exchanging relation with said heater means for sensing the ambient temperature of the fluid in said channel;
• means employing the output of said third thermal sensor to control the temperature of said heater means to a predetermined elevated temperature relative to the ambient temperature of the fluid as sensed by said third thermal sensor means;
• means for comparing the output of said first and second thermal sensor means to derive an output representative of the rate of fluid flow in said channel for flow rates up to a first predetermined flow rate;
• and
• means for measuring an electrical quantity which is a function of the electrical power dissipated in said heater means to derive a second output representative of the rate of fluid flow in said channel for flow rates above said first flow rate.