Patent ID: 8746032

Claim:
An apparatus for measuring a rate of flow of a fluid in a primary conduit, the apparatus comprising: a first flow sensor operable to provide a first measure of the rate of flow in the primary conduit; a flow impedance in the primary conduit, the flow impedance causing a pressure drop in the primary conduit when the fluid is flowing; a bypass conduit extending from a point upstream of the flow impedance to another point downstream of the flow impedance so that the pressure drop in the primary conduit causes fluid to flow through the bypass conduit; and a thermal flow sensor comprising: at least one electrically heated temperature sensitive element disposed in the bypass conduit; a flow modulator operable to modulate flow rate in the bypass conduit to thereby cause the at least one temperature sensitive element to generate a cyclic signal indicative of heat removed therefrom by the flowing fluid; and thermal signal processing circuitry operable to receive the cyclic signal and to generate therefrom a second measure of the rate of flow in the primary conduit.