Patent ID: 7780343

Claim:
A fluid concentration sensor comprising: a heater thermistor connected to a first electronic circuit; a reference thermistor connected to said first electronic circuit with said heater thermistor together for measuring heat dissipating rate of said heater thermistor under a constant temperature operation thus to deduce concentration of a fluid; and two temperature sensing thermistors connected to a second electronic circuit for measuring a flow speed of said fluid, and wherein said flow speed of said fluid is applied to a microcontroller to calculate and eliminate influence of said flow speed of said fluid on said heat dissipating rate of said heater thermistor in a dynamic flow situation since said heat dissipating rate of said heater thermistor is depended on both said flow speed and said concentration of said fluid; wherein said heater thermistor and said two temperature sensing thermistors are disposed on a thermally isolated suspending membrane which is extending over a cavity and formed by silicon bulk etching from backside of a silicon substrate, and wherein said reference thermistor is disposed on a non-membrane region of said silicon substrate to measure ambient temperature.