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The flow rate is detected by the MEMS thermal mass flow sensor.The sensor chip, produced in MEMSIC proprietary CMOS compatible technology, is composed of a central heater source (micro heater) and two temperature sensors (thermopiles), which are placed symmetrically upstream and downstream of the micro-heater.If no gas flows over the sensor surface, the symmetric thermopiles measure the same rise in temperature, resulting in the same output voltage of the two thermopiles.If a non-zero gas flows from the inlet to the outlet of the meter,the velocity of a fully-developed laminar air flow unbalances the temperature profile around the heater and heat is transferred from upstream thermopiles to the downstream thermopiles, causing a change in the voltages of the thermopiles.Larger gas flow rates result in larger asymmetry in the temperature profile.Precision analog circuitry in the ASIC converts the temperature difference to a differential analog voltage at the output pins.