THERMALLY_BASED_ANEMOMETER（AIR_FLOWMETER）

Published:2009/7/1 0:32:00 Author:May | From:SeekIC

This design used to measure air or gas flow works by measunng the energy required to maintain a heated resistance wlre at constant temperature. The positive temperature coefficient of a small lamp,in combination with its ready availability,makes it a goodsensor,A type 328 lamp is modified for this circuit by removlng its glass envelope,Thelamp IS placed h a bridge whichis monitored by A1. A1's output iS current amplifiedby Q1 and fed back to drive the bridge,When power IS applied,the lamp IS at a lowresistance and Q1's emitter tries to come fun on．As current flows through the lamp,its temperature quickly rises,forcing its resistance to increase.This action increasesA1's negative input potential．Q1's emltter voltage decreases and the circuit finds a stableoperating point.To keep the bridge balanced,A1 acts to force the lamp's resistance,hence its temperature,constant,The 20 k － 2 k bridge values have been chosen so that the lamp operates just below the incandescence point.To use this circuit,place the lamp in the air flow so that its filament is at a 90°angle to the flow.Next,either shut off the air flow or shield the lamp from it and adjust the zero flow potentiometer for a circuit output of 0 V.Then,expose the lamp to air flow of 1000 feet/minute and trim the full flow potentiometer for 10 Ⅴ output.Repeat these adjustments until both points are fixed.With this procedure completed,the air flowmeter iS accurate within 3％over the entire 0－1000 foot/minute range.



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