This design used to measure air or gas flow works by measuring the energy required to maintain a heated resistance wire at constant temperature. The positive temperature coefficient of a small lamp, in combination with its ready availability, makes it a good sensor. A type 328 lamp is modified for this circuit by removing its glass envelope. The lamp is placed in a bridge which is monitored by AI. AI's output is current amplified by Q1 and fed back to drive the bridge. When power is applied, the lamp is at a low resistance and Q1’s emitter tries to come full on. As current flows through the lamp, its temperature quickly rises, forcing its resistance to increase. This action increases its negative input potential. Q1's emitter voltage decreases and the circuit find a stable operating point. To keep the bridge balanced, Al 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 shields the lamp from it and adjusts the zero flow potentiometer for a circuit output of 0 Volt. 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 flow meter is accurate within 3% over the entire 0-1000 foot/minute range.

   Schematic

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