Operating description

The air flow meter (Debimeter) is hot film type. In this system, a heated membrane is placed in a measurement channel through which air is taken into the engine.

The film membrane is maintained at a constant temperature (~ 120°C above air temperature) by an adjacent heating coil.

The mass of air flowing through the measurement channel tends to remove heat from the membrane. To keep the membrane at constant temperature, a certain current must therefore flow through the heating coil. This current is measured by means of a Wheatstone bridge.

The measured current is therefore proportional to the flowing air mass.

1. Connector
2. Measuring channel
3. Hot film sensor
4. A= Air intake

NOTE This debimeter measures air mass directly (not volume) and thus eliminates problems of temperature, altitude, pressure etc.

Operating description

The Wheatstone bridge (R3, R2, Rs, Rt+R1 format) is in equilibrium when Rs is about 120°C more than air temperature.

The air flowing through the membrane removes heat from Rs, and the bridge becomes imbalanced. This situation is deteced by the circuit connected to IC1, which controls transistor T1 to an extent proportional to bridge imbalance. T1 then sends more current through Rh to heat Rs and thus restore the bridge to equilibrium.

Circuit IC2 measures the current flowing through Rh. This current maintains the bridge in equilibrium and is thus proportional to the mass of air flowing through the air meter.

Wiring diagram of air flow meter