Description of the operation

The air flow meter is the "heated film" type; the operating principle is based on a heated diaphragm which is placed in a measuring channel through which the intake air which enters the engine flows. The film diaphragm is kept at a constant temperature (~ 120°C higher than the temperature of the air) by the heating resistance which is in contact with it.

The mass of air which passes through the measuring channel tends to remove heat from the diaphragm therefore, in order to keep it at a constant temperature, a certain current should flow through the heating resistance; this current is measured by a suitable Wheatstone bridge.
The current measured is therefore proportional to the mass of air flowing through.

1. Connector
2. Measuring channel
3. Hot film sensor

A= Air intake

NOTE This flow meter directly measures the mas of air (and not the volume) thereby eliminating problems of temperature, altitude, pressure, etc.

Description of the operation

The Wheatstone bridge (made up of R3, R2, Rs, Rt+R1) is in equilibrium when Rs is at around 120°C higher than the temperature of the air.

The air which passes through the diaphragm removes heat from Rs, therefore the bridge is not in equilibrium. This situation is detected by the circuit at IC1, which operates transistor T1 in a manner which is proportional to the imbalance of the bridge with the result that more current passes through Rh to heat up Rs and the equilibrium of the bridge is restored.

Circuit IC2 measures the current which passes through Rh. This current makes it possible to maintain the equilibrium of the bridge and is consequently proportional to the mass of air which passes through the air flow meter.

Air flow meter wiring diagram