Operation

The hot film mass air flow sensor has a hot film sensor placed in the intake air flow. The resistors R, R1 and Rs as well as R2 and R3 are combined to form a bridge circuit. The heating resistor RH is installed outside the bridge. The heating current which is necessary to maintain the heating resistor at a constant temperature determines the bridge voltage. The heating resistor RH and the sensor resistor Rs are dependent on temperature, and their resistance is reduced with increasing temperature (NTC).

The sensor resistor Rs is fitted to the heating resistor RH and therefore adopts the temperature of the heating resistor. In the operating state the heating resistor RH is supplied with a current H that causes it to heat up to a temperature that constantly is 16°C/32°F above the instantaneous intake temperature. When the air flow rate increases, the heating resistor is cooled down, resulting in an increase in its electrical resistance. In order to maintain the temperature difference between the intake air and the heating resistor RH, the current H is regulated accordingly.

The temperature of the outlet air acts on the resistor R; the compensation resistor R1 is connected in series to it and compensates for the temperature differences of the bridge circuit across the whole operating range.

The servo amplifier Rv compares the current flow between the temperature resistor R and the sensor resistor Rs and influences the current flow lH to the heating resistor RH, in order to maintain a constant 16°C/32°F temperature differential between the intake air temperature and the temperature of the heating resistor RH. Regulation of this constant temperature differential is performed within in milliseconds.

A change in current is registered as a voltage drop at the measurement connection UA. This voltage drop is used by the DME control unit as the measuring para meter for the intake air volume. The hot film mass air flow has no moving parts and generates only minimum flow resistance in the intake passage.