Alfa Romeo 159 Training Material page 142

Fiat Auto S.p.A.
ENGINE
ALFA 159 TRAINING MATERIAL
After Sales Training
The Wheatstone bridge in addition to being used to measure resistance is specifically employed as a measuring
circuit for resistive sensors.
A thermo resistor whose values are known is employed instead of an unknown resistance (Rx). The resistance of the
thermo resistor varies with the temperature and causes imbalance of the bridge.
There is therefore a proportional link between voltage imbalance, unknown resistance and temperature.
The first resistive bridge (P1), with a thermo resistor, controls and stabilises the heater temperature (R1) of the
sensor (hot film heating).
The second resistive bridge (P2), with two thermo resistors exposed to the air flow, detects and converts the voltage
air flow variation also defining the direction of flow.
TEMPERATURE SIGNAL
The temperature is measured by a NTC in analogue air mass flow meters. The characteristic is shown above.
The measurement of a digital air mass meter is a constant frequency duty cycle modified PWM signal.
The working voltage is 5 volts. The measuring range is from -50 ºC to +150 ºC. The duty cycle is from to 10% al 90%.
AIR FLOW RATE SIGNAL
HFM5
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