Airflow Measurement

During operation a heated platinum wire is subjected to the flow of intake air in the inner tube of the hot wire air mass meter (HLM). Heat is dissipated from hot wire by the flow of air. This heat is compensated by control of the heating current. The supplied current also flows via a precision resistor whose voltage drop represents a direct measure for the airflow drawn in. Air temperature fluctuations are detected by way of a compensation resistor and taking into account in the measurement. Approximately 5 seconds after switching off the engine a short current surge is applied to the hot wire in order to burn the wire clean of impurities.