zato kaj alfe ne koriste takav debimetar, već na vrući film....
A evo nešto malo i o vručem filmu iz jednog engleskog stručnog časopisa.
Hot Wire
Unlike air flow meters, hot wire or hot film mass air flow sensors directly measure air mass, as the convective qualities of air are affected by factors like temperature, humidity and density. Hot wire sensors create an analog signal and hot film sensors a digital frequency signal.
Most common hot-wire MAF sensors use a platinum wire or filament heated to a prescribed, maintained temperature above ambient, located centrally in the incoming air stream. These sensors function on the electrical principle that resistance increases with temperature. As intake air moves past the wire or film, the cooling effect causes a measurable drop in resistance, and thus lower voltage is required to maintain the prescribed temperature. The hot-wire MAF control unit is sent a reference voltage of 5V, and returns around .4V to.5V at idle and from 4.5V to 5V at full throttle. Based on a fixed data set, an accurate assumption of air mass is made.
Readings can stray from the target values as a result of contamination of the wire and for the sake of accuracy, an additional input from an integrated intake temperature sensor is sometimes used. Many hot wire MAFs incorporate a burn off cycle when the ignition is switched off, heating the element to over 1800-degrees F to clear it of contamination. Hot wire sensors are the most physically delicate and easily contaminated of all the air flow and mass sensors.
Hot Film
Hot-film MAF sensors function much like a hot wire sensor, and used a centrally-heated film or metallic grid-type element. One side of the film encounters cooling airflow, while the shielded backside maintains a consistent temperature, and the current differential between the two is measured and relayed as a square-wave digital frequency output, between around 30Hz at idle and 150Hz at wide-open throttle. Hot film sensors tend to be more robust and less susceptible to contamination than hot-wire types.
Mass Air Flow Sensor Contamination:
Contamination of hot-wire type sensors does occur. Usual suspects include substances like silicone potting compound, dirt, oil and spider webs.
Potting compound, used in the manufacture of the sensors to environmentally seal them can migrate onto the wire.
Oil most often enters a MAF in the form of vapor via an engine’s PCV system. In an effort to lower emissions, positive crankcase ventilation systems use a PCV valve to draw fuel and oil vapors from the crankcase, this can allow blow-by gases to make it past the piston rings, and reintroduces them into the intake system upstream of air flow metering devices. When a throttle body or airflow meter needs cleaning, the oily film that is removed is largely a product of this system.
