The MAF "wires" (thin film resistors) heat the incoming air a fixed number of degrees (it has to know the incoming air temp to do that). A Wheatstone bridge measures the power (BTU/unit time) required to do that. Knowing the heat input (BTU/min), the delta T (deg-F) and the specific heat of air (BTU/LB/deg-F), the sensor calculates the mass air flow rate (LB/min) - some constants required to make the units of measurement compatible.

But, its only heating/measuring the small sample of air that is actually contacting the wires. The ratio of the sample to the total flow through the sensor is a function of the cross-sectional area, and the velocity distribution across that area. That's why the screen is there - to promote uniform, repeatable flow velocity distribution across the full area. The ratio of the sample heated air to the total air is part of the meter calibration.

There is no "IAT" output function in the LT1 MAF sensor, its only used for the internal calculations.

 

I am not sure I understand what you mean it heats up a sample of the air x degrees. I always figured it kept just track of the power required to maintain a temp differential between the iat temp and the maf wire temp. I would have always thought they would just come up with non dim numbers and use correlations to develope the calibration as much as possible in order to keep the response of the system fast so they could run a cheaper processor. I could just be talking out of my rear though I am crap at EE stuff, and have always just assumed thats how it was done from what I know about thermal measurements, heat transfer, and fluid mechanics.

I know you're an EE and all but where is this particular info coming from, I would not mind reading it if its available.

 

I'm not an EE.... ME. I can give you an excellent reference on automotive electronics:

http://www.amazon.com/exec/obidos/AS...0target=_blank

 

I always thought you were EE, guess that was another fella with a similar name. I finish my ME stuff up in spring. Me and electrical work do not get along well, I can handle basic circuits, wiring, and analysis but when it comes to anything more complicated I am a total waste. I appreciate the suggestion though, have one ordered.

 

Funny.... I only got 4 C's in 6 years of college, and two of them were in "electricity".

BS is in Mechanical, MS in Management Engineering, which is more "industrial" related. I did process plant design for the first ten years out of school, and picked up a lot of the intrumentation and control knowledge. It helped when they decided to operate car engines with a computer......

 

I am big into thermodynamic processes, and materials science (high temperature operation), and I can design complex mechanisms very quickly and efficiently. The job offers I have had are for industrial two stroke diesel design, and brayton cycles for cooling use. I think when I graduate i'll take night classes on the employers tab and get my civil degree as well it'd take me about a year. I'm one of those insufferable guys who has to know everything down to the finest detail if i'm interested.

 

I feel alot of engineering love...

 

this is how I thought air flow sensors work:
"The MAF has a thin wire that heats up. The air rushing past it cools it. The computer measures how much current it takes to keep the wire at a constant temperature. Some MAFs have a temp sensor. Once the computer knows the outside temp and the "wind chill" it can determine the density of the air. It uses air density to calculate how much fuel to deliver"

 

The computer doesn't do that. Its all done internal to the MAF sensor. What you have described is typical of a hot-wire anemometer, used by HVAC mechanics to determine air velocity. If you follow the description I wrote, there is no need for an air density calculation, as long as you measure heat input, and delta T.

 

I meant actual core/base air flow measurement works like this: The MAF has a thin wire that heats up. The air rushing past it cools it. The computer measures how much current it takes to keep the wire at a constant temperature, this current is used for the base/core airflow value.

Your describing the MAF is heating the incoming air a fixed number of degrees and using that current to determine air flow.

Either way it's basically the same theory and all the other stuff I dont have a clue about but I do know everything else is used to apply the real world offests to make the core measurement accurate. We have $5K airflow meters at work that use the same "heated wire/measure current" principle.

 

I think everybody here has the same basic understanding of the MAF, but some are using different terminology than others which perhaps confuses things.

What you just described, is the MAF calibration table in the PCM: frequency -> g/sec, magically.

The MAF itself just measures a voltage and then outputs a signal with a frequency which represents that voltage.