MAF reading goes down instead of up with increasing rev - is that okay?

[onepound]

Using PSA-COM to monitor live readings MAF reading goes down instead of up with increasing rev - is that okay?

I have just replaced the MAF and the error code is gone but I dont understand this result

[mickthemaverick]

If the MAF is measuring pressure then it will reduce as the airflow speeds up but if it is measuring airspeed then that is not right. I've always assumed it measures the pressure but I could be way off track, however there are people on the forum who will know for sure and hopefully one of them will see your post and confirm or deny my thinking!!

[myglaren]

I'm fairly certain the the MAP measures the pressure while the MAF measures the airflow.

[ozvtr]

MAP; Manifold Absolute Pressure. Measures the pressure with respect to absolute 0 pressure. Measured in millibars. Just the same as the numbers on the weather maps. The air pressure on a fine warm day is about 1000 millibars. A stormy day measures about 900 millibars. FYI. The engine acts like a vacuum pump, sucking in air. With the throttle closed the "vacuum" in the inlet plenum increases and the measured pressure drops toward 0 pressure. As the throttle opens, more air enters and the pressure increases away from 0.
So an easy way to test these things (if you have a scanner) is to find out the barometric pressure of the day (from the weather bureau) and compare it to what the MAP sensor indicates (without the engine running). The MAP sensors also have a temperature sensor inside them, but I won't bore you with the whole Boyles law thing.

MAF; Mass Air-Flow sensor. Uses a number of ways to measure the volume of air entering the engine. It's measured in grams per second. At idle the measured amount should be about the same in g/s as the displacement of the engine in litres. So a 2 litre engine should give a result of about 2g/s at idle. These also tend to have a temperature sensor in them. Boyles law, blah, blah.
The volume of air entering the engine is proportional to engine revs. More revs, more air.

Onepound, what units are those results in?

[wheeler]

Yes the units are important here, most PSA stuff & Diagbox/Lexia measure in mg/cp (Milligrams per stroke)
Same principal as Oz says, a 2000cc 4 cylinder engine will give a reading of 500 mg/cp at idle (if the EGR is fully closed) This will lower when the EGR is opening.