Testing a MAF

[2seater]

This is somewhat related to the question of using a MAF for flow measurement, but is a different angle on it. I am not using the MAF as the flow element, but I am testing it on a pitot style flow bench. The MAF is mounted to the test hole and the frequency output is used to get the mass of the air flowing through it. I have the GM charts for grams per second vs frequency output, and there appears to be a substantial difference between flow rate in cfm vs the calculated weight of that same flow. I did find the mass of the air I was using for the conversion was too high, and this has moved the figures closer together but a difference remains. This brings up this question: since depression is not used as part of the calculation for air flow, should the density of the air used to calculate cfm vs grams be reduced to account for the lower atmosheric pressure inside the flow tube? I know the Excel formulas account for barometric pressure and temperature, converting them back to standard but I can't get around the lower pressure under the test hole makes the air less dense downstream of the mass measuring instrument? Does this make sense and if so, what should be done?

[75-1138604505]

2seater,
My thoughts on this is that it should be tested with a variable restriction to flow between the MAF and your bench. This would better simulate the OEM positioning of the MAF prior to the throttle body. Testing with the MAF in a depression I think would really alter the electrical signal. The resistance of any electrical device is temperature sensitive. Pressure or vacum will change the temperature of the electronics considerably.
Just my 2 cents
Kurt

[Tony]

If it is truly a MASS air flow sensor, air density (pressure), and temperature should already be fully compensated for.

But if it requires a whole lot of different external correction factors to be very carefully applied, then it is probably not going to be terribly accurate or repeatable.

[2seater]

I appreciate the input. It is a question that makes my brain hurt The MAF I am using for my base measurements is relatively small, so a fair amount of depression is required at higher flows, about 11.5"w.c. at 350 cfm on the bench. Acting on my hunch, I simply reduced the air density in the calculation to make the mass vs cfm match. It did show a fairly steady decrease in density needed as the flow and depression increased. 30.5 gr/ cu.ft. at 56 cfm and 1/2" depression and down to 24 gr/cu.ft. at the highest measured flow mentioned above. The MAF "should" be self compensating, the stock air intake tube is in place on the inlet, the stock ECM is powering it, and even the air intake temp. sensor is in place and connected to the ECM. If this conjecture about depression is true, the larger MAF I want to compare the stock one to should show a better result since it will not cause much restriction and hence depression at the flows I need. Thanks.