I'm missing something...

[Jon Snoddy]

You are close, but doesn't the "flow as volume/time" definition assume
something less compressible than air, like a liquid?  With a liquid, your
argument holds :a glass of water at sea level has roughly the same volume as
a glass of water on a mountain.  So if you were drawing water through the
air flow meter, the density wouldn't change much and the measurement would
be pretty much the same at sea level as on a mountain.  But if you replace
the uncompressible water with a compressible gas, doesn't something have to
change?  (remember that "gas law thing" from high school physics).  Drawing
a gas at low pressure through that spring loaded door will have a different
effect than drawing that same gas through the same spring loaded door, under
a higher pressure...I think.

-jon

-----Original Message-----
From: owner-diy_efi at diy-efi.org [mailto:owner-diy_efi at diy-efi.org]On Behalf
Of rob files
Sent: Monday, November 12, 2001 1:07 PM
To: diy_efi at diy-efi.org
Subject: RE: I'm missing something...

Hi Mike,

It is called an air flow meter for a reason...it can only measure air
*flow*.  The units of flow are volume per time....so the AFM can tell the
ECU, "I am measuring 20 cubic meters of air/hour."  Now you can see why you
would need a density correction for this, 20 m^3 is a volume and depending
on temp and pressure, you could have a varying amount of oxygen molecules
(which is all the ECU wants to know).  It's very likely that the vehicle you
were riding in does not do continous atmospheric correction and that is why
the a/f ratio went out of wack with the increase in elevation.

BTW, Mass Air Flow meters (the hotwire type) measures the mass of air
directly without the need for density correction.  MAP sensors can only
provide the manifold pressure which the ECU uses along with density
correction, RPM and some constant VE value to calculate the mass of oxygen
entering the engine.

That is how I understand tthings anyway
-Rob

>From: "Diehl, Jeffrey" <jdiehl at sandia.gov>
>Reply-To: diy_efi at diy-efi.org
>To: "'NickG '" <nikog at mediaone.net>,        "'diy_efi at diy-efi.org '"
><diy_efi at diy-efi.org>
>Subject: RE: I'm missing something...
>Date: Mon, 12 Nov 2001 01:41:52 -0700
>
>I'm not trying to be arguementitive, but I really want to understand this.
>
>My understanding was that the mass of the incoming air was what forced the
>flapper door open.  If this is so, then it should compensate for density,
>right?
>
>If that isn't how it works, then is it vacuum which opens the door?
>
>Any comments are more than welcome.
>
>Thanx,
>Mike Diehl.
>


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