Working of the AFM Meter as I understand it! Why it measures volume flow rate.

["Neil Deshpande" <Ndeshp@xxxxxxxxxxxx>]

First, I should say that the AFM does measure volume and since I knew this =
to be true I decided to try and reverse understand how this was true =
rather than forward prove it was not.  In that sense I've been down Mr. =
Kiborts' road first and Mr. Conforti's road second and can understand the =
frustration on both sides!

As I understand it, here is how the AFM works:

When an orifice is placed in an airflow a pressure drop is created across =
it. =20

In the case of the AFM it is this pressure drop across the flap that =
deflects the flap against the spring.  The flap blocking the flow route is =
what creates the orifice.

A greater flow would mean a greater pressure drop and a greater deflection =
of the flap.

I know that a deflection of the flap would result in an enlargening of the =
orifice and a resultant reduction in the pressure drop, but at some point =
they would reach an equilibrium unique-for-that-flow-rate value.  The =
meter can, therefore, be calibrated.

As a practical design aside, I should mention that the sensor is (physicall=
y and electrically) designed so that the relation between the output and =
the flow rate is logarithmic.  No doubt Jim C. will be able to tell us =
why.

The reason variable orifice meters are used instead of fixed orifice ones =
is that they can accurately handle a greater ratio of minimum to maximum.  =
Fixed ones handle about 10:1.  The ratios in cars can be 100:1.  For those =
who like numbers the absolute value of max. flow in an automobile lies =
between 400 and 1000 kg/h as per the Bosch Automotive Handbook, 4/e.

Please resist the temptation to send me private flame mail!  I've thought =
this through and am still thinking without e-mail prods!

Sincerely,

Neil Deshpande
Flow Meter from 86 325 lying on kitchen counter for last 6 months!
1988 325 iS
1989 325 iC

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