Re: MAF vs AFM my take

[Brian M Kennedy <kennedy@xxxxxx>]

Chris Guy wrote:
> 
> I think Jim C and Mark K. are both wrong on this one (though Jim is right
> 99% of the time).
> 
> Dyn Press = 1/2 * air density * velocity^2.
> Volume flow rate = Velocity * Area of duct.
> Mass flow rate= density * velocity * area
[snip]
> This is all theoretical and there is a _LOT_ of assumptions here, but it
> does show that in theory the flapper does not measure mass flow rate
> directly.

Well, I don't know if your formulae are right in this case, but even if they
are, your conclusions are wrong.  The velocity-squared term will dwarf the
density term in the dynamic pressure equation.  To get mass flow, you will
need either a separate velocity measure _or_ separate density measure (the
latter is what is typically used).


So, I can see no faults in Jim's post... other than perhaps he didn't derive
the answer from the base formulae, which most of the list probably doesn't 
care about.  However, your formulae do show how Mark got confused, how some
of his facts were correct, though his conclusions were wrong.  Unfortunately,
nothing can excuse his poor judgement in presenting his arguments so rudely,
or in dodging Richard's efforts at moderating, even though Richard chose to
take the tough route of moderating rather than the easy route of booting Mark.


I'd fear endless bickering in this thread, but don't have to in this case...
the poor digest won't be around that long.  Oh well,

Brian

P.S.  Richard, many thanks for hosting the digest to date.  It has been 
extremely informative for me, and usually enjoyable.  I'm sure most people
regret the heat you've gotten lately.  I hope you'll join most everybody on
the other lists -- your inputs are valued and welcome.  Although you took 
the beating, it is the list as a whole that needed to learn the lesson.
For Rob's sake, I hope we have.

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