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Re: ~ long fogging and AC (was: Re: 1st Snow in My A4 (1/2 long))

On Dec 11, 10:16am, marriott@Summa4.COM wrote:

> > Had I been allowed (by Hans und Klaus) to use the AC, the same
> > air would've been dry.
> If the outside temp was below ~40F and you were heating the air
> before you blew it into the car, the air _was_ dry.

No!  "Dry" is a very relative term here.  First, let's get rid
of the "heating the air" part, because though heating changes
the relative humidity, the absolute humitidy (i.e. the amount
of water in a given volume of air) isn't changed by heating.
What counts when worried about windshield fogging is the
relative humidity in the air, but not relative to the air's
temperature... relative to the *windshield* temp, which will
be close to the outside air temp!

OK.  Now, how about an outside air temp of 35F.  Dry?  What if
it's raining out?  That's 100% relative humidity.  Suck in some
air.  Call it 80% relative humidity.  At 35F, it takes *very*
little moisture to reach high relative humidities.  Now breath
out.  You just added moisture to the air, and since it doesn't
take much to affect the relative humidity at this low temp,
blammo, your windshield is fogged.

> > In every air-conditioned car that I ever owned - a Volvo, a
> > Nissan, an Isuzu and  my father's Oldsmobile - I could run the
> > AC at any temperature.

This is NOT your father's Oldsmobile... :-)

> I'll bet you could _select_ AC at any temperature, but the
> compressor wouldn't run at 5  F. That's the way the AC cars I've
> had (two non-CC Audis, Subaru, Mercury) worked.

My experience is that two Hondas, two Nissans, and a Toyota
would all let me engage the compressor (not just "select" it)
at low temps.

Now, will someone please find out what microcontroller is in
the climate control so we can chip it?!?

Dan Masi
'96 A4Q