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

```On Dec 11,  3:00pm, marriott@Summa4.COM wrote:
> Subject: Re: ~ long fogging and AC (was: Re: 1st Snow in My A4 (1/2
long))

> > 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.
>                       ^^^^^^
> Did you mean "given Masi" perhaps? ;-) That given volume of air
> will change by heating/cooling if the pressure remains constant.

Yes, but it's not important to this discussion.  Use air mass
in the above statement.

> The ability of air to hold water vapor is dependent on the air's
> temperature above 32 F/  0C _raised to the fourth power_. That
> relation's not a fundamental law, it's just a curve  fit of
> experimentally measured values. If you can heat that cm^3 up 30 or
> 40 degrees, it  will hold thousands of molecules of water.

Fine.  And as that cm^3 passes by a cm^2 of almost-freezing
windshield, what happens?

> A small change in temperature makes a _huge_ change in
> spongeability. That's why the relative humidity in your house is so
> low in the winter. Outside, maybe it's 40 F and  80%. By the time
> that same air comes in your house and gets heated to 70 F, the
> relative  humidity has gone down to 0.16%. Yes, less than 1%.

Exactly.  Look at it the other way.  Consider a cold, rainy
winter day, where the interior of your car is 60F at a balmy
50% RH.  Run that air by that chilly windshield.

> A clothes dryer works by heating both the air, to lower the RH, and
> the clothes. I  haven't seen one yet that worked with a condenser
to
> dehumidify the incoming air.

Wouldn't be too efficient, would it?  Of course, we're not talking
about drying clothes.  We're talking about keeping condensation from
forming on a chilled plate.  If there were such a plate inside a
clothes dryer, and it were critically important to keep said
plate fog-free, we'd surely see some different dryer designs.

> > > > 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.
>
> Your compressor would run at 5 F?

Not sure about exact temps, but they would all definitely,
without question, run below 32F.

Really, this can be argued academically in all kinds of ways.
But Jim, have you ever actually been in a car where you could
in fact turn on the compressor during a cold day when the windshield
was fogged?  Honestly, you would be absolutely astounded at the
immediate and thorough defogging result.  Maybe I haven't explained
it in principal, but the result is *VERY* definite.  That's why
those of us who've been doing this are screaming so loud now that
we have cars that won't allow us to.  It is MUCH more effective
than heating.

> > Now, will someone please find out what microcontroller is in
> > the climate control so we can chip it?!?
>
> 2.2 bar max without heater-core milling! And infinite O2 sensor
> life. ;-)

Careful with the wastegate adjustment, though; might end up
blowing out a window...

-dan

```