Date:         Sun, 9 Mar 2003 00:30:39 -0500
Reply-To:     Marc Perdue <marcperdue@ADELPHIA.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Marc Perdue <marcperdue@ADELPHIA.NET>
Subject:      Re: window tint physics
Content-Type: text/plain; charset=us-ascii

Doug Demarest wrote:

> Marc,
>
> I've been enjoying reading the discussion of window
> tinting and transmittence of uv light, etc.  I
> disagree on a couple of things, one on principal only.
>  Light most definitely does not slow down. You can
> always count on light to go exactly the same
> speed--ie, the speed of light.
>

Yes, Bill N. already pointed out my error on light slowing down.  I
misspoke on that one.  It gives up its energy as heat.

>
> I suspect you are right that after some period of time
> (and 6 hours seems reasonable) the vans will be the
> same temperature.  But if two materially identical
> objects are the same temperature, it will require
> equal removal of heat (ie, AC power) to cool them.
>

The number of BTUs required to cool them both to the same temperature
would be the same in a STATIC situation.  However, the van without the
tinting would have a higher cooling load because it has a higher energy
input.  You're not just removing a fixed amount of heat, but that fixed
amount PLUS the net energy inflow.

>
> The useful experiment, I think, would be to test HOW
> LONG it takes each van to heat up.  If a tinted van
> takes 6 hours to heat to, say 130, but a non-tinted
> van hits 130 in 2 hours, that is a useful result.
>

I can see that.  It really would depend on what you're trying to
assess.  How long it takes the van to reach a given temperature would be
useful if you're concerned about how long you can leave your dog or
musical instrument in the van before it's likely to suffer brain damage
or delaminate, respectively.  On the other hand, I typically don't leave
animals or instruments in the van any length of time when there's that
possibility.  What I want to know when I get into my 120 degree F van is
how quickly is it going to cool down!  :^)

Thanks for the counterpoint.
Marc