[CAUT] FW: Temperature/Humidity range

[Chris Solliday]

This is very clear mud, thanks Fred. Related to this, I have a client who
insists on setting his room humidifier by watching the condensation on his
window instead of the numbers on his hygrometer or my Dickson Data Logger
for that matter. He is usually low (lower than 42% at 68F anyway) and his
piano says so too. How can I best explain this to him, so he'll do it my
way? Please forgive further maieutic play but this is a serious question and
I'm sure others confront this situation as well.
Chris Solliday
----- Original Message ----- 
From: "Fred Sturm" <fssturm@unm.edu>
To: "caut" <caut@ptg.org>
Sent: Wednesday, January 18, 2006 8:41 PM
Subject: Re: [CAUT] FW: Temperature/Humidity range


> On 1/18/06 8:32 AM, "Chris Solliday" <solliday@ptd.net> wrote:
>
> > Thanks for the clarification Alan, the inverses challenge my dyslexia
too. I
> > should have said "should" not "will." So 70/40 is the same as 80/30 and
> > aprox 68/42? The link Alan sent is great, everyone should check it out
if
> > you find explaining this issue challenging. And what is absolute
humidity?
> > Anyone?
> > Chris Solliday
>
>     Probably the best "absolute" measure of humidity is dew point. It
tells
> you how much moisture is in the air in a fairly absolute way, though
> indirectly. It means the temperature at which the water vapor in the given
> air mass will condense/precipitate (form dew). You could take that air
mass
> and raise its temperature, or lower it, and the dew point would stay
> constant. The RH, OTOH, would drop when the temp rose, and would rise when
> the temp dropped.
>     Another way of looking at it, and this has practical applications, is
to
> take an object with a temperature lower than the air (like a glass of ice
> water) and observe the condensation on it. If that glass is precisely just
> below the dew point, condensation will begin. Raise its temp above dew
> point, and there won't be condensation. Apparently this is used in the
> humidistats for some HVAC systems with humidity control. They set the
system
> for a particular dew point, and they measure it with a mirror (polished
> metal) which is lowered to the dew point temp. When it clouds over, the
> misters are turned off. When it is clear (reflects light to a sensor), the
> misters go on.
>     But RH is what matters for us in terms of effect on pianos. The RH
more
> or less determines whether water will go from air to wood, or from wood to
> air (given a particular moisture content in the wood).
>     Clear as mud, right?
>
> Regards,
> Fred Sturm
> University of New Mexico
>
>
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