[CAUT] FW: Temperature/Humidity range

[Fred Sturm]

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