Relative vs. Absolute Humidity

[DAMPPCHASR@aol.com]

To Avery Todd, thank you for thinking of Dampp-chaser relative to
harpsichords and humidity.  Let me add just a bit to what Gayle passed on
this morning and yes, please forward it to the harpsichord list.

RH goes up when the temperature drops.  There has to be some other factor
affecting the environment to make it go down.  The phenomina is that the air
has the capability to hold moisture at the reduced ( or dropping)
temperature.  A cloud forms or fog sets in for the most part not because of a
change in moisture content of the air but because the temperature has dropped
to dew point.  This is the temperature at which the air cannot continue to
hold the amount of water present and therefore starts to come out in the form
of visible water vapor.

I suspect that in Mr. Feeser's situation the hygrometer he was sing to
measure RH did not wrk very well.  Without going into a long disertation
about hygrometers, let me say that they become terribly inaccurate if not
"excercised" periodically.  Klaus Fenner does a very good job of explaining
this phenomina in the July 1993 ( I think) PT Journal.  If anyone would like
a copy of this please call me on 1-800-438-1524.

If Mr. Feeser's RH was 35% at 68 degrees F, the psychometric chart tells us
it would be 82% at 45 degrees F, and certainly this wouldn't be considered
"dry".  On the other hand, if his hygrometer was current, the 45 F/ 32% RH
would yield a very dry 15% RH at 68 degrees F as Bud Finc points out.

While temperature is important in itself and its effect on string length,
it's most  significant import comes from its influence on relative humidity.
What we have found in so far unpublished test results is that temperatures
effect on tuning is moderate and immediate, while humidity' s effect ( caused
by temperature change) is significantly more severe and begins to take place
several hours after the temperature change.

Far more information on hygrometers and psychrometers let me provide the
following.  Radio Shack handles a small electronic hygrometer that  sells for
about $25.00.  From what I've seen of this, it's reasonably accurate.  The
problem is long form reliability.  A film will develop on the sensor in about
a year and its accuracy gets into the questionable area.  Comfort Staation is
handled by Piano Climate Control at 800-443-7509 and PianoTek at 800-347-3854
for the same price or perhaps slightly less.
For long term reliability but somewhat harder to use, is a sling psychrometer
available from Abbeon Cal Inc. My choice is model # 10-140.  It was priced in
my somewhat old catalog for $41.90 with replacement wicks (socks) at $4.00
for 12.  This will provide good long term accuracy.  Either one will put you
in the ball park.

To get better than ball park will cost a lot of money and it simply isn't
worth it for general humidity measurements.

                                                     Thanks again,
                                                           Bob Mair