Temperature Change affecting pitch

[Richard Brekne]

List... Since we are on about it with humidity.. I thought I might just send this
along to all interested. This should give you a pretty good idea about the woes
of Humidity problems. How they touch upon the instruments we work on is of course
only part of the story. Here is some of the rest of it... grin..

Found on the net.  Hotbot search engine... term... "Humidifiers"



Understanding Humidity

An on-line dictionary gives the following definitions:

Humidity: a moderate degree of wetness especially of the atmosphere.

Absolute Humidity (AH): the amount of water vapor present in a unit volume of
air.

Relative Humidity (RH): the ratio of the amount of water vapor actually present
in the air to the greatest amount possible at the same temperature .

If you are having problems with high or low humidity you are not alone.

Let's start with how the atmosphere gets, "a moderate degree of wetness". Two
hydrogen atoms join with one oxygen atom to form one molecule of water (H2O).
The atomic weight of the (H2O) molecule is much less than the atomic weight of
Gasses that form our atmosphere, mostly Nitrogen, so the (H2O) molecules float
freely in our atmosphere.

That's right water molecules are lighter than air. Remember (H2O)   has three
forms Gas, Liquid (Water) and Solid (Ice). We are discussing the Gaseous form of
(H2O). If you set a dish of water outside the water disappears. It evaporates
into the air. Look up and you will see clouds. They are water vapor floating in
air. We
refer to (H2O) as water because that is the form in which we use it to survive.
The term Humidity does not apply to water, nor to ice, it applies only to (H2O)
vapor
or gas.

Nature or more correctly the laws of physics, imposes limits on how much (H2O)
gas the atmosphere can hold. When air is heated it expands. The molecules move
further apart making it less dense. This space between the normal atmospheric
molecules may now be occupied by (H2O) molecules.

As the atmosphere (Air) cools it can no longer hold as many molecules. Finally it
gets to the point it squeezes out the excess molecules in the form of Fog or
Water
(condensation).

The ability of warm air to hold more (H2O) than cool air is the basis for the
simplest measurement of (H2O) in the atmosphere. This measurement is called
Relative
Humidity (RH). It is based on the amount of moisture in the atmosphere compared
(relative) to the amount of moisture the atmosphere can hold at it's present
temperature. This measurement differs from Absolute Humidity, which is the actual
amount of (H2O) in a given volume, such as a cubic foot. Relative Humidity is
expressed in Percent ie 50%RH, while Absolute Humidity is expressed as a volume
ie .375 grams per cubic foot.

Molecules of air are bouncing around in all directions at all times. They bounce
off each other and any objects they come in contact with. We here at Wizard's
view
are concerned only with controlling these factors in enclosed areas, so we have
left out any discussion of altitude. Atmospheric pressure does affect humidity
but in
an enclosed area the interaction of air molecules and (H2O) molecules create
their own pressures. The molecular movement that naturally occurs will cause the
RH in
any enclosed space to eventually equalize.

Naturally the equalization time will depend on how and where the (H2O) molecules
are being introduced or removed. Circulation devices such as fans may be used
to speed up the equalization process. For example in a room 10 feet wide by 40
feet long if you add (H2O) molecules at one end it will take much longer to
equalize
the entire volume than it would if the air were being circulated by a fan. If the
(H2O) molecular percentage is kept constant by adding or removing (H2O) molecules

as required every item in the enclosed area will also equalize. A museum storing
precious art treasures must maintain a constant temperature and humidity level.
The
size of the museum necessitates  the use of circulation equipment. The art
treasures are kept at the proper temperature and humidity not in spite of the
added
circulation but because of it.

Moving air can add or remove humidity. For example a home maker hangs wet clothes
on the line to dry. The clothes are wet or 100% RH. The wind (moving air) is
at a lower humidity and the sun is heating the clothes and the air around them.
Then quite  naturally occurrence the (H2O) molecules leave the clothes and float
away
in the wind, until the RH is exactly the same as the air around them. If the air
around the clothes is 50% RH the clothes will absorb the same amount of moisture
they
would in a box with no circulation at 50% RH.

Evaporation or the addition of (H2O) molecules to the air lowers the temperature
of the entire mass or atmosphere. Evaporative cooling systems work on that
principle. If you spray water onto a substance that absorbs it, such as a sponge,
and blow air across it the temperature of the air will be lowered and the RH will
rise.



If you would like more information on this subject Email us at
wvi@wizardsview.com and we will provide you with more in depth explanations of
the actions and
effects of the (H2O) molecule.



--
Richard Brekne
Associate PTG, N.P.T.F.
Bergen, Norway