Wood will change dimensions with its moisture content. It will swell in high humidity and shrink in low humidity. The different type
of woods used in the manufacture of pianos need a humidity between 42% and 45% to
produce stable wood moisture content (WMC) of 8% to 12%. This is a WMC that is
ideal for what jobs each wooden part is designed for.
Relative Humidity
Many people don't realize is that the ‘measurement of
moisture in the air’ is called "Relative Humidity" because it is “related”
to temperature. When temperature changes, so does the humidity. A change
of 15 degrees Fahrenheit can either double your relative humidity, or cut it in
half.
Because humidity control systems are an investment in your
piano, I like to see evidence of the need for a system before I
recommend one. Each time I tune a piano, I record the temperature and humidity.
This way I have a history and can use these readings to help you make a
decision that fits your needs. By rule of thumb, I start leaning towards a
system when I see a change in humidity of 10% over the year. This is the point
where nearly everybody knows a piano is out of tune.
Why?
In the Portland area I have measured humidity as low as 23%
(where the glue joints on piano parts are breaking from the shrinkage) and 78%
(the piano is stuck and nothing moves).
You may have already invested thousands of dollars in a fine
piano, a humidity control system costs very little in comparison and will
protect it from damage for the rest of its life and help keep it in tune.
Considering the damage that can occur to your piano
from humidity changes, a humidity control system is the best insurance policy
for your piano.
More information regarding humidity control systems
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