I think we may tend to have the misconception that an undercover or a string cover "traps" air that has a different RH from the environment. I am not aware of any actual, rigorous research that has been done, but my own modest research suggests that this is not at all the case. The exchange of moisture in the air happens through either the loose weave of the undercover or through the tighter, thicker wool of the typical string cover essentially as if it weren't there at all.
(My research has consisted in simply placing a hygrometer within the cover, facing toward and positioned close to the soundboard, leaving it there for a period of weeks, and comparing its minimum and maximum readings with readings of the air in the room. This has been done on grands with full DC systems installed. I have found zero difference. I would welcome more rigorous research, using data loggers inside and outside to track more minutely).
What the cover does is reduce air movement. Since a Dampp-Chaser system is focused on creating a microenvironment within the inch or two just below the bottom of the soundboard, air motion that affects the cavity can upset that equilibrium. An undercover can reduce that problem (especially where there are outside doors being opened and closed constantly). With respect to a string cover, I think the major affect is to avoid motion of air at a different temperature from blowing across the strings, causing temporary pitch fluctuation (which will be greatest on the bass strings because of the copper windings, and thus will make the piano out of tune with itself as well as changing pitch).
I don't believe a string cover keeps in moisture from below the board, because I don't think enough moisture actually migrates through the board to create a significantly different microclimate above the soundboard. In any case, all the measuring I have done (on pianos with full systems whose lid is always down, placing the sensor tip of my hygrometer through plate holes toward the tail) I have never measured a higher RH than that of the room.
Note that I live in a dry climate, with 5% - 60% fluctuation, so my experimental data is skewed accordingly.
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Fred Sturm
University of New Mexico
fssturm@unm.eduhttp://fredsturm.nethttp://www.artoftuning.com"We either make ourselves happy or miserable. The amount of work is the same." - Carlos Casteneda
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Original Message:
Sent: 03-18-2017 10:51
From: Peter Stevenson
Subject: To cover or not to cover
I have not tried using string covers, as I do not live in an area that has high-moisture problems. However, I definitely see an marked improvement in the effectiveness of grand pianos that have an undercover, so much so that all of my installations now regularly include the undercover.
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Peter Stevenson RPT
P.S. Piano Service
Prince George BC
250-562-5358
ps@pspianoservice.com
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Original Message:
Sent: 03-18-2017 09:10
From: Ruth Zeiner
Subject: To cover or not to cover
I should clarify what I just wrote about speeding up the exchange
between air and wood. I meant the exchange of moisture.
Ruth Zeiner