August 21, 1997 It seems that the consideration at hand provides for 2 options, control the humidity levels surrounding the piano or control the humidity within the piano and of course, expect this treatment to migrate throughout the piano. The following provides several facts that deal with this. We have 3 room dehumidifiers that are in regular use to provide dehumidification to our test and research areas. We use controls that cost $1200 each to provide the precision required for these areas. The dehumidifiers have the following capabilities and power requirements: Pints of Water Removed each 24 hrs Amps 50 7.3 40 7.5 25 5.9 25 5.0 I've listed a 4th unit (the 5.9 amp one) that is used in the lower level of my house. I see its effects on a daily basis and monthly when the bill from Carolina Power & Light comes in. If you look at the operating cost of this unit you find the following: 5.9 amps x 120 watts/amp 708 watts 708 x 24 hrs 16990/1000 = 17 KW hrs/day 17 KW hrs/day x 30 days/month 510 KW hrs/mo x .09 $/KW hr $45.90 /month This is how much it costs me to run the 5.9 amp dehumidifier continuously throughout the month. Fortunately I have to do this only for the high humidity months of late spring, summer and early fall. I've provided the above calculation so that anyone can apply this to his or her particular situation and make a similar calculation. The goal of all technicians should be to stabilize the piano throughout each day of each week, each week of each month and each month of the entire year. You do this by bringing RH levels down during periods of high humidity, to humidity levels that you can bring the piano up to during periods of low humidity. For the sake of simplicity, assume that you have high humidity only in the summer and low humidity in the winter. If you can raise the environment to 42% in the winter, then you should try to get the summer humidity down to 42%. An air conditioner won't get it to 42% in the summer unless you live in a relatively arid part of the country. An air conditioner and a room dehumidifier generally won't get it to 42% either because what you are taking out is being replaced by the transfer of water through the walls, floor or ceiling. I won't get deeply into the mechanics of this, but it has to do with higher vapor pressure on the hot, humid side of the structure trying to equalize with the lower pressures on the cooler, dryer side of the structure. This holds true whether it's summer or winter. The struggle to equalize is constantly going on within your home if one side of the wall is cool and dry compared to the other side. In most climates winter brings the need to raise the relative humidity to the same level that you were able to attain in the summer when you lowered it. For the sake of this example, let's say you were able to get to 42% in the summer, then this is all you need to raise it to in the winter. Not a problem. Just bring in 2 good size room humidifiers and let them go to work. Unfortunately it's not as simple as this. Our old friend, the difference in vapor pressure between inside and outside, creates a very significant problem. Here the higher vapor pressure inside is struggling to equalize with the lower pressure outside. It does so within the wall itself. If the temperature within the wall is sufficiently low, the moisture moving from inside to outside condenses and freezes creating expansion pressure between the walls' components. Eventually, the frozen moisture thaws and begins to rot the wall components. Now this is a pretty slow process, but each time it occurs, it adds a little to the wall's deterioration. Aprilaire, makers of the Cadillac of furnace humidifiers, provides the following table for indoor humidification levels at various outside temperatures: Outside Temperature (oF) Recommended RH +40o 45% +30o 40% +20o 35% +10o 30% 0o 25% -10o 20% -20o 15% You can see from this that at outside temperatures below 30oF, humidity levels in the 40% area are not recommended. The conclusion I draw from all of this is that it just plain makes sense to control the humidity within the piano rather than trying to control the humidity surrounding the piano. Unlike David A. Vanderhoofven, I do have an axe to grind, however, if what I have said is not correct, I'm sure someone will let me know.
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