Actually, no, I am referring to freon type regular a/c units. What I am saying seems counter intuitive which is why I posted the warning. How does moisture condense out of the air? It is the dew point principle. You may have heard the evening weather man talk about the dew point temperature. For example, the current outdoor temperature may be 90 degrees F with a RH of 60%, the dew point temperature would be 74 degrees. What that means is, assuming barometric pressure remains constant, if the temperature would cool to 74 degrees the RH would approach 100% and water would begin to condense out of the air. This is exactly how dew forms on the grass overnight. http://en.wikipedia.org/wiki/Dew_point_temperature This is also how the a/c units pull moisture out of the air. As the air goes over the evaporator coils the temperature drops below the dew point (which means RH goes to 100%). For every degree below the dew point the air is cooled, water must condense out. So for our example of dew point at 74 degrees, the air coming off the coils may be around 40 degrees F. Air at 40 degrees and 100% RH won't hold as much water as air at 74 degrees and 100% RH. The difference is the amount of water that is condensed out. What is important to realize, though, is the RH of the air coming off the coils is still at or near 100%. This is the air that comes out of the vents, and if it is blowing on the piano, you are blowing really moist air on it (cold, but moist) and the piano will pick up moisture accordingly. I encountered 2 last week. The rooms were cold, air was blowing directly on the pianos, and they were 40 cents sharp at F3. Now, after the 100% RH of the 40 degree air from the vents warms up to your 75 degree room temperature, the corresponding RH drops to around 45%, and you can say, yes, the a/c is "drying" the room air, it is pulling humidity out of it. So if you encounter a piano that is in an air conditioned home but it is still going really sharp, take a look to see if the vent is too close, or the fins are pointed to allow the air to blow on the piano. Dean Dean May cell 812.239.3359 PianoRebuilders.com 812.235.5272 Terre Haute IN 47802 -----Original Message----- From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of Terry Farrell Sent: Friday, August 28, 2009 9:47 AM To: pianotech at ptg.org Subject: Re: [pianotech] Clarification Question: i'll take a pass I presume you are referring to what are called "swamp" AC units? Yeah, but the swamp AC units are not common at all. In fact I've only run into one of them so far down here. It was at a church and the piano was in the sanctuary with the swamp unit for years. They built a new sanctuary, moved the piano into it and wanted it tuned for service the next day. I did that and then was surprised to hear back from them a week later that the piano had gone WAY out of tune. I went there expecting it to be close to where I left it, but found that the darn thing had made a huge drop in pitch - it's been a while, and I forget exactly how much - but I think it was like a good 50 cents or so. I asked the guy about the nature of the old AC unit and he told me it was a swamp job. That was my first rude introduction to swamp AC units. A modern AC unit DOES remove water from the air. My shop will usually go up to about 50% RH if I'm not in there for a couple days - but then if I run my AC unit for a few house, the RH will drop to 40% or lower if I run it long enough. Terry Farrell On Aug 28, 2009, at 8:13 AM, Dean May wrote: > Be careful about thinking the a/c will keep RH down. Conditioned air > coming > off the A/C evaporator coils is at near 100 % RH. If that air is > blowing on > the piano, i.e., the piano is too close to a vent, then the piano is > going > to pick up significant moisture, even if A/C is on full blast and > the room > is cold. > > Dean > > Dean May cell 812.239.3359 > > PianoRebuilders.com 812.235.5272 > > Terre Haute IN 47802 > > > > -----Original Message----- > From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] > On Behalf > Of Terry Farrell > Sent: Tuesday, August 25, 2009 8:05 PM > To: pianotech at ptg.org > Subject: Re: [pianotech] Clarification Question: i'll take a pass > > Hey Wim - I think you and I experience similar evnironmental > conditions. I find that modern air-conditioned homes in Florida > provide a very stable environment for a piano - most folks don't hit > the heat much in the winter and the AC running most of the time in the > summer keeps the RH down - pretty darn stable environment. > > It makes me happy. :-) > > Terry Farrell
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