Not too scientifically minded, but interested in such things. Mind if I pontificate more? (I know, too much today . . .)
>From Answers.com:
"The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage."
At any given pressure and temperature, air has the capability of holding a set amount of moisture. This represents 100% RH, and this is a very tenuous percentage, as only a very slight drop in temperature will make the humidity start to come out of the air, as rain or dew.
If the air is made to be absolutely dry, it would be at 0%RH, no matter what the temperature of the air is. So anhydrous is anhydrous - totally dry is always totally dry, no matter what the temperature is. Air is never truly totally dry, but I suppose it gets close.
The warmer and more dense the air is, the more moisture (by weight) it can hold. So if you are at 10ºc outside and it is raining, so the humidity is close to 100%, you take that same air into the house and heat it to 22ºc and it will be, oh I am guessing, maybe 60%. Now none of the water has left the air - as a percentage of weight, the water content is exactly the same, but the air is now capable of holding much more water than before because it is warmer, so the relative humidity has gone down.
If you then take a shower and turn on all your humidifiers to max, you can bring the warm air up in humidity - maybe up to 80% at 22ºc is doable. Then you open up a window and put a fan blowing the air out the window, and stand outside and watch. You will see clouds form, and even micro-rain as the warm humid air cools back down to 10º, and the moisture has no choice but to fall out.
Interestingly, as the water vapor condenses into clouds or rain, it releases heat, warming the air a little. So the process of raining from clouds makes the clouds get warmer, raising the RH a little, allowing the storm to continue for a long time. Otherwise the water would all fall out in a big drenching rain and be done - kind of like thunderstorms in Florida.
At low temperatures, there is a fairly narrow range of water (by weight) that the air can hold. The humidity gauge will move really fast from 10% to 90% if any moisture is introduced into the air without changing the temperature. {I wonder - there must be some point where the air is so cold it cannot hold any moisture - would that be at freezing? Yet I know that it can snow at -15ºf, but that may just mean that it is warmer up in the clouds. There's a gap in my understanding that I need to fix}
I read that on Mars, the air is very cold and totally dry, and when there is exposed ice the water evaporates directly, and is lost into space because the air cannot hold it. Hmmm.
Cold temperatures outside always mean dry air inside. The more you add moisture to the air inside, the more it will get sucked outside and try to condense everywhere and cause problems. I remember visiting the University of Manitoba last winter, where the building is actually maintained at about 35% humidity even when it is way below zero (it was -24ºc when I was there). The pianos were in great shape, but the walls and windows had all kind of moisture damage from water condensing in the plaster and on the window sills. This is in a well constructed, concrete building with double-paned windows and all.
Getting back to pianos, there is a direct relationship between how much moisture is in a piece of wood and how much is in the air around it, of course. But EMC of wood is a different measurement than RH in air, and I find many technicians get this confused. Wood is a different material with it's own sort of RH curve, but EMC measurements give the percentage of moisture by weight, not as a percentage of how much moisture the wood is capable of holding at that temperature, like RH is. A fresh cut log is something like 200% moisture - meaning if you weigh the log, 1/3 of the weight will be wood and 2/3 will be water. As the wood sits around it looses moisture, and will eventually settle down to less than 20% EMC - and usually will have to crack along the length, across the grain, in order to relieve the tension from shrinking.
My little quick rough guideline I try to give piano dealers and schools for soundboard health is the following: if you hold a room at 22ºc and 25%, the Spruce in the soundboard will be dried to about 5% EMC (very roughly - it does vary by species and density). So when the humidity goes below 20% at comfortable room temperatures, then the soundboard is being dried down farther than the factory did originally, and the chance of cracks opening up is very high. Some soundboards can handle this better than others, but eventually they will seek relief from the shrinkage and will need to crack.
OK, enough. Maybe this will trigger someone with a more scientific perspective to respond and fill in my gaps.
Don
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From: caut-bounces at ptg.org [mailto:caut-bounces at ptg.org] On Behalf Of Jim Busby
Sent: Tuesday, December 09, 2008 9:30 AM
To: caut at ptg.org
Subject: [CAUT] RH? Please enlighten me.
List,
The recent post on relative humidity has me wondering what RH really means. Don Ms funny post about "death" with humidity being too low shows that many (myself included) don't understand RH.
For instance, when I heard that the RH was 100% in Louisiana I figured the whole state was under water.
And, obviously life can be sustained when RH registers at near zero, as is does occasionally in our music building. (Interestingly the static electricity in such low humidity makes it essential that you "discharge" the charge in your body before touching an ETD, or before pumping gas, as multiple fires at gas stations have shown.)
Any scientifically minded tech out there care to enlighten me?
Thanks.
Jim Busby
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