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Relative Humidity? Is there an Absolute?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=27349		 Page 1 of 1
Author:  runamuck [ Tue May 11, 2010 5:44 pm ]
Post subject:  Relative Humidity? Is there an Absolute?
Humidity readings seems to always be given relative to temperature. We all know that
the warmer it is the more moisture the air can hold.

But let's say it's 75 degrees today and the rel. hum. is 40%. And then yesterday it was 60 degrees
with the rel. hum. also at 40%.

It seems that there is a difference in the actual moisture content yesterday then the moisture
content today even though the relative humidity is the same. But I'm wondering this:

What does the wood, and its expansion and contraction properties know about relative humidity?
Author:  Andy Birko [ Wed May 12, 2010 7:57 am ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
Since no one's answered after 70+ views....

Wood only sees relative humidity. It doesn't care about absolute.
Author:  Frank Cousins [ Wed May 12, 2010 8:24 am ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
I would imagine there have been no answers because I suspect its complicated :shock: eg I would imagine that at different temperatures the wood expands and contracts as most materials - does its ability to absorb mositure also depend thus on temperature and well as relative humidity? :shock: we need a :ugeek: to let us have teh definitive answer - afterall they say 72 and 42%Rh is good as a constant... but if your temp is 80 or 60 can you adjust teh RH to keep the moisyture content in the wood constant? and if so how do you calculate it? :shock: :geek: ;)
Author:  runamuck [ Wed May 12, 2010 8:54 am ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
Andy Birko wrote:
Since no one's answered after 70+ views....

Wood only sees relative humidity. It doesn't care about absolute.



Can you explain how you know that?
Author:  Andy Birko [ Wed May 12, 2010 8:57 am ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
I don't have any cites on this but let's be logical about this.

The reason we care about RH is because of dimentional stability. If we look at the original and arguably most accurate hygrometers, they used a natural hair from a human or an animal. The length of the hair changes solely due to changes in RH, not due to temp. e.g. if the hair is 1" long at 60% RH @ 90o, it's going to be 1" long at 60% RH @ 40o. Even if this isn't entirely true and there is separate effect due to absolute humidity is pretty darn insignificant. I see no reason to think that wood behaves any differently.

I think that the fact that all the charts, graphs etc. only use RH as an input are a strong clue that RH is what maters and that temp and pressure are "worked into the mix" through RH.
Author:  SteveCourtright [ Wed May 12, 2010 9:03 am ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
http://en.wikipedia.org/wiki/Equilibrium_moisture_content
Author:  Andy Birko [ Wed May 12, 2010 9:18 am ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
SteveCourtright wrote:
http://en.wikipedia.org/wiki/Equilibrium_moisture_content



That's great but did you read the equation? I can't seem to cut and paste right now but everywhere you see a "T" there's also a x 10E-2 all the way to x10E-7!! Temp is not that important.

For all practical purposes - RH is the only thing that matters.
Author:  npalen [ Wed May 12, 2010 9:59 am ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
I have a hygrometer (cheapy) hanging where the forced air from the AC hits it directly. The RH reading increases dramatically as soon as the cold AC air hits it. Seems logical in that the cold air doesn't hold as much moisture but the absolute humidity hasn't changed materially in that short amount of time.
I too have wondered how the wood sees humidity at different temps.
Nelson
Author:  JohnAbercrombie [ Wed May 12, 2010 12:12 pm ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
The 'quick answer' (refer to that link above) is that the water 'in' the wood is in equilibrium with the water vapor in the air.
Once you understand the idea of this equilibrium, it all seems much simpler.
The RH is a measure of how much of the 'maximum possible' water vapor is actually in the air. That's the only thing you really have to be concerned about, if you are looking at simple wood storage.

For drying wood, operating wood kilns, etc. it gets more complicated. ("Daddy, how can they dry wood with steam?" [uncle] )

Cheers
John
Author:  SimonF [ Wed May 12, 2010 1:16 pm ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
All you need to be concerned about is RH. The RH determines whether the wood will lose water or gain water. At a low temperature the air will not be able to hold as much water vapor. But even though the air has less water in it, that doesn't mean the air will want to draw more moisture from the wood. That is why Relative Humidity is important. It really helps to have a background in Physics and Chemistry to really understand the nuances of the subject; however, a little bit of online reading about meteorology will probably give you all the information you need to have a good enough understanding of the subject.

But the bottom line is that relative humidity is all that matters.
Author:  Colin North [ Wed May 12, 2010 2:23 pm ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
runamuck wrote:
Andy Birko wrote:
Since no one's answered after 70+ views....

Wood only sees relative humidity. It doesn't care about absolute.



Can you explain how you know that?


Thoughts/musings out loud-
First, there are no dumb questions, only dumb answers. ( Not that I am suggesting that there are any of those here)
RH is just that, relative humidity, ie relative to the (absolute) maximum moisture air can hold at that temperature (already said).
The term relative must compare to a known.
It can be confusing, eg absolute temperature, ie is temperature elevation relative to absolute zero.
Absolute humidity - well, there is such a term and it is is the quantity of water in a particular volume of air.
See http://www.humidity-calculator.com/ for a calculator if you want to know what that value is, eg 45% HR at 1 atmosphere and 72 degrees Fahrenheit is 8.88702.
But still RH is what affects/concerns us in guitar making.
This is of course, as said above, due to the tendency towards equilibrium (stability of a system in this case) of wood's moisture content, (causing swelling and contraction of the wood) with this relative water vapour concentration in the air surrounding it.
So while the absolute humidity does not change with temperature, the relative humidity does, due to air's capacity to hold more or less water vapour.
Sorry folk's just trying to get it clear in my own head.
Author:  JohnAbercrombie [ Wed May 12, 2010 2:36 pm ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
And, thinking about this stuff leads inevitably to the conclusion that it is much better to have lots and lots of wood piled in your shop to help stabilize the RH! (This rationale may work better with your spouse than mine...)
laughing6-hehe

John
Author:  Colin North [ Wed May 12, 2010 2:39 pm ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
Not neccessarily!! gaah
Author:  Fred Tellier [ Wed May 12, 2010 2:54 pm ]
Post subject:  Re: Relative Humidity? Is there an Absolute?
If you really want to add to the complexity consider Dew Point which described herehttp://en.wikipedia.org/wiki/Dew_point We used this for absolute humidity reading in the air supplied to the Spray Booths in the Auto Plant where I was a controls electrician before retirement. All of the measurements from the very expensive sensors we had coupled to the PLC computers were in dew point that were then translated to RH in the display screens for human viewing but moisture addition or removal was controlled in dew point temperature. If you think Luthiers are anal about humidity try automated painting with water born paint or powder coats. We used to have to run the chillers so cold to remove the moisture in hot humid days that we had to reheat the supply air back to the required temperature. Gas burners and chillers both running, the bean counters used to go nuts at the cost.

Fred
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