Official Luthiers Forum!

Ok in another post Someone made some comments to the effect...

"These values all represent the same absolute humidity (the water vapor that is present in the air. And they are all DRY!!"

Temp   RH
30c - 19%
20c - 34%
10c - 66%
5c - 93%

I spent all morning trying to put my head around this but can't. Oh I get the basic idea. (generalization follows) If at 40f the air can have a maximum moisture content of x(absolute humidity?) and at 80f it can have a maximum of 2x then it follows that 1/2x at 40f would be 50% RH and 1/2x at 80f would be 25% RH. We cool?

Ok what I dont get is how do I know that 20c at 34% is equal to 10c at 66%? Is there a chart? Is the relationship linear? What is the reference point? Why do we not just use absolute humidity? What is the average air speed of an unladen swallow?

Someone also said "A better way to keep tabs on how moist the air is is to watch dewpoint." Ok I'm feeling really dense... Dew point 40f and temp 40f equals 100% humidity? If so, what does this have to do with the price of tea in China? Say temp is 70f and dewpoint is 36f... I don't get it.

In the summers I have really low RH say 100f and 18% right now it is 61f at 53%. Normally this time of year 50f-60f at about 50%-60% is the range. I haven't made any attempt to control it other than to build between say 25%-40% and not build otherwise. I had considered some sort of control but now I'm not so sure it's gonna be so easy.

Intuition says with a swamp cooler in the summer to raise RH and I should be able to build all year, but I'd really like to know for sure how this works...

Any thoughts?









RCoates39092.119525463

Unless you are working in meteorology or air conditioner design, absolute
moisture content of the air means absolutely nothing, has no impact on
anything, and should be of no concern.

All you ever need to look at is the relative humidity. The rest is just numbers
that will have no impact on wood, finishing, dry lips, or anything else. If you
want to know more about it for the sake of pure curiosity then great, but
absolute moisture content, dewpoints, etc., need not be of much concern to
the woodworker.

When the relative humidity is low, add moisture to the air. When it's high you
take moisture out. That's about all there is to it.

_________________
Eschew obfuscation, espouse elucidation.

Ok, but relative humidity being relative I have to believe there is more to it than just "build at 45%RH" 45% at 30f and 45% at 70f and 45% at 112f are not the same.

Since I live on a God foresaken part of the planet that has temerature swings from 20f to 115f across any given year and I can't possibly afford to temperature control (70f) my garage (shop) all year I'm hoping that a firm understanding of temperature/humidity (RH) will allow my to build guitars that might not self destruct if shipped to another climate.

Maybe I'm overthinking this (i'm often guilty) but perhaps not. I do know that I don't know...

Ronn-
Unless you enjoy physics for the intellectual stimulation (I'd guess that you do) the details aren't necessary for building guitars.
Details at
http://en.wikipedia.org/wiki/Relative_humidity
and many other spots.
Basic idea: Air is a solution, and water vapour (gas form of water) is dissolved in the air. Temperature affects the solubility of water vapour in air. etc etc...
Remember that water in your wood stash is in equilibrium with the water vapour in the air, so the relative humidity will indicate the water level in your lumber, once it's reached equilibrium. Since the tonewoods are thin, equilibrium happens fairly quickly. 'Air Drying' lumber is the process of letting this equilibrium be reached....
Cheers
John

I mis-spoke...
Chemistry, not physics- but let's not fight about that!
John

[QUOTE=JohnAbercrombie] Remember that water in your wood stash is in equilibrium with the water vapour in the air, so the relative humidity will indicate the water level in your lumber, once it's reached equilibrium. John[/QUOTE]

Exactlly my point. Since RH is is the percentage of total possible moister in the air at a given (current) temperature and because warmer air can hold more moister, it stands to reason that my wood is going to hold substantially more moisture when RH is 45% and temp is 100f than it will when RH is 45% and temp is 40f.

So we all know that 45% at 70f is good, I'm just trying to figure out how that relates to the temp extremes I live through.

If the humidity is 45% at 100 degrees, or 45% at 20 below, the wood will
not be able to tell the difference and will remain perfectly stable. The total
amount of water in the air by itself makes no difference, and is usually
measured in grams of water per cubic meter of air. The only thing that
matters is the ratio of how much water is in the air relative to how much
it is capable of holding, ergo, relative humidity.

Example - Put a spruce top in a completely sealed box with 10 grams of
water per cubic meter of air (I'm making these numbers up for purpose of
example because I don't feel like working through psychrometric charts -
they won't be truly accurate but the main point is the same). If 10 g/m3
worked out to 50% relative humidity at 20 degree celcius then the wood
would be fine. Cool the sealed box to 5 degrees and the wood absorb
more moisture and swell, even though there is still the same 10 g/m3
water in the air. Now heat it to 75C and it will dry out and crack like mad,
even though there is still the same 10 g/m3 of water.

If I may personify Air, if the air is warmer, it is capable of holding more
water, it feels thirsty and sucks water from your wood. When it's cold it
can't hold enough, it feels and releases it's moisture to your wood, or as
dew on grass. So if your wood is acclimated to 45% at 40F and is moved
to 45% 100F condition it will still be in perfect equilibrium and will neither
absorb or give up any moisture at all.


edit: I reread what I wrote, and realized I probably didn't do the best job
clarifying the points you wanted. Trust me anyway - wood will remain at
the same moisture content whether the RH is 45% at 40f or 45% at 140f.
Maybe someone else out there can explain it better.


David Collins39091.9842592593

_________________
Eschew obfuscation, espouse elucidation.

Wow...GREAT explanation. That's what I was told: "45% humidity at 50f, or 455 at 90f will not make any difference to the wood.

Also, Ronn...this is a tired paragraph that I repeat from time to time but here goes: you DO NOT have to heat/cool/humidify your shop/garage/house/etc. You only have to heat/cool and humidify a very small space, meaning closet-sized in order to properly maintain your work pieces. Build a small, tiny closet of a room that you can keep relatively stable...say 65 - 85f, and 42-46% humidity and keep your current work pieces in there. That shouldn't be "too" difficult, no matter where you live. Then bring those pieces out into your shop area while you are working the pieces, and return then when you are done. The pieces, once stabilized to the proper humidity, won't gain/lose enough moister to fundamentally alter the piece, or pieces, you are working on. Give that some thought. It is very easy to control the temp/humidity in a 4 foot square room vs. a "shop", or something similar. I know a well respected builder who does this very thing...quite successfully.

_________________
Eagles may soar, but weasels don't get sucked into jet engines.

Bill, would it be best to keep the wood you're working with in a closet type space with just a lightbulb to keep the wood in the best of humidity conditions?

Yep, Ron is correct. The relative humidity is what is important. This
might seem counter-intuitive because you would think that the ACTUAL
"wetness" of the air is all that really matters.

As I am sure everybody knows that the higher the temperature the more
water will exists in the gas phase -- the cooler the temperature the
more it will shift to the liquid and eventually the solid state. This is the
physics behind why for the same amount of molecules in the air, the
relative humidity is not the same at different temperatures - because the
relative humidity is calculated in reference to the saturation point.

To put this another way, if you take air and heat it, you will lower the
relative humidity even tho there is still the same amount of water
molecules in the air. However, because the water vapor saturation point
has been elevated with the temperature change, evaporation can be more
severe. The result is that your skin dries out and more importantly, our
guitars can dry out.

That is why relative humidity is the important thing to worry about - not
the absolute humidity value in the air.

Peace Out,SimonF39092.0086458333

It's really the EMC (Equalibrium Moisture Content) of the wood that's important. This is the amount of free water in the wood and that seeks equalibrium to the surronding environment. As the RH changes the wood takes on or gives up water in order to stay in equalibrium. For example spruce in a RH of 45% has a EMC of about 8%, at a rh of 55% the emc is about 10%. So we control the EMC by the RH. EMC is effected a little by temperature but not much. Jim_W39092.0566435185

_________________
Jim Watts
http://jameswattsguitars.com

First off, Thnx to everyone. And thank you David for taking the time to offer a detailed response. your sealed box analogy makes great sense... to a point.

Simon hit the nail on the head. You see this is the trouble I'm having. I'm looking at this from a standpoint of "how wet is the air"? Perhaps that is where I'm going astray.

Looking at the Psychrometric Chart below... (rough numbers to follow). I see that at 50f and 50% RH you have .004 pounds of moisture per pounds of dry air (PM:PDA). At 110f and 50%RH you have .028 PM:PDA. A substantial increase (x7). So how does my guitar know how much moisture to suck up?

Allow me to clarify and lets put this in David's box. In the two examples above given the same RH, the actual moisture content of the enviroments are substantially different. So, again in the sealed box, what keeps our piece of spruce from being wetter in the .028 PM:PDA box than the other. It seems there would also be a PM:M3S (pounds of moisture per meter cubed of spruce) equation.

Now here I'm guessing (who am I kidding? I'm guessing through this whole thing), but is seems to me our spruce doesn't really care what the temperature is (within limits) so if you put it in a wetter enviroment it is going to take on more moisture. Like any desiccant our spruce will absorb moisture from the surrounding air and if it does not reach its saturation point it will come to equalibrium with the surrounding air. If this is true then that equalibrium point will result in a much higher moisture content for our spruce in a .028 PM:PDA inviroment than it will in a .008 PM:PDA enviroment. Higher moisture content equals swelling in our spruce. Houston we have a problem...

Now please don't take my comments as argumentative. I'm willing to be wrong here. Problem is I'm from Missouri. You need to show me.

Again Thanx to all

Ron, going back to my original comment about relative humidity being
important. My background is in biochemistry/chemistry and this
question really belongs in meterology.

So I am not absolutely certain but I think the following is correct. Going
back to my example of taking air and just raising the temperature and
thus lowering the relative humidity. Even tho the air still has the same
amount of actual water vapor, the relative humidity decreases. The result
is that the air acts as though it is drier. So even though the air has the
same amount of actual moisture in the air, your skin still dries out.

I think the problem you are really having is with your thinking that higher
moisture content in the air means more water in our wood - this is not
necessarily correct. In the field of thermodynamics (study of energy), we
know that just because a reaction or process can happen it doesn't mean
it will happen. From what I have briefly read about this particular topic is
that the relative humidity is a very good indicator of the air's ability to
transfer moisture to nearby objects. If the relative humidity is high, the
air is "wet" and will act wet. If the relative humidity is low, the air is "dry"
and will act as if it is dry.

Think of this scenario, you have a super-saturated (more than 100%)
moisture content in the air - which can happen under certain
circumstances. Due to certain governing principles (primarily entropy),
you would expect the air to really want to get rid of this extra moisture.
However, if you raise the temperature enough, this same environment
could have very low relative humidty. You would then expect that it
would want to take on moisture and not give it away. Hopefully, this
shows a different way of looking at this situation.

If I can re-emphasize the idea of air being wet and dry. The extent to
which air is wet or dry is just another way to state the relative humidity.
The most practical example of this I have already given. Despite having
the same actual moisture content, a low relative humidity will dry out
your skin while a high relative humidity will not. Put yet another way is,
"You need more water content in the air for a higher temperature to
prevent the air from drying out your skin." It seems to me that the effect
for skin and a lovely spruce soundboard would be the same.

The truth is too really understand the physics of this scenario, you need
to be intimately acquainted with entropy, concepts of Gibbs free energy
and chemical potential, mole fractions, phase chemistry, etc... There is a
lot of physical chemistry here, which happens to be one of the more
difficult fields for most folks. And honestly, a lot of the understanding
comes down to just knowing that a certain equation is going to need a
postive or negative answer. Which for me, is sometimes as satisfying (or
rather dis-satisfying) as saying, "just because". In other words,
sometimes the reasons are very straightforward and understandable and
the only way you can make sense of it is by relating it to something that
is understandable. Which kind of reminds me of some of my undergrad
classes that dealt with quantum mechanics, at some point you have to
loose a hands on approach and just realize that it is the math that needs
to make sense.

Peace Out,
SimonSimonF39092.1048958333

Simon thanks again that does go right along with a lot of stuff I'm reading. I would guess it's similar to what happens when taking a hot shower. The room size, water temp and surfaces present are the same summer or winter, but in the winter the colder temps alow the fogging of the mirror etc. Same moister content but different RH. Winter scenario reaching saturation or beyond. The only variable is air temp. and consequentely RH.

As to my original question/theory, I'm certain that I'm not the first guy to come to such conclusions so the fact that my hours searching have not turned up any corroborating information (at least none of any consequence) I must assume for now that temperature as it relates to moisture content and RH for luthiers is of little import. Our concern should indead focus solely on RH irrespective of temperature. This just as David Collins first said.

The validity of my theory I presented was infact bolstered by the search that followed Jim Watts post on EMC. I found that just a Jim said temperature AND RH do affect the EMC of wood... It's just that temperature doesn't affect it much at all. Again, just as Jim stated. I found that information all over the place but Here is a nice little PDF that illustrates it well, across a 60f spread. For reference I found other similar info elswhere.

So, the question for me is, why does temp at any given RH not affect the EMC of wood any more than it does as I first theorized? I don't know. If you do tell me. If not I'll keep my eyes open.

One last time thank you all. This has been fun.

Oh and Bill I do plan to seal off a "small" area maybe 100 sq' to climatize. That space has to include my workbench. I know I'm greedy.

I have to agree in simple terms RH% is a good general
gauge to indicate safe conditions for wood storage.
Or where a finished instrument lives. Somewhere between
40%-50% is fine for most circumstances. Temperature has
less to do with it. As Luthiers, the real number that
matters is the moisture content of our wood.

This chart may be of some help. It shows the moisture
content of wood. It is easy to see that 100 deg. of temperature change at the same RH will only change the
moisture content of wood a little over 1%
1% of moisture content change in a 15" spruce top, can
change the width by .020 (shrink or expand)No big deal.
13% of RH change = about 1% Moisture content change.


Now lets say your building guitars and you plan to ship to some other area. We need to think about the conditions the instrument lives in. As an example the average moisture content(MC) of wood objects in the home would be around 8%. Again this is an average.
That said, does it not make sence that if we build at
8% MC our guitars will live a happy life. The Big Boys
all know this and take the steps needed To control MC.
After all it is common that there builds are shipped worldwide. About 20 yrs ago Jean Larivee admitted he learned this the hard way. He has paid attention to MC
ever since.

Many here already know this also, for most of us maintaining idea conditions in the shop is not possible.
But as it was already said. we can keep our wood and builds in a closet, small room, cabinet that we control
to ideal conditions.

I have made my living as a woodworker for 35+ years.
So when it comes to this subject I can go on and on.
My brother said it best "Oh no he has his Tech Talk Tom
hat on again" I laughed and knew it was time to shut up

But if still want to discuss this,I think I still have that hat and you've been warned.

Tom

_________________
http://www.moriciguitar.com

Thnx, thats a better chart than the PDF I posted a link to. Yours shows exactly what I was refering to only over a 100f spread. At 45% RH about 1.5% delta in the EMC so not much to be concerned with it seems.

[QUOTE=Tom Morici] I have to agree in simple terms RH% is a good general
gauge to indicate safe conditions for wood storage.
Or where a finished instrument lives. Somewhere between
40%-50% is fine for most circumstances. Temperature has
less to do with it. As Luthiers, the real number that
matters is the moisture content of our wood.
[/QUOTE]


Interesting thread!!

One observation though. If I go into my general workshop area these January mornings, the rh is around 80-90% and the temperature is about 11°C. If I turn on the de-humidifier and leave it running I can maybe get it down to 70-80% rh. If instead I put on the heater and get the temperature up to around 20°C, the rh drops to around 45%.

So I understand that emc is responsive to rh for a given temperature, and not much responsive to temperature at a given rh. BUT if my problem is how to control the best environment to build and keep guitars, my solution here in January is to switch on the heater not switch on the de-humidifier.

rh responds pretty dramatically to temperature and the emc responds to rh, so in a way emc IS responsive to temperature when you remove the ceteris paribus of rh staying constant.

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Sorry if I steps on some sacred toes with my humidity comments. But..... The point remains:

When you are faced with a situation like Mattia is, that is, The temp outside is around 40 degrees and the RH is 88, don't despair. Do the math, if you warm up your shop up to 73 degrees the RH drops to 30%. Not because it is any drier, just hotter. So don't go out and get a dehumidifier... get a heater. If you don't want to do the math look at the dewpoint, (there he goes again). If it is under 50 degrees then the heater will do it. Over 50 degrees dewpoint and you'll need a heater and dehumidifier.

In Fact Dave White said it even better!!!! He's lived it

_________________
Joe Beaver
Maker of Sawdust

[QUOTE=Joe Beaver] Sorry if I steps on some sacred toes with my humidity comments. But..... The point remains:

[/QUOTE]

Wasn't trying to single anybody out. I just hoped for answers to what seemed to be a complex issue. Within the next year or two I plan to address the humidity monster in my shop. I was just trying to "know my enemy" (Sun Tzu).

Ronn, is that a european or an african swallow?

-j.

_________________
“If God dwells inside us like some people say, I sure hope He likes enchiladas, because that's what He's getting”
-jack handy

[QUOTE=j.Brown] Ronn, is that a european or an african swallow?

-j.[/QUOTE]

I don't know... AHHHHHHHHHHH!