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I've been pondering this one for a long time and finally found a some data to show what differences exist between the moisture content in the air at two different temperatures.

Because I can't afford to heat my shop all winter long, I turn the heat on Saturday morning and work out there during the weekends. Then during the week the temp gets back down to 30 or cooler. With a ventless nat. gas wall heater, my temp will not get to 70 unless I crank it for a very long time. At temps cooler than 70, my humidity hangs in the 70-90% range, which frightened me. However, as the temp rises, the humidity goes down (which is to be expected.) What I didn't realize was that in order to maintain the same moisture content in the air and everything else in the shop, at 30 degrees f, I need to keep the humidity at 90% in order to hold the same moisture as 25% and 70 degrees.

Interesting data.
I thought it would be good to share and see what others think of this, especially if my interpretation of the data is completely wrong.

http://www.engineeringtoolbox.com/water-vapor-air-d_854.html

We've always said that we need to keep the humidity up during the winter (at least we do in Minnesota.) I just didn't realize just how much this is really true.

Dave

Here, in Montreal... the weather goes from 30° in summer to -25° in winter, and this is not the extremes. During the winter, we heat our houses a lot, of course. Heating the air, whitout addind anyter moisture in the air, makes the RH% go doooown, dooown... In my appartment, the RH% is around 60-70% is summer, and go down to almost 20% in winter.

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Francis Richer, Montréal
Les Guitares F&M Guitars

I have seen these charts and I never get it, they must be talking controlled scientific environments, because I know I have seen ( talking about the weathere here) in florida 90 degrees with 86% humidity , and in las vegas 114 with 15% humidity . these charts do not take into consideration the weather and various fronts that are pushed around by other fronts.
what I do know is here in new york ( most of the time ) in the summer I am removing moisture from the air, and in the winter I am adding moisture, to keep a constant average of about 45% . Jody

Jody... Amount of moisture changes a lot. The capacity of moisture that the air can take will vary according to the temperature.
Let suppose...

There's X moisture in the air, and that represent, let say 50% of RH (that means the air now contents 50% of the amount of moisture it can takes).

Then you keep the same amount of moisture, but boost the temperature (put some heating in a house, best example). The amount will stay the same, but the RELATIVE humidity will drop in %, because hot air has a bigger capacity of moisture.

My english is bad.. hope that's clear.
Francis

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Francis Richer, Montréal
Les Guitares F&M Guitars

RH is what matters.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

It's physics and not lab tricks. I'll give you two situations. You can have 100% RH air in Alaska at -30f and 100% RH air in Miami at 97f. Which one has more moisture in the air? The air in Miami has loads more energy and it takes much more water to saturate it than the very cold air from Alaska. If you take a big balloon of that air from Alaska and heat it up to 97 without adding any water will it be 100% RH? No way Jose! It will be <.1%RH. Now change locations to Las Vegas 97f, 10% RH and Miami 97f, 100% RH. Same temperature but not the same amout of water vapor or the same amout of energy.

Relative Humidity is "relative". It's the actual amount of water vapor in a gas compared or related to the maximum amount (Dewpoint) the gas can hold at that temperature. The more energy in a gas, the more water vapor that gas can hold. As the energy falls, the water vapor molecules start coming together and falling out as condensation.

The problem you can get into trying to add humidity to a 70 degree shop and then allowing it to cool is rust on your Cast Iron and Steel tools. The air will eventually exceed its dewpoint and condensation forms as the temperature drops.

For woodworking, as Howard said, Relative Humidity is what matters. Mostly. Temp has an effect on the equilibrium moisture content (EMC) of wood, but it's a pretty small effect compared to RH.

E.g., using the chart from the OP's link:
At an RH of 50% and Temp of 70F, there are 44 grains of water vapor per lb of dry air.
For the approx same 44 grains, you need 50F and 80% RH, or 90F and 20% RH. Big differences in RH to get the same amount of water vapor in the air.

But for guitar building, EMC is what matters to control shrinkage and dimensions, and EMC in wood is mostly a function of RH.
See Table 4-2 at this link: http://www.fpl.fs.fed.us/documnts/fplgt ... ter_04.pdf
Say we're at 40F and 40% RH: this gives an EMC of 7.9%. For the same 7.9% EMC, we could be at 100F and 45% RH, or 130F and 50% RH. I.e., EMC is pretty tightly correlated to RH even over large temp changes.

Or, say we keep RH at 45% and just vary temp:
degF__EMC
30____8.7
60____8.6
90____8.1
So, keeping RH stable, temp can go from 30F to 90F and EMC only changes a little, and most of that change happened towards the 90F end.

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David Malicky