Trying to understand M C

tomtrees · March 29, 2009, 03:02:52 AM

The house reads between 50 and 60 % Hummidty. The wood in my house reads about 15 % MC with an electronic meeter. Is there a relationship between the two and how can I calculate this. If I could only work out an ideal MC for the wood that is going into my house (I know there will be fluctuations between winter and summer). I get lost of diffrent people say diffrent thinks (some not very reliable) and I know the norm is less than 15 MC.

Any comments? Sorry for the spelling

Tomtrees

LeeB · March 29, 2009, 03:32:51 AM

I would say that if the wood in your house is at 15%, you would be in pretty good shape using wood in the 10 to 12% range coming in. I use air dried at around 10 to 12% and don't have problems as long as I remeber to design for any movement into my projects and my house is much drier than what you are quoting.

tomtrees · March 29, 2009, 04:07:19 AM

I should mention that the 50 to 60 % hummidity is air hummidity

Hi LeeB Any hints for air drying I think I woud have a job to get 10% mc but I may be wrong.

Tomtrees

Den Socling · March 29, 2009, 10:27:08 AM

To understand MC, you need to know about a fancy sounding term called Equilibrium Moisture Content (EMC). When wood reaches EMC, The number of water molecules leaving the wood is equal to the number of molecules returning to the wood. The result is equilibrium.

Equilibrium changes with temperature and humidity. You can find EMC tables in drying manuals or with a web search.

scsmith42 · March 29, 2009, 12:51:59 PM

To add to what Den wrote, EMC is also relative to temperature. Not a whole lot though - perhaps 2/10's of a percent between 60 and 100F.

SwampDonkey · March 29, 2009, 03:03:29 PM

To add, MC in the air or in the wood does not equal RH%. Relative humidity tells you how close dew point temperature and actual air temperature are converging. In other words, how close you are to having the moisture in the air precipitate out, thus 100 % RH. The higher the dew point temperature, the higher the moisture in the air. Dew point is also wet bulb temperature you here about in the kiln process. RH in itself doesn't tell you much about moisture content. However, high dew point for my region is in the 60's and it very muggy, saps your strength. It would rarely, almost could say never get that high in my climate in January. For instance, last winter we got 12 feet of snow, that's 13" a water. We had 58" of water from precipitation for the whole year (a record) and our winters are about 7 months long. Winter is much dryer here. In my region EMC is around 16 %. I would say dew point temperature is guiding factor more than anything.

jimF · March 30, 2009, 08:01:48 PM

Dew point and wet bulb temperature are not the same. Dew point is the temperature at which condensation starts - as was said. Wetbulb temperature is the temperature a wet cloth or object will attain if enough air flows over the object. It is true the higher the wetbulb the higher the dewpoint. Dewpoint is always lower than wetbulb. There is no direct linear relationship between the two. There are calculators and tables or charts that will give you the conversions.

A real good and easy calculation is RH% /5 is approximately equal to MC% - for the range of about 50F - 130F - the temperature range most of you will experience in your drying setup. Ex. 80%RH/5 is approximately 16% MC.

SwampDonkey · March 31, 2009, 06:30:05 AM

Yeah, my mistake on wet bulb temperature. It measures the temperature from evaporative cooling. Dew point temperature is less than wet bulb as long as the air is not saturated to 100 % RH. A wet bulb thermometer, is just a regular thermometer with a "sock" sleeved over it, kept wet by whicking action. Dew point is still a major factor for rising and falling air moisture, and then beyond that one step would be pressure.

LorenB · April 15, 2009, 02:33:40 AM

Quote from: tomtrees on March 29, 2009, 03:02:52 AM
The house reads between 50 and 60 % Hummidty. The wood in my house reads about 15 % MC with an electronic meeter. Is there a relationship between the two and how can I calculate this. If I could only work out an ideal MC for the wood that is going into my house (I know there will be fluctuations between winter and summer). I get lost of diffrent people say diffrent thinks (some not very reliable) and I know the norm is less than 15 MC.

Any comments? Sorry for the spelling

Tomtrees

Tomtrees,

Don't worry about the spelling. I can't even count to ten in French.

The US Department of Agriculture produced a book called Drying Hardwood Lumber. You can download the entire book in PDF format here:
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr118.pdf

On page 15 of that book, there is a discussion of EMC (Equilibrium Moisture Content) as well as a table that shows what EMC your wood will stabilize at for a given RH (Relative Humidity).

That book can explain it better than I can.

Sorry, I'm sure it's only available in English.

Loren