EMC calculator - how dry do you need your wood

[DaveO]

I found this surfing another site. It is a very interesting source of info. Everyone wants kiln dried wood at 6-8%, then lets it acclimate to their shop, makes something, then moves the finished piece into their home. This will tell you the moisture content the wood will be in whatever environment it is in. My house, 76° with AC and around 60% humidity = 10.9 EMC, my shop, 78° and around 70% humidity = 12.9 EMC. Letting the wood acclimate to my shop will result in shrinkage once the piece is moved inside :eusa_thin

http://www.csgnetwork.com/emctablecalc.html


Dave

 

[jeff...]

I always though it was E=MC2

But now I find out it's this :eusa_doh:

M = 1800/W [ KH/(1-KH) + (K1KH + 2K1K2K2H2) / (1 + K1KH + K1K2K2H2)]

Where:

M = moisture content (%)
T = temperature (oF)
H = relative humidity (%) W = 330 + 0.452T + 0.00415T2
K = 0.791 + 0.000463T - 0.000000844T2
K1 = 6.34 + 0.000775T - 0.0000935T2
K2 = 1.09 + 0.0284T - 0.0000904T2

 

[McRabbet]

Dave,

I'd suggest you add this to the Link Library. Thanks.

 

[DaveO]

Rob, great idea...done :-D

Dave

 

[Gofor]

I always though it was E=MC2

But now I find out it's this :eusa_doh:

M = 1800/W [ KH/(1-KH) + (K1KH + 2K1K2K2H2) / (1 + K1KH + K1K2K2H2)]

Where:

M = moisture content (%)
T = temperature (oF)
H = relative humidity (%) W = 330 + 0.452T + 0.00415T2
K = 0.791 + 0.000463T - 0.000000844T2
K1 = 6.34 + 0.000775T - 0.0000935T2
K2 = 1.09 + 0.0284T - 0.0000904T2

What is T2?

Go

 

[DaveO]

What is T2?

Go

In this application I believe it is T (temperature (oF)) x 2, elsewhere the sky the limit :lol:

Dave

 

[Gofor]

Thanks. After seeing K1, K2, etc, was not sure it wasn't another factor.

Go

 

[cpowell]

I found this surfing another site. It is a very interesting source of info. Everyone wants kiln dried wood at 6-8%, then lets it acclimate to their shop, makes something, then moves the finished piece into their home. This will tell you the moisture content the wood will be in whatever environment it is in. My house, 76° with AC and around 60% humidity = 10.9 EMC, my shop, 78° and around 70% humidity = 12.9 EMC. Letting the wood acclimate to my shop will result in shrinkage once the piece is moved inside :eusa_thin

http://www.csgnetwork.com/emctablecalc.html


Dave

So, are you saying that "shrinkage" is inversely proportional to temperature? :lol: :lol:

Chuck

 

[MikeH]

So, are you saying that "shrinkage" is inversely proportional to temperature? :lol: :lol:

Chuck

He said shrinkage.....:rotflm: :rotflm: :rotflm:

 

[Kyle]

In this application I believe it is T (temperature (oF)) x 2, elsewhere the sky the limit :lol:

Dave

I would think that temperature could not exceed 260 degrees C in any case. Barbquing could occur at that point

 

[cpowell]

I stuck the equation in a spreadsheet that performs the same calc. If you look at the cells you can see how the equation is used.

Change either Temperature or RH and EMC is updated.

Chuck

 

[Gofor]

Thanks Chuck. Really made it simple.:eusa_clap:eusa_clap

Go

 

[cpowell]

Another post on WN got me interested in looking at this a little bit more. I revised the spreadsheet and generated EMCs with temperature fixed and RH varying, then generated EMCs with RH fixed and temperature varying.

Revised spreadsheet attached. If you look at the chart with 45% constant RH and temp varying from 45 to 90 deg F, the EMC changes by 0.6 percent. Now look at the chart with temp fixed at 70 deg F and RH varying from 40 to 85 percent. The EMC changes by 10 percent. :shock:

The temperature and RH fields can be modified and the charts will update accordingly. I protected the sheet "EMC Raw Data" but it can be turned off. Go to "Tools", "Protection", "Unprotect Sheet" to turn off.

This makes me realize that I should be careful sizing wide RPs made in the summer which will be moved into a house with wood heat during the winter. My tendency is always to allow for panel expansion. Never thought about allowing for shrinkage.

One day when I'm bored I'll add wood movement to the sheet, maybe by species, just to get a feel for how Temp, RH, movement interact.

Chuck

 

[McRabbet]

Chuck, can you add this to the Download Library, please?

Thanks.

 

[cpowell]

Chuck, can you add this to the Download Library, please?

Thanks.

Added...hopefully in the right category.

Chuck

 

[McRabbet]

I moved it to the Wood Reference Category and added a little to the beginning for folks to know it is an EXCEL file. Thanks.