MadSci Network: Engineering

Re: Are humidity and moisture content of drying lumber, the same numbers?

Date: Mon May 22 23:56:26 2000
Posted By: Graeme Bushell, Faculty, Chemical Engineering, The University of New South Wales
Area of science: Engineering
ID: 958329390.Eg

In short, no they are not the same.

Relative humidity is the amount of moisture carried in the air, relative to 
the maximum amount that can be carried at that temperature and expressed as 
a percentage. For example, air at a temperature of 60F has a carrying 
capacity of approximately 0.011 pounds of water per pound of dry air. If  
this temperature is actually carrying 0.0055 pounds of water per pound of 
dry air then the humidity will be 50%. The carrying capacity is a strong 
function of temperature.

The second part of your question is harder. It is a question about 
equilibrium distribution. Generally speaking, if you have two different 
phases, such as air and water, any chemical compound will distribute itself 
between the two phases. For example, if you have air and water in contact, 
some of the oxygen will exist in the air as a gas, and some of it will 
dissolve in the water. The amount which goes into each phase depends on the 
chemical nature of the phases and the compound of interest, in this case 
oxygen. The partition coefficient for the particular system will tell you 

For your case, the two phases are air and wood. The compound of interest is 
water and you want to know its equilibrium distribution between the two 
phases (partition coefficient). It might even be different for different 
kinds of wood. Good luck with your search!

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