Computing Air Moisture Calculator

Computing Moisture Content of Wood

Computing Moisture Content of Wood

The moisture content of wood is directly related to the humidity and temperature of the surrounding air.  The equilibrium moisture content (EMC) occurs when the wood has reached an equilibrium with its environment and is no longer gaining or losing moisture. Here is a calculator for computing the EMC of wood at or below the fiber saturation point (about 30% moisture content) given the temperature and relative humidity:


Temperature:     Rel. Humidity (%):         
Fahrenheit Celsius

   EMC:


The table below provides EMC values for a fairly representative range of atmospheric conditions that wood is likely to be exposed to.  Values in this table are applicable to wood of any species for most practical purposes.

Relative Humidity % Ambient Air Temperature - degrees Celsius and Fahrenheit
(Celsius rounded to nearest degree)
C: -1 4 10 16 21 27 32 38 43 49 54
F: 30 40 50 60 70 80 90 100 110 120 130
5 1.4 1.4 1.4 1.3 1.3 1.3 1.2 1.2 1.1 1.1 1.0
10 2.6 2.6 2.6 2.5 2.5 2.4 2.3 2.3 2.2 2.1 2.0
15 3.7 3.7 3.6 3.6 3.5 3.5 3.4 3.3 3.2 3.0 2.9
20 4.6 4.6 4.6 4.6 4.5 4.4 4.3 4.2 3.0 3.9 3.7
25 5.5 5.5 5.5 5.4 5.4 5.3 5.1 5.0 4.9 4.7 4.5
30 6.3 6.3 6.3 6.2 6.2 6.1 5.9 5.8 5.6 5.4 5.2
35 7.1 7.1 7.1 7.0 6.9 6.8 6.7 6.5 6.3 6.1 5.9
40 7.9 7.9 7.9 7.8 7.7 7.6 7.4 7.2 7.0 6.8 6.6
45 8.7 8.7 8.7 8.6 8.5 8.3 8.1 7.9 7.7 7.5 7.2
50 9.5 9.5 9.5 9.4 9.2 9.1 8.9 8.7 8.4 8.2 7.9
55 10.4 10.4 10.3 10.2 10.1 9.9 9.7 9.5 9.2 8.9 8.7
60 11.3 11.3 11.2 11.1 11.0 10.8 10.5 10.3 10.0 9.7 9.4
65 12.4 12.3 12.3 12.1 12.0 11.7 11.5 11.2 11.0 10.6 10.3
70 13.5 13.5 13.4 13.3 13.1 12.9 12.6 12.3 12.0 11.7 11.3
75 14.9 14.9 14.8 14.6 14.4 14.2 13.9 13.6 13.2 12.9 12.5
80 16.5 16.5 16.4 16.2 16.0 15.7 15.4 15.1 14.7 14.4 14.0
85 18.5 18.5 18.4 18.2 17.9 17.7 17.3 17.0 16.6 16.2 15.8
90 21.0 21.0 20.9 20.7 20.5 20.2 19.8 19.5 19.1 18.6 18.2
95 24.3 24.3 24.3 24.1 23.9 23.6 23.3 22.9 22.4 22.0 21.5
98 26.9 26.9 26.9 26.8 26.6 26.3 26.0 25.6 25.2 24.7 24.2

EMC Equation

The EMC calculator and table employ the following equation to derive moisture content:

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

where

M = moisture content (%)
T = temperature (oF)
H = relative humidity/100

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

 

Source: U.S. Forest Products Laboratory


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