Ratios: Relative humidity to moisture content

      Common conversions and instruments used in measuring wood moisture content. October 31, 2000

Q.
What is the conversion scale between RH and MC? For example: 75% RH = what amount of MC? Also what do you call the gauge used to measure these? I am trying to control the moisture in a room, and I would like to see which of my methods works the best.

A.
A hygrometer measures humidity. A thermo-hygrometer measures humidity and temperature. A psychrometer has two thermometers (one wet and the other dry) that measures temperatures and can be used to calculate humidity.

The property of the air is called the equilibrium moisture content (EMC) and it is numerically equal to the MC that wood will achieve during drying if you wait long enough and if the conditions do not change. (Example: At 30% RH, wood will achieve 6% MC. So the air has an EMC of 6%.)

Common conversions:
0% RH = 0% MC = 0% EMC
30% = 6%
50% = 9%
65% = 12%
80% = 16%
99% = 28%

Look in the archives for more discussion.

Gene Wengert, forum technical advisor



For a great table on the relationship between relative humiduty, room temperature, and equillibrium moisture content visit www.furniturecarver.com/emc.html.

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