Equilibrium Moisture Content Is Determined by Relative Humidity

Woodshop EnvironmentEquilibrium moisture content is the moisture level where the wood neither gains nor loses moisture since it is at equilibrium with the relative humidity of the surrounding environment.

In order to properly understand the effect of relative humidity on the moisture content of a piece of wood and the equilibrium with its environment, a basic understanding of some commonly-used terminologies is extremely important. Before going into any details, one should simply keep in mind that the relative humidity (along with temperature and other factors) does affect the moisture content of wood as well as the equilibrium value of moisture content, as long as the fiber saturation point, discussed later, is not reached. The mutual relationship of relative humidity “h” or “ϕ”, moisture content “MC”, temperature “T”, and equilibrium moisture content “EMC” is governed by complex mathematical equations.

What Is the Moisture Content of a Piece of Wood?

The moisture content of a piece of wood is the total amount of water (or moisture) contained in that piece of wood. Mathematically, moisture content “M” or “MC” is the difference between the mass of wood with moisture “m” and without moisture “mod” (oven-dry mass), divided by the mass of wood without moisture “mod”. It can be expressed as:

MC = (m – mod)/mod

What Is Fiber Saturation Point?

Several types of WoodIn almost all kinds of wood, moisture can exist as either free water or bound water. The water vapors or liquid water in the cavities and cell lumina is termed as free water, whereas the water that is held by the intermolecular attraction within cell walls is termed as bound water. Fiber saturation point “MCfs” is the point at which no water exists in the cell lumina but the cell walls are completely saturated.

Another way to think of fiber saturation point is to think of it as the moisture content level beyond which the properties of wood do not change as a function of moisture content. Typically, the value of MCfs is about 30% for wood but can change from species to species and piece to piece.

What Is Equilibrium Moisture Content?

As long as the fiber saturation point is not reached, the relative humidity “h” and temperature “T” of the atmosphere affects the moisture content of wood considerably. The moisture content at which wood neither gains nor loses moisture is known as the equilibrium moisture content, or “EMC”. The equilibrium is dynamic in nature because of the changing relative humidity and temperature.

When a piece of wood is placed in a certain environment, over a period of time it tries to achieve equilibrium with the environment. The moisture content changes to adjust to the relative humidity and temperature of the surroundings. After a certain period of time, the moisture content stops changing. This moisture level is termed as the equilibrium moisture content “EMC”. It stays as it is as long as the relative humidity and temperature of the surroundings are not changed.

What Is Relative Humidity?

Generally expressed as a percentage, relative humidity “h” or “ϕ” is the ratio of partial pressure of water vapors (H2O) to the saturated vapor pressure of water at a particular temperature in an air-water mixture. Temperature and air pressure affect the relative humidity of the air. The following equation is generally used to calculate the relative humidity of air:

ϕ = (ew/e*w) ×100%
where ew is the partial pressure of water vapors and e*w is the saturated vapor pressure of water at a particular temperature.

EMC, Relative Humidity, and Temperature

The relationship between moisture content (as well as the equilibrium moisture content “EMC”) of wood and relative humidity can be studied and approximated for a given temperature. It is clear that for each value of relative humidity “h”, there is a corresponding value of EMC. Therefore, EMC can be plotted as a function of relative humidity. True for most of North America, 30% to 50% relative humidity corresponds to 6% to 9% EMC. The EMC values of solid wood are generally greater than wood composites.

For a reasonable estimation of the true target EMC at any value of relative humidity and temperature, the following equation may be used:

EMC = [ -ln (1 – ϕ) / 4.5 x 10-5 ( T + 460 ) ] 0.638

ln = natural logarithm

ϕ = relative humidity expressed as a decimal

T = temperature in oF

Coastal AreaThe Hailwood-Horrobin equation can also be used to mathematically estimate the complex relationship of EMC, relative humidity, and temperature.

A general trend, as you may also have noted from the equations discussed, is that temperature doesn’t affect the moisture content as appreciatively as relative humidity does. Coastal areas like Miami, Seattle, and Japan have higher values of relative humidity and subsequently higher values of EMCs than non-coastal areas.

When Is Wood Wet and When Is Wood Dry?

The moisture content of a piece of wood at a particular time determines how wet or dry that piece of wood is. The lower the moisture content, the more dried the wood and vice versa. A Wagner moisture meter can be used to determine the moisture level in wood. Relative humidity and temperature affect the equilibrium moisture content, as is evident from the equations discussed before.

The Importance of Correctly Measuring Moisture Content

The moisture content of a piece of wood and the equilibrium moisture content can be determined by the relative humidity of the air surrounding the piece of wood. The stakes of unknown moisture content are way too high. At the worst, excess moisture content in wood can cause total flooring disasters. Wagner moisture meters are designed to measure any excess moisture content in a timely manner so that million-dollar mistakes are avoided. Moisture content in wood can be measured accurately and as often as one might need before making any important decisions. It is a nice way to ensure that moisture content in wood is in accordance with industry standards.

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Larry Loffer

Larry Loffer is a senior technician at Wagner Meters, where he has over 30 years of experience in wood moisture measurement. With a degree in Computer Systems, Larry is involved in both hardware and software development of wood moisture measurement solutions.

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