How to Calculate the Moisture Content of Wood: A Comprehensive Guide
The moisture content of wood is an important factor not only for woodworking but also for other purposes like construction and firewood use.
Properly measuring and understanding the moisture content of wood is essential to these applications.
Contents
How to express moisture content in wood?
There are two ways to express the moisture content in wood:
Moisture content on a wet basis
This method calculates water mass as a fraction of the total mass of wood plus water.
This approach is employed in the chemical and pharmaceutical sectors. This moisture content is always less than 100% and is frequently referred to as moisture content on a wet basis.
So the formula for moisture content on a wet basis becomes:
Moisture content = (wet weight – dry weight) / wet weight x 100%
Moisture content on a dry basis
The mass of water is a percentage of the oven-dry mass of wood. In other words, the moisture content of wood is the ratio of the weight of water in the wood to the dry weight of the wood.
If the amount of water exceeds the oven-dry mass of the wood, the moisture content will be greater than 100%.
This occurs in the greenwood of many lower-density species, such as pine sapwood. This method is frequently referred to as moisture content on a dry basis.
So the formula for moisture content on a dry basis is:
Moisture content = (wet weight – dry weight) / dry weight x 100%
The second way of expressing moisture content (measuring on a dry basis) is the standard way. It is used on worldwide basis and is the preferred way of expressing moisture content in wood.
How to measure the moisture content of wood?
There are a few ways that you can measure the moisture content of the wood. Here are some of the most common methods:
The oven-dry method
Oven drying is the most common method used to determine the moisture content of the wood. It is also the most accurate method.
To use this method, you need to:
The difference in weight will give you the weight of water that has been lost during the drying process. To calculate the moisture content, you need to divide the weight of water lost by the dry weight of the wood.
When using this method, it is important to note that the wood must stay in the oven for 24 hours to ensure that all of the water has been removed.
Although this method removes most of the water, there will still be some water left in the wood. However, this method is accurate enough for most purposes.
The moisture meter method
Another common method used to determine the moisture content of wood is the moisture meter method.
These meters measure the electrical resistance of the wood. The more water in the wood, the lower the resistance.
Moisture meters have two probes that are inserted into the wood. The meter then calculates the moisture content based on the resistance between the probes.
The accuracy of moisture meters can vary, so it’s important to calibrate the meter before use. Moisture meters can be calibrated with wood that has a known moisture content.
This method is less accurate than the oven-dry method but it is much faster and easier to do.
To use this method, you need to:
The advantage of this method is that it is much faster than the oven-dry method. The disadvantage is that it is less accurate.
Karl Fischer titration method
Karl Fischer titration method is used to determine the moisture content of wood on a microgram scale. This method is used in the laboratory to measure very low moisture contents.
The principle of this method is that the water in the wood reacts with the Karl Fischer reagent to produce a conductive solution. The conductivity of the solution is proportional to the amount of water present in the sample.
Water reacts with iodine and sulfur dioxide quantitatively under the presence of alcohol and N-base.
The resultant solution has a conductivity that is proportional to the amount of water in the sample.
Karl Fischer titration can measure moisture contents as low as 0.01%. This method is very accurate but it is also very time-consuming and expensive. This method can utilize two different techniques:
Coulometric titration
Coulometric titration involves injecting a weighted sample into an anolyte containing iodine, alcohol, and N-base in the titration vessel.
Iodine reacts with the water in the sample to produce iodine ions. Water content is estimated using the amount of electricity used to create iodine from iodine ions.
Volumetric titration
Volumetric titration involves injecting a weighted sample into the titration vessel’s dehydration solvent. Iodine-containing titrant is dropped into the titration tank in stages.
The volume of the titrant reacting with the water is used to compute the water content.
Distilling off the water and measuring its volume
When a sample of wood contains terpenes, oils, or preservatives, the oven-drying process is replaced with the distillation method.
To assess the moisture content, the specimen is chipped, pooled into a sample of 20 to 50 grams, and then placed in a hot flask with a solvent, such as xylene or toluene, added.
The water in the chips evaporates and ascends. The water vapor condenses in a cooler on the flask’s side and is collected in a pipe from which the reading is obtained.
This approach is not suitable for monitoring concepts since it entails chipping the wood material, i.e. the destruction of sections of timber elements. It requires complicated apparatus and it is very time-consuming. It is a slow process and isn’t commonly used.
Microwave method
The water content of the wood affects the dielectric constant of the wood. The microwave method uses this principle to determine the moisture content of the wood.
This method is based on the fact that water molecules are polarized and when a microwave field is applied, they rotate. The amount of rotation is proportional to the moisture content of the wood.
Once the specimen is placed in the microwave, the dielectric constant of the wood is measured. The moisture content is then calculated from the dielectric constant.
The advantage of this method is that it is very fast and easy to do. The disadvantage is that it is not very accurate and it can only be used on small samples.
Sorption method
Sorption is the process of uptake and accumulation of water vapor by wood. This method is used to determine the equilibrium moisture content (EMC) of wood.
The sorption method can be used to measure both the hygroscopic moisture content (HMC) and the bound moisture content (BMC).
To measure the hygroscopic moisture content, the specimen is placed in a sealed chamber with known relative humidity. The equilibrium moisture content is then determined by measuring the weight of the specimen.
To measure the bound moisture content, the specimen is first oven-dried to remove all the free water. It is then placed in a sealed chamber with known relative humidity. The equilibrium moisture content is then determined by measuring the weight of the specimen.
The advantage of this method is that it is very accurate. The disadvantage is that it is time-consuming and can only be used on small samples.
Radiometric method
The radiometric method uses the principle of radioactive decay to determine the moisture content of the wood.
This method is based on the fact that water molecules are transparent to gamma rays but not alpha particles. When a sample of wood is irradiated with gamma rays, the water molecules in the wood will absorb some of the radiation.
The amount of radiation that is absorbed is proportional to the moisture content of the wood. The advantage of this method is that it is very accurate. The disadvantage is that it is expensive and time-consuming.
Spectrometric method
The spectrometric method uses the absorption of light by water molecules to determine the moisture content of the wood.
This method is based on the fact that water molecules absorb light in the infrared region of the spectrum.
The amount of light that is absorbed is proportional to the moisture content of the wood. The advantage of this method is that it is fast and easy to do. The disadvantage is that it is not very accurate.
Thermogravimetric method
The thermogravimetric method uses the principle of weight loss on drying to determine the moisture content of the wood. This method is based on the fact that water molecules are removed from the wood when it is heated.
The weight of the wood is measured as it is heated. The moisture content is then calculated from the weight loss. The thermogravimetric method is very accurate but it is time-consuming.
Color-reaction method
Color-reaction methods are based on the fact that water molecules change the color of certain dyes. The most common color-reaction method is the iodine-starch test.
In this test, a sample of wood is impregnated with an iodine solution. The iodine solution reacts with the starch in the wood to form a blue-black complex. The intensity of the color is proportional to the moisture content of the wood.
The advantage of this method is that it is fast and easy to do. The disadvantage is that it is not very accurate.
This method is applicable for wood moisture concentrations of 6 – 20% percent and temperatures ranging from 15° to 25° C.
The test is not recommended for use on woods with high extractive content such as redwood and cedar since the extractives may interfere with the test.
