Like many other natural materials, wood has several species that are used in construction or restoration projects. In the process of revitalizing a structure or building a new one from the ground up, it’s important to take into account the species of the wood. Otherwise, you may be left with an incorrect moisture content (%MC) reading that can affect your final project.
When measuring moisture in wood, the end result has to be species corrected. This means that the number that is collected from the moisture reading, to ensure its accuracy, must be adjusted to the type of wood being measured.
Measuring moisture in wood means that one also has to take into account factors such as specific gravity (SG) when using pinless moisture meters, and electricial propoties, when using pin-type moisutre meters. Fortunately, for users of moisture meters, many meters already come equipped with the capability to correct for the species of wood being tested.
How to Correct Moisture Readings for Different Wood Species
When dealing with wood, you have two options: either use a species correction chart, or use a meter that has a built-in ability to correct for specific wood species. The chart lists the conversions for the most popular wood species. As pinless meters differ in measuring techniques, their chart will include conversions for the different SGs that are within the meter’s range.
While the chart is convenient, it lacks the ability to give users the exact amount that they may be looking for. Other than that, the chart is an extremely useful tool to get quick moisture answers.
For meters that have the built-in species correction function, the method is quite simple. To get a solid reading, just program the meter to the species of wood being measured. If you’re having difficulty finding a way to get to this mode, refer to the device’s manual. Once this is done, the meter will correct the measurement to reflect the correct moisture content of the wood.
What if There’s No Data on a Wood Species?
When there’s no species correction information on the species you’re looking for, there are still ways to ensure the moisture of the wood is accurate. What’s the trick? Establishing the equilibrium moisture content (EMC).
To do this, gather a couple of wood sample that are about 1/4- to 1/2-inch-thick, 6 inches long, and 2 inches wide. Leave the samples in the environment where they’ll be used for a few days, checking their moisture daily. Make sure the meter is set to its basic parameters and monitor the %MC until the readings no longer change.
This achieves equilibrium moisture content with the environment for the wood, thus working with tolerances between +/- 2% should ensue no warping, cracking or shrinking of the wood.
OR, one can use the oven test to measure a wood surface’s moisture which requires a scale and oven. Make a few samples measuring 4 inches by 6 inches and under 1 inch in thickness.
The Oven Test
You may also perform an oven test to determine moisture content. If a set of lab equipment (Scale & Oven) is not available to handle very small pieces, make several samples (four to five pieces), and treat them as a single sample.
Record several moisture readings and the average of the samples. Only keep the meter readings that are within 2% of one another.
Total the weight of all the samples. Dry the samples and weigh them again. Weigh the samples again after 12 hours. Put them in the oven and check the weight after 4-5 hours. Take the sample out to check the weight in close time intervals, then take them out the oven once the weight is stabilized and doesn’t fluctuate.
In much simpler terms, the formula for % moisture content is
(Initial weight — dry weight) / (dry weight x 100).
Although the oven test is not convenient in most instances, it should allow you to establish the %MC of the wood by its water weight with a fair degree of accuracy.