Why Woodworkers Should Measure Moisture
What if there was one simple step in your woodworking process that could prevent a significant number of issues—about 80% of them?
Turns out, there is! Moisture testing.
Because moisture is one of the biggest causes of failure in woodworking and wood-product manufacturing, it pays to test moisture from the start. You’ll save yourself time and money not having to make repairs. And your clients will come to trust your craftsmanship even more.
Wondering if moisture is really that big of a deal?
Find out as we cover:
What Wood Moisture Content Is All About
In the simplest terms, wood moisture content (MC) refers to the amount of water present in any given piece of wood. To be more specific, it’s the weight of the water in a piece of wood expressed as a percentage of the weight of the wood after it’s been completely dried in an oven.
This process of calculating the MC involves weighing the wood sample and then drying it in an oven (usually at about 103°C or 217°F) until it reaches a constant weight. The difference between the green and oven-dry weight gives the amount of water that was in the piece of wood.
This weight is then divided by the oven-dry weight and multiplied by 100 to give the MC as a percentage.
Factors Influencing Wood Moisture Content
Several factors can influence MC, each of which can contribute to the complexity of working with wood:
- Environmental conditions: The primary environmental factors that affect MC are temperature and relative humidity. Warmer air can hold more moisture than cooler air, and high relative humidity means there is more moisture in the air that could potentially be absorbed by the wood.
- Type of wood: Different species of wood have different cellular structures and therefore, absorb and release moisture at different rates, and shrink and expand to different degrees.
- Time of year: The time of year when the tree was felled can affect the initial MC of the wood. Trees tend to contain more moisture during the growing season than during the winter months. All freshly cut wood, though, requires significant drying before it’s useful in woodworking.
Why Measure Wood Moisture Content
As a hygroscopic material, wood absorbs and loses water to achieve a balance with the surrounding atmosphere. This dynamic process can cause the wood to expand and contract—a phenomenon known as wood movement. If you work with a piece of wood that hasn’t achieved balance with the environment, the wood can shift after the project is finished, causing problems like warping, cupping, or twisting.
It all comes down to the concept of equilibrium moisture content, or EMC.
EMC is the point at which wood neither gains nor loses moisture relative to the air surrounding it. When wood is at its EMC, it’s “in balance” with its environment.
EMC is influenced by two factors:
- Relative humidity.
For instance, in a cold and dry climate, the EMC of wood will be lower, causing the wood to release moisture and shrink. On the other hand, in a warm, humid climate, the EMC will be higher, causing the wood to absorb moisture and expand.
Wood that is not in equilibrium with its intended environment can lead to damage, such as warping, checking, splitting, and dimensional changes. If a piece of furniture is built with wood at a higher EMC than in its intended environment, it might shrink and crack as it loses moisture.
That’s why measuring the MC of wood is essential. It’ll help you determine whether the wood has reached its EMC so that you can prevent issues like the following.
Common Moisture-Related Problems
Here are several common problems due to improper MC levels in wood:
- Warping: Warping can appear in the form of a bow, crook, twist, or cup in the wood. It occurs when different parts of a wood piece dry at different rates, causing uneven shrinkage. This is one of the most common moisture-related issues in woodworking.
- Checking and splitting: Checking refers to small cracks that appear on the surface of wood as it dries, while splitting involves deeper, more significant cracks. Both can weaken the wood and detract from its aesthetic appeal.
- Swelling and shrinking: These dimensional changes can lead to fit and function problems in finished products. For example, a drawer might become stuck if the wood swells due to high humidity, or a musical instrument might warp if the wood shrinks in a dry environment.
- Decay and fungal growth: Wood with a very high MC is a conducive environment for fungi, leading to decay. Certain types of fungi can also cause discoloration.
Beyond the stability of the wood, MC also affects various woodworking processes. For instance:
- Wood that is too wet can clog saw blades and make precise cuts more difficult.
- If the wood’s MC changes significantly after joining, it can cause joints to loosen or tighten, impairing the structural integrity of the piece.
- A high MC can prevent finishes from adhering properly, leading to peeling or blotchy appearance.
Next, we’ll take a look at how to measure MC and avoid these issues.
How to Measure Wood Moisture Content
The most accurate and scientific way to measure moisture is by using the traditional oven-dry method. However, this method is impractical and time-consuming, so woodworkers usually opt for a high-quality moisture meter.
We’ll give you an overview of each method and then cover some best practices for using a wood moisture meter.
The oven-dry method involves weighing the wood, drying it in an oven at about 103°C or 217°F until it reaches a constant weight, and then weighing it again. The moisture content is then calculated by comparing the weight before and after drying.
While this method is accurate, it’s not a time or cost-efficient way because it takes hours or days and involves destroying a sample of the wood.
We recommend the next method instead.
Moisture meters provide a much quicker way to measure wood MC. These devices work by measuring the electrical resistance (for pin-type meters) or dielectric properties (for pinless or non-invasive meters) of the wood, which are related to its moisture content. Here’s how.
Pin-Type Moisture Meters
Pin-type meters send an electrical current between two pins that are inserted into the wood. Because water conducts electricity, the level of resistance to the electrical current can indicate the approximate amount of moisture in the wood.
These meters give fairly quick readings but require making small holes in the wood. They also tend to measure the surface of the wood—unless you use a hammer probe with insulated pins.
Because pin meters measure resistance, they are very sensitive to the chemical composition of the wood, which often varies from tree to tree, and even more between species. Setting your meter’s species setting will remove some of this inaccuracy from the reading.
Pin meters are also affected by temperature (which pinless meters are usually not). To account for this, you’ll need to use a temperature correction chart, unless the wood is near room temperature.
Pinless Moisture Meters
Pinless meters, also known as non-invasive meters, work by sending an electromagnetic signal into the wood and measuring the signal’s reflection. Unlike pin meters, these meters can scan a larger area in a much shorter time and do not damage the wood.
However, they are affected by wood density, which varies quite a bit between species. To account for this difference in species, high-quality meters will have species settings that ensure you still get accurate readings.
One example of a quality pinless meter is the Wagner Meters Orion® 950. It offers features like:
- A temperature and humidity sensor: It has a built-in sensor that measures the ambient temperature and relative humidity (RH) of any room the device is in. This feature essentially combines an ambient conditions data logger and a moisture meter in one device.
- An EMC calculator: The meter uses the temperature and relative humidity to determine the EMC of the wood. This makes it easy for you to compare it with the readings you get from the wood.
- Dew point calculations: The Orion 950 can calculate the dew point of the environment, utilizing its built-in temperature and humidity sensor. It will calculate and display the current dew point whenever and wherever you need it.
Factors Affecting Measurements
Several factors can influence the accuracy of moisture measurements. Knowing these factors can help you adjust so you avoid skewed readings. They are:
- Wood species: Different species of wood have different electrical resistances and dielectric properties. Most moisture meters allow you to adjust the readings for different species, so make sure you know what type of wood you’re measuring.
- Wood temperature: The temperature of the wood can affect its electrical properties, especially the resistance that pin meters measure. Pinless meters are generally not affected by temperature.
- Surface moisture: If the wood’s surface is wet, this can significantly affect the readings, especially with pinless meters. To avoid inaccuracy, wipe off any visible moisture before measuring. The Orion moisture meters have a patented IntelliSense circuit that corrects for surface moisture.
- Depth of measurement: Pin-type meters measure the moisture between the pins, so the depth at which the pins are inserted can affect the reading. It’s hard to insert most handheld pin meters deeper than ¼ of an inch. Pinless meters, like the Orion 950, typically measure the moisture around ¾ of an inch below the surface. Some others, including the Orion 930, 940, and 950, have two depth modes.
Best Practices for Using Moisture Meters
If you’re all set with a moisture meter, here are some best practices to keep in mind as you begin using it:
- Account for wood species: As mentioned earlier, different wood species have different properties that can affect moisture meter readings. Be sure to input the correct species into your meter if it has that capability.
- Check calibration: Regularly check and calibrate your moisture meter according to the manufacturer’s instructions to ensure its accuracy. The Orion 950 comes with a calibration check block to make this process simple and straightforward.
- Measure multiple areas and depths: For the most accurate understanding of a wood’s MC, measure multiple areas and, if possible, different depths. This is especially important for larger pieces of wood.
- Use your measurements for wise decision-making: Be aware of what the moisture content readings mean for your woodworking project. If the MC is higher than the EMC, you may need to allow more time for the wood to dry before using it.
And there you have it! Following these practices of moisture testing, you’ll improve the quality of your work and prevent issues in the long term. That means less hassle for you and greater satisfaction for your customers.
Moisture Damage Doesn’t Have to Be Your Story
Moisture has a major impact on wood. Too high of a moisture content could mean beautiful cabinet doors warping and showing gaps soon after installation. Or a newly built dresser with drawers that won’t close after a few months.
As a result, you get a call from an unhappy customer, demanding that you repair the problem. Wanting to provide good customer service, you oblige—at your own expense and time.
Thankfully, that doesn’t have to be your story.
By using a high-quality moisture meter to check the MC of your wood, you can protect your project, time, money, and reputation.
So, what are you waiting for? If you don’t already have a moisture meter, we walk you through the process of buying one with our moisture meter buying guide.
Wagner Meters is a family-owned American business that aims to provide solutions in moisture measurement technology that will enhance the quality and value of each customer’s project. With an almost 60-year legacy of innovation, Wagner continues to be a resource for both individual craftsmen and high-performance commercial endeavors.
Last updated on February 12th, 2024