The Fastest Way to Measure the Moisture Content of Wood

How to measure the moisture content of wood

There are two main ways to measure the moisture content (MC) of wood—or the amount of water in a piece of wood. The first way, known as oven-dry testing, is the scientific way. The second way is by using a wood moisture meter—the fastest and easiest method for those installing wood flooring or doing woodwork.

How does each method work? Let’s find out!

Oven-dry Testing

The most accurate method for measuring the MC of wood is oven-dry testing. The ASTM (American Society for Testing and Materials) D4442 standard specifies that the oven-dry method is the primary method for research.

It involves drying a sample piece of lumber until its weight no longer changes and then comparing the original weight with the dry weight. The difference is the amount of moisture that the wood contained.

Let’s follow those steps in more detail:

1. Take a sample piece of wood and weigh it.

2. Put it in an oven that has good ventilation and a constant temperature of 220° F (about 104° C). Allow the sample to dry out completely.

3. Check the weight of the sample regularly until the weight stops decreasing (no more than 0.1% change over a couple of hours).

4. Weigh the sample again.

5. Calculate the MC of the wood with the following formula:

   

Though accurate, this method has drawbacks.

For one, it can take many days, depending on how much water the sample contains and how dense the wood is (denser wood holds more water in its cells). This means that your project could be stalled.

Talk about a headache, especially if you’re in a time crunch!

Second, most people don’t have the specialized oven needed for this test. The oven must have proper ventilation to allow moisture to leave. It also has to be very accurate and able to maintain a constant temperature for long periods.

Then, there’s the sample wood itself. It will most likely be unusable once it has dried in the oven.

The dry wood will end up damaged or burned. And it probably won’t be at the MC level needed for your project.

Who wants to waste their time drying a piece of wood for days when the wood won’t even be usable at the end?

And finally, the oven-dry testing method isn’t a very portable method since it requires large tools, such as an oven and a scale.

Wouldn’t it be so much easier to use a method that fits into your pocket?

That’s where moisture meters—the second method—come in!

Moisture meters

Moisture meters are the fastest and easiest way to check the MC of wood (ASTM D4444). A high-quality wood moisture meter will provide you with accurate readings in a matter of seconds. There are two types of moisture meters to choose from: a pin meter or a pinless meter.

Pin meters

Pin meters get their name because they have two pins, or electrodes, that must be pushed into the wood. An electric current runs between those two pins.

Conductivity is the key for pin-type meters.

Since wood is a poor conductor of electricity while water conducts electricity well, the amount of resistance to the electric current determines how much moisture is in the wood. The greater the electrical resistance, the lower the MC.

Pinless meters

Pinless moisture meters, on the other hand, use an electromagnetic sensor to measure MC. All you have to do to take moisture measurements is hold the sensor plate in full contact with a flat piece of wood.

Whether using a pin meter or a pinless meter, keep the following in mind to ensure that you get accurate results:

• Get a high-quality moisture meter. The Bessemeter DS500 is one example.
• Verify calibration using a calibration reference tool.
• Adjust your meter for the correct wood species.
• Adjust for temperature if the wood is not at room temperature (for pin meters only). Some meters may have a built-in correction function for temperature, but you will likely need to enter the temperature. If your meter doesn’t have that function, you’ll need a temperature correction chart.

Using a wood moisture meter will help you get the quick results you need and help you along the drying process. But getting a moisture meter only makes sense if you know why you need it in the first place.

Why measure the moisture content of wood

Measuring MC is an essential practice for woodworkers because it guides their decisions when preparing their building material. It prevents moisture-related problems—warping, buckling, crowning, or cupping—that can result if the wood doesn’t have an MC level corresponding to its environment.

If a wood cabinet is not at the target MC, here’s what can happen.

It’ll increase or decrease in moisture and, whether through swelling or shrinkage, become warped so that the drawers are impossible to close.

And too much moisture in a piece of wood could even lead to mold or fungal growth.

That’s why it’s important to measure the MC before beginning a project. Knowing the MC of your wood helps you to know how far it is from reaching the EMC (equilibrium moisture content), which is the target moisture level for a given location. Then, you can help acclimate the wood to its environment.

The result?

You end up with a quality project that won’t show moisture-related damage down the road and touts the durability that you would want associated with your reputation.

As an example, let’s say that you want to install a wood floor in a home in Arizona. You look up the EMC chart (find it in the Wood Handbook) and learn that the EMC of the dry climate there is 7%.

Then, you use a moisture meter to sample about 40 boards of the flooring. The moisture readings indicate that they have an MC of 11%. This tells you that you will need to allow the flooring some time to acclimate to its final environment before you install it.

To learn more, read our article about why you should measure moisture content.

But now, we’re going to turn our attention to one more question you’re probably wondering about:

How often should I do it?

When to measure the moisture content of wood

To avoid moisture-related damage, check the moisture content of your wood at a few key points throughout your project:

• When purchasing the wood
• During the acclimation process
• When getting ready to use it for your project
• When dealing with flooding or water leaks

Before purchasing your wood, you’ll want to check its MC right away to determine how close it is to the EMC of the final location. That way, you’ll know whether it’s worth purchasing or whether it’s wet wood and you need to look for drier wood. It’ll also indicate if you need to acclimate the wood.

For a small project, it’s not difficult to check the MC of all your wood. But when dealing with a larger project, choose a random sample.

For flooring, the NWFA (National Wood Flooring Association) recommends that flooring installers check 40 boards for every 1,000 square feet of flooring and then average the numbers.

If the wood requires acclimation, you’ll want to use your moisture meter periodically to check progress. Is it coming close to its EMC?

Then, when you’re ready to use the wood for a project, check again—just to be sure!

Lastly, moisture meter readings can be very useful when dealing with flooding or water leaking onto wood surfaces. They can show where damage has occurred and where drying needs to take place.

Measure away for a successful project.

Checking MC is essential to achieving a quality result, whether for woodwork, construction, or floor installation.

We’ve learned that we can measure the wood’s MC in two primary ways:

1. Oven-dry testing
2. Moisture meters

The fastest way?

Moisture meters get the credit for that! If they are made well, they will save you time, money, and effort.

So invest in a high-quality meter—whether pin or pinless—to ensure that you get accurate results. Whether you are a professional woodworker, flooring installer, or a prosumer who enjoys a DIY here and there, a moisture meter will ensure that you have a project free from moisture damage.

Then, measure away!