What Do These Moisture Meter Readings Mean?
Anyone who works with wood, whether they’re woodworking professionals, wood flooring installers, furniture or cabinet manufacturers, sellers of building materials, or musical instrument manufacturers to name a few, must know its moisture content. The moisture content of wood is defined as the ratio of the weight of the water in the piece of wood to the weight of the piece of wood without the water.
You cannot know if a piece of wood is right for your project unless you know its moisture content. Depending on the type of project and where you live, the required wood moisture content is usually in the 6-12% range. You may need to do some research to determine the correct moisture content for your geographical location and particular wood project.
Why Is Moisture Content Important?
The moisture content of wood is important because wood will shrink or expand according to its moisture content. In other words, its physical size will actually change. This has the potential to cause a host of problems. Imagine spending hundreds of hours building a beautiful wood cabinet only to see it start to deform later. These problems can be easily avoided if you know the wood’s moisture content.
So, how much does wood shrink or expand?
While the amount of shrinkage or expansion varies from species to species, wood will generally shrink or expand about 1% across the grain when its moisture content changes by 4%. However, even with large moisture changes, wood barely shrinks or expands at all along the grain.
Why is it important for woodworkers to know about moisture content?
You might be thinking that the 1% shrinkage caused by a 4% change in moisture content isn’t that much. However, even a shrinkage this small – maybe only several thousandths of an inch – can cause after-construction gluing problems, especially with cracks and joints. This can happen with furniture, cabinets, or wood flooring.
With furniture or cabinets, a 1% shrinking or swelling can result in poor-fitting drawers, doors, and joints. It can even cause warping and cracks in the finish.
Wood flooring will also shrink or expand with moisture changes and even small changes can produce big effects. For example, a small 1/32” change per 2” multiplied across an 8-foot room equals 1 ½”. A change this big will be easy to see.
Know Wood’s Equilibrium Moisture Content
Once wood acclimates to its environment, it reaches what’s called its equilibrium moisture content or EMC. This happens when the moisture inside the wood reaches a balance or equilibrium with the relative humidity (RH) and temperature of the wood’s surrounding environment. It stays that way as long as the RH and temperature don’t change.
The air’s RH affects the EMC. Here’s how that works:
If the air’s RH is 40% and the temperature is 68F, the wood will eventually attain an EMC of just under 8%.
Here are some key RH values and the corresponding EMC:
|Relative Humidity (RH)||Temperature||Equilibrium Moisture Content (EMC)|
The first two values above are fairly typical interior EMC values for heated and air-conditioned offices and homes in most of North America. The 50% RH level would be at the higher end of normal interior EMC values.
In cold weather climates when a heater is running, interior conditions may be even drier than 6% EMC unless the air is humidified. On the other hand, in humid summer months, especially without air-conditioning, moisture levels in homes, offices, and manufacturing facilities may sometimes exceed 9% EMC.
In most of the U.S., when wood is outdoors but protected from the rain, the wood will attain an EMC of 12% when the RH is 65%. In coastal areas, including parts of Florida, Texas, Louisiana, Washington, and island climates, the EMC is higher. In these more humid locations, wood indoors can reach a 12% EMC, while wood outdoors can attain a 16% EMC.
Wagner Meters’ Orion® 950 Smart Pinless Wood Moisture Meter has a built-in EMC calculator. This makes it easier than ever for professionals to calculate the EMC while they’re on the go.
Managing Wood Moisture
The best way to manage wood moisture content is to use a quality moisture meter and conduct a moisture test. Consider it good insurance for anyone involved with woodworking or flooring projects.
According to Dr. Gene Wengert, Professor Emeritus at the University of Wisconsin-Madison, 75% of all wood manufacturing and quality issues are related to moisture content. That means that a quality moisture testing meter is the perfect tool for spotting wood moisture issues before they turn into big problems.
Moisture meters are used to measure the percentage of water in a given piece of wood and are most useful when the percentage of moisture ranges from 5% to no more than 30%. When the moisture content is around 30%, wood reaches its fiber saturation point, or FSP. Moisture meter accuracy is greatly reduced when the wood moisture content is above the fiber saturation point. In fact, above this point, most readings are no longer even useful.
Wood in buildings usually has a 5% to 15% moisture content. The best range of moisture for interior furniture projects is usually 6% to 8% moisture content, with interior softwood millwork tolerating a slightly higher moisture content.
Depending on the type of meter (pin or pinless), either the naturally-occurring chemicals or minerals or the wood’s density can affect the accuracy of the moisture measurements.
Types of Moisture Meters
A quality moisture meter is the quickest way to moisture test wood. They come in two varieties: pin-type and pinless.
Pin-type meters use small electrodes (pins) that physically penetrate the wood’s surface. An electrical current then moves from pin to pin measuring the resistance.
Pin-type meters are able to measure the moisture content of wood in this way because while water conducts electricity, wood doesn’t. So, you can determine how dry wood is simply by measuring the resistance to an electrical current. The drier the wood, the more resistance.
Pinless meters use an electromagnetic sensor to scan the wood’s surface in order to determine its moisture content. Because they’re pinless they don’t damage the wood and are good for measuring things like expensive hardwood floors. Unlike pin-type meters, pinless meters are able to quickly scan large areas of wood.
We include more information on pin-type and pinless meters in the next section, including important information on how to read them.
Wagner’s Pinless Moisture Meters
Our pinless digital moisture meters are well known for their accuracy and reliability. They are widely used by everyone including professional flooring installers, cabinet makers, quality control inspectors, and hobbyists.
Our Orion series of digital moisture meters come with a 7-year warranty and include — depending on the model — features such as in-field calibration capability, the ability to measure moisture content from the surface to either .25” or .75”, the ability to store up to 100 readings (and store the minimum, maximum, and average values), Bluetooth®, an app, and the ability to calculate the EMC
How to Read a Moisture Meter
Pin-Type Moisture Meters
Pin-type meters are more sensitive than pinless meters to naturally-occurring chemical and minerals in the wood as well as wood temperature variations. A quality pin-type meter should always come with temperature correction charts. These are especially important when conducting moisture tests on wood that hasn’t been stored at room temperature.
Pin-type meters are also sensitive to the wood’s chemical makeup, which varies according to species. Therefore, pin-type meters should require the user to input the wood species before taking a moisture reading.
Pinless Moisture Meters
Pinless meters are more sensitive to differences in wood density (or specific gravity). Since density varies according to wood species, pinless meters require the user to set the meter to the correct average specific gravity before taking a measurement.
Wagner’s pinless moisture meters allow you to easily set the specific gravity from .20 to 1.0. This range allows you to measure the moisture content of nearly every currently known wood species including softwoods and temperate and tropical hardwoods.
What’s Your Targeted Wood Moisture Level?
For nearly all interior wood applications (and even some outdoor ones), wood needs to be dried to a moisture content appropriate for the environment where it will ultimately be used. The target (i.e., ideal) moisture content will depend on how the wood will be used and the average RH at the location where the wood will be used.
According to Dr. Gene Wengert, Professor Emeritus at the University of Wisconsin-Madison, wood should be dried to a moisture content that’s within 2 percentage points of the EMC of the location where it will be used. (The EMC of air is equal to the moisture content the wood will attain when stored indefinitely at a particular humidity level. A plus or minus 10-degree variation from normal 70-degree ambient temperature has no significant effect on the EMC.) Targeted moisture content will vary from region to region.
The average RH in most homes and offices in the U.S. — except for coastal regions and dry areas like the desert Southwest — is between 30% and 40%. This means the EMC ranges from 6.2% to 7.7%. In other words, wood in interior locations will equilibrate in this range. Therefore, lumber intended for interior use should be dried to a moisture content of approximately 6% to 8% and should be kept at this moisture content both prior to and during manufacturing.
NOTE: Softwoods machine better at a slightly higher moisture content and shrink and swell less than hardwoods when the moisture content changes. Therefore, the target moisture content for softwoods is usually higher.
Can a Moisture Meter Be Wrong?
So, how accurate are moisture meters? Well, that depends on the type of moisture meter (pin-type or pinless), the quality, and how it’s used. Even the best moisture meters can produce inaccurate readings if they aren’t used properly.
If your moisture meter isn’t calibrated, you won’t get an accurate reading. Therefore, all wood moisture meters must be calibrated before use. All Wagner moisture meters are factory-calibrated and ready to be used.
Accuracy of Pin-Type Moisture Meters
Because pin-type moisture meters are sensitive to variations in temperature, they should always be used with temperature correction charts.
Pin-type moisture meters are also sensitive to the chemical makeup of wood, which varies from species to species. Therefore, all pin-type wood moisture meters should ask the user to input the type of wood before taking a reading.
Failure to account for temperature variations and a wood’s chemical makeup will reduce the accuracy of even the best pin-type moisture meters.
Accuracy of Pinless Moisture Meters
Pinless moisture meters are sensitive to variations in wood density (also known as ‘specific gravity’). Therefore, they need to be set to the proper wood density prior to taking a moisture reading. Obviously, if the user fails to do this, the reading won’t be accurate.
Wagner’s Orion series of meters are the most accurate wood moisture meters on the market. They allow you to set the wood density so you can accurately measure every single wood species. This includes both tropical and temperate hardwoods as well as softwoods.
What Is a Normal Moisture Reading?
Furniture, cabinets, wood objects, and other woodwork used indoors typically require a wood moisture content from 6-8%. However, in high humidity coastal regions or other locations near large bodies of water this range may be extended to 10%.
Wood musical instruments, microphones, bowling pins, and other specialty items made of wood usually require a wood moisture content of 6-9%.
For hardwood floors, the moisture content typically ranges from 6-8%. When installing hardwood floors, make sure to check the moisture content of both the finished hardwood and the wood subfloor. Depending on the width of the finished hardwood, the moisture content of the hardwood flooring must be within 2-4% of the subfloor moisture level.
The moisture content of lumber used for framing, exterior construction, and exterior furniture usually ranges from 12-19%. Anything above 19% is susceptible to mold and decay.
Remember, wood experts say that moisture is to blame for at least 75% of all failed wood projects. In other words, the wood was either too wet or too dry.
Whatever the type of wood or project, a quality moisture meter will allow you to ensure that your wood is dried to the correct moisture content level prior to use.
Our Orion 950 Smart Pinless Wood Moisture Meter is the best in the industry and trusted by professionals. It comes with Bluetooth, an app for your smart device, an RH sensor, and an EMC calculator. Unlike other meters, the Orion meter can also be calibrated in the field to make sure that it continues to be accurate over time.
As Sales Manager for Wagner Meters, Ron has more than 35 years of experience with instrumentation and measurement systems in different industries. In previous positions, he has served as Regional Sales Manager, Product and Projects Manager, and Sales Manager for manufacturers involved in measurement instrumentation.