How to Understand Moisture Meter Readings
You’ve measured the moisture content of your wood using a moisture meter, and you’re looking at the number on the screen.
What does the number mean? And how does it make a difference in your project?
We’re here to help you understand just that so you’ll be informed and confident about the following key ideas:
- What moisture meter readings mean
- Factors that affect moisture readings
- What to do with moisture readings
What do the moisture meter readings mean?
Moisture meter readings indicate the moisture content of a piece of material—wood, or sometimes concrete, gypsum, or drywall. Moisture content (MC) is the amount of water in a piece of wood. It is the weight of the water divided by the weight of the wood, expressed as a percentage. This number shows up on the moisture meter.
Here’s an example:
A piece of wood weighs 12 lbs. But 2 lbs of that is water weight. Thus, the wood itself weighs only 10 lbs and has an MC of 20%.
Depending on the moisture meter, the MC may display this number on an analog scale or on a screen as a digital number.
Let’s understand the reading scales for wood versus those of other materials.
Most moisture meters have a reading scale, ranging from about 6% to 32%, that’s dedicated to wood. Before using the meter, be sure to verify calibration with a calibration reference tool.
You’ll need to know what type of wood you’re going to measure so that you can adjust the meter for an accurate reading. (Manufacturers usually calibrate moisture meters for Douglas fir.) Some meters will allow you to enter the species number, and then they will automatically correct the readings for the species. If the meter does not have that function, then you will need to use a species correction chart.
In general, moisture meters are accurate for wood moisture readings up to about 32%.
Why this number?
It is the fiber saturation point (FSP) of wood.
When a tree is first cut down, its wood holds water as both free water and bound water. The free water is liquid water in the cavity of the wood and is the first to evaporate. On the other hand, bound water is bound within the structure of the wood cells and takes much longer to release.
The wood is at its FSP when it no longer has free water in its cavity but all the cell walls are completely saturated with bound water. At this point, it’ll have an MC of about 32%. Moisture meters work well for readings below this.
How about with other materials?
Though many moisture meters are designed for wood, they can also measure the moisture in other materials, such as concrete or gypsum. The reading will display on a relative scale. In other words, the number does not indicate the actual percentage of moisture in the material. Instead, it’s an estimate that helps you to know whether the moisture level is high or low.
If you measure the moisture content of concrete, some meters may show the reading on a relative scale of 0 to 100, with 0 being completely dry and 100 being completely saturated.
For measuring the MC of sheetrock or drywall, meters may have a relative scale for gypsum that goes from 0.2 to 50.
Aside from the material itself, other factors affect MC readings too. We’ll look at those next.
Factors that affect wood moisture meter readings
Factors that affect moisture meter readings include wood species, temperature, surface moisture, and depth of measurement. Most of these factors are specific to the type of meter you have—either a pin meter or a pinless meter. Knowing their differences will help you understand what impacts their results.
Pin meters have two electrodes (pins) that have to be pressed or pounded into the wood when measuring moisture content. The amount of electrical resistance between the two pins determines how much moisture is in the wood.
This kind of meter is sensitive to temperature, so it may need to be adjusted accordingly if the wood is not already close to room temperature. Sometimes, there’s a way to do this on the meter itself. But if not, you can use a temperature correction chart after measuring the wood.
Pin meters can have either insulated pins or uninsulated pins. The insulated pins are useful for giving you the moisture gradient of a piece of wood since MC can vary from the surface to the center of a piece of wood. The tips of the pins can be inserted to different depths to measure the MC at those points.
This feature of insulated pins is particularly useful when the wood has water on its surface. The pins are able to get readings beyond the surface of the wood.
Pinless meters use an electromagnetic sensor, instead of an electrical current, to measure moisture content.
Unlike pin meters, changing temperatures have little effect on pinless meters. However, surface water and uneven surfaces can affect readings.
To get the most accurate reading with a pinless meter, be sure that you place it on a flat, dry surface.
Some pinless meters will allow you to adjust the depth at which you measure, while others are set at one depth. Check out the Bessemeter DS500, which can measure at either ¼” or ¾” deep.
What to do with wood moisture readings
The readings you get from your wood moisture meter will determine the next steps you take in your project. They help you to know whether the wood is ready for use or needs to go through a process of acclimation. The wood’s readiness will also depend on the type of project it is being used for.
Follow these two simple steps after you’ve taken your moisture meter readings:
- Determine the final destination and purpose of the wood.
- Compare the moisture content reading with the EMC (equilibrium moisture content).
1. Determine the final destination and purpose of the wood.
The final destination and purpose of your wood will tell you its required EMC. The EMC is the MC that the wood will eventually reach at a certain temperature and relative humidity. If the wood’s MC is at or close to the EMC, this will prevent your final product from becoming damaged by losing or gaining too much moisture.
What will the wood be used for? Will it be indoors or outdoors?
And if outdoors, what is the EMC for the area it’ll be in?
Wood used in the construction of buildings is generally about 15%.1 But it cannot exceed 19%, the point at which it’ll start to become susceptible to mold growth.2
Lumber that is used in wood products needs to be much lower. For many locations, the lumber needs to be dried to between 8 and 12%.
For wood products that will be used outdoors, the EMC may be 9–14%, though the region will narrow this number down. Use an EMC chart of the US to help you in this step.
A common target moisture content is between 6% and 9%—typical for an indoor environment that has a relative humidity of 30–50% and a temperature of 60–80° F.
This EMC applies to furniture, specialty items (e.g. musical instruments, bowling pins), and wood flooring.
When it comes to flooring, keep in mind that the wood must have an MC within 2–4% of the subfloor’s MC.
2. Compare the moisture content reading with the EMC.
After measuring the moisture content of your wood, compare that number with the EMC you determined for your project.
How close is your wood to reaching the EMC?
The answer to this question will govern the next steps you take.
If the wood’s MC is far from the required EMC, consider selling the wood and purchasing something with an MC that fits your needs. Doing so will save you the time and effort required to acclimate the wood.
If the wood’s MC is fairly close to the EMC, you may decide to acclimate the wood yourself so that you can use it. See our article on acclimation for more details.
And if the wood’s MC has reached the EMC, then you’re good to go! You can begin your project.
Measure moisture content with confidence
Now that you’ve gotten an overview of moisture meter readings, you’re ready to launch into your project with confidence. You know what those numbers on your meter mean. And you know how to use them to make wise woodworking or flooring decisions.
But remember—no matter how well you understand your moisture meter readings, they won’t benefit you if you have a low-quality moisture meter. Inaccurate readings may hurt your project instead of helping it.
To avoid this challenge, check out our shop for a moisture meter that you won’t have to stress about.
Less time and money dealing with problems, and more time and money to perfect your project!