What Are Moisture Meters?
Moisture meters are small, hand-held instruments — either analog or digital — designed to measure the moisture content of various materials including wood, concrete, paper, soil, and drywall.
Both professionals and hobbyists use moisture meters to make sure they produce quality work. This is because too much moisture (and sometimes too little) can cause problems with materials such as wood and concrete. These problems can end up costing both professionals and hobbyists a lot of money. Also, nobody wants to spend valuable time (hundreds of hours sometimes) on a project only to see it ruined later because the material used wasn’t at the proper moisture content.
Wood Moisture Meters
Why We Measure the Moisture Content in Wood
Wood is hygroscopic. That is, it’s something like a sponge. It both absorbs moisture from the environment and releases moisture back into the environment. As it’s doing this, it both expands and shrinks in size. When wood absorbs moisture from the environment, it expands. When it releases moisture back into the environment, it shrinks.
This absorption and release go on until the wood reaches a state of balance with the ambient environment. At this point, wood is neither absorbing nor releasing moisture. We call this the Equilibrium Moisture Content (EMC) of wood. Place any piece of wood into an environment and it will eventually come into balance with it. This might take a few months or even longer. However, it will eventually happen.
Wood’s hygroscopic properties mean that anyone who works with wood needs to make sure the wood they’re using for their projects has already reached its EMC before they start working with it. If they don’t, the wood will continue to either release or absorb moisture after the project is complete and this could very well result in serious deformities like buckled wood flooring or problems with cabinet doors and drawers.
One very simple way to determine the moisture content in wood — and avoid these problems — is to use a quality moisture meter that provides accurate moisture content readings. However, before we talk about moisture meters, we should talk a bit more about the EMC of wood.
An Introduction to the EMC
If you work with wood, whether as a professional or hobbyist, you need to understand how it interacts with its ambient environment. This is because — as we noted above — wood is hygroscopic. That is, it gains or loses moisture as the relative humidity of the environment changes. When this happens, wood either expands as it absorbs moisture from the environment or shrinks as it releases moisture back into the environment. When this process stops, wood has reached its EMC.
Dr. Eugene Wengert, professor at the University of Wisconsin Madison’s Dept of Forestry says wood should be dried to within 2% of the EMC where it will be finally used. In other words, wood should be dried to within 2% of the moisture content it would reach if it were left at that location long enough to come into balance with the environment.
|Relative Humidity of Environment||Equilibrium Moisture Content (EMC)|
As you can see, the moisture content wood will reach if left long enough in an environment is the same as the EMC. This is the point where the wood is in balance with the environment and is no longer absorbing or releasing moisture.
Looking at this chart we see…
If the relative humidity of the environment is between 33-39%, wood will eventually reach a moisture content (or EMC) of 5%.
At higher humidity levels, we see that the wood will eventually come into balance with its environment at a higher moisture content: 9% if the relative humidity is between 47-52%, 8% if the relative humidity is between 40-46%, and so on.
So, Dr. Wengert is saying that if the relative humidity of the environment where the wood will be finally used is — let’s say, 42% — the wood will need to be dried to somewhere between 7-11% moisture content. That is, within plus or minus 2% of 9%.
And, it will need to be kept at this moisture content during construction as well.
Now let’s take a look at the two different kinds of moisture meters…
Pin-type Wood Moisture Meters
Pin wood moisture meters have two pins that the user needs to push into the wood in order to get a reading. In other words, the pins need to physically penetrate the wood. When the meter is then turned on an electrical current flows between the two pins and measures the resistance.
Pin-style meters work because water with certain impurities conducts electricity, but wood itself doesn’t. Drier wood, therefore, offers more resistance.
The depth of the reading depends on the length of the pins. An external device called a hammer probe can be used for deep readings.
Pin meters are sensitive to wood’s chemical makeup and temperature. So, they not only come with temperature correction charts but will also ask you to specify the species of wood before taking a reading.
Because pin wood moisture meters poke holes in the wood they aren’t that useful for measuring the moisture content of things like fine wood furniture. However, they’re good for measuring the moisture content of lumber, firewood, and sometimes hardwood floors.
With denser species, it can be difficult to push the pins into the wood. Over time, pins tend to either bend or break and will need to be replaced. The cost of this can really add up if it happens often. Also, when the wood is wet a small amount of moisture can enter the wood as the pins are being pushed in and produce an inaccurate reading.
Pinless Wood Moisture Meters
Unlike pin-style meters, pinless wood moisture meters don’t need to poke holes in the wood to take a reading. While pin moisture meters only measure the moisture content between the two pins, pinless meters can quickly scan large areas of wood in no time. You might need to take several readings with a pin meter to cover the same area and each reading creates two holes in the wood.
Pinless meters are sensitive to wood density — also known as ‘specific gravity’ — and include a setting that allows you to specify the wood species prior to taking a reading.
Pinless meters can measure moisture content from the surface down to 1.5 inches, depending on the brand and model. While surface moisture on the wood can have an extreme effect on most pinless meters and will, therefore, need to be wiped off, there are a few high-end meters on the market that can ‘’overlook’’ surface moisture.
Wood Moisture Meter Calibration
Both pin and pinless meters must be correctly calibrated for an accurate reading. Meter manufacturers often sell a calibration verification device that you can use to check if your meter is in calibration. Some meters also include a built-in calibration check.
A moisture meter that’s out of calibration usually needs to be sent back to the manufacturer for re-calibration. However, a few high-end pinless moisture meters on the market offer a self-calibration feature that allows the user to re-calibrate the meter without sending it back to the manufacturer.
How Wood Moisture Meters Work
How to Use and Read a Moisture Meter
Pin Wood Moisture Meters
To use a pin-type moisture meter, insert the pins into the wood, turn on the meter and take a reading. Remember, the meter is only measuring the moisture content between the two pins. Therefore, unless you have a very small piece of wood you may need to take several readings and then average them in order to measure the wood’s moisture content.
Pinless Wood Moisture Meters
To use a pinless moisture meter just turn it on, place the scanning plate on the wood and take a reading. Move the meter to take another reading, and so on.
Moisture Meter Accuracy
Moisture meter accuracy depends on the type and quality of the meter and the material being tested. Moisture meters are useful for measuring moisture contents that range from a low of 5% to a high of 30%. Moisture content above 30% is the Fiber Saturation Point (FSP) of wood and at this point, accuracy goes way down. In fact, when the wood’s moisture content is above 30%, a moisture meter is essentially useless.
Accuracy of Pin Moisture Meters
Pin moisture meters are sensitive to wood temperature and cheap pin moisture meters are extremely sensitive to temperature changes in wood. This is why all pin meters should include temperature correction charts.
The accuracy of pin-type moisture meters is also affected by wood’s chemical makeup and before taking a reading you’ll need to tell the meter what species of wood you’re measuring.
If you’re using an external hammer probe with an attached cable for a deep reading you’ll need to be careful how you hold the cable. This is because it has an electromagnetic field around it that could affect the accuracy of the reading by up to 2%.
Finally, when measuring relatively high moisture content, pin-type meters are susceptible to drift. In other words, the reading is unstable and will change before your eyes. Manufacturers recommend taking a reading right away before it starts to drift. However, depending on where you’re working, this might be difficult to do.
Accuracy of Pinless Moisture Meters
As we mentioned above, pinless meters are sensitive to wood density, also referred to as ‘specific gravity’. Because this varies according to wood species, you’ll need to input the correct specific gravity setting before taking a reading.
Surface moisture can affect the accuracy of pinless moisture meters and should be wiped off the wood before taking a reading. However, some high-end pinless meters do have the ability to ‘see past’ small amounts of surface moisture to ensure greater accuracy.
Quality wood moisture meters are accurate if used properly. Cheap meters can be quite inaccurate.
If you’re measuring the moisture content of firewood only, you can probably get away with buying a cheap wood moisture meter since accuracy isn’t that important. Even if it’s several percentage points off, that’s probably OK for firewood. However, professionals and hobbyists need accuracy, something cheap moisture meters can’t offer.
How to Use a Moisture Meter on Drywall
Drywall is very sensitive to moisture. In fact, it’s so sensitive that even 1% moisture change can damage drywall. If that happens, it will need to be torn out and replaced. Therefore, it’s important to find the moisture and remove it before it has a chance to damage the drywall.
If you’re using a moisture meter designed for drywall…
Push the pins into the drywall and turn on the meter. If you notice any moisture, continue taking measurements until you get an idea of where the moisture boundary lies.
If you don’t have a meter designed for drywall…
Even if you don’t have a meter designed to measure the moisture content of drywall you can still get a qualitative picture of the moisture problem. It’s not precise, but it will tell you if there’s indeed a moisture problem. All you need to do is take a moisture content reading on an area of drywall you’re sure is completely dry and then any area that differs significantly from this has a moisture problem that needs to be corrected.
Features to Look for in a Quality Moisture Meter
Accuracy is the most important feature to look for in a wood moisture meter. Higher-end meters are the most accurate. Cheap meters are the least accurate.
It’s Built to Last
Quality moisture meters are built to last. They have durable cases that hold up under heavy use.
Pins Are Sturdy and Easy to Install/Replace
The pins on higher-end pin-type moisture meters typically last longer because they’re sturdier. However, they still break rather easily. In fact, over the lifetime of your meter, you’ll probably end up spending as much on the pins as you did on the meter itself. This is especially true if you use the meter regularly.
Reads Moisture Content up to 30% and All the Way Down to 5%
Quality meters can read moisture content up to 30% and down to 5%. The higher the quality of the meter, the lower the moisture content it will be able to read.
It’s Easy to Use
Quality moisture meters are easy to use. You’ll like the meter’s size, shape, and the way it feels in your hand. It will easily fit in your pocket or attach to your belt.
It Comes with Good Documentation
Good documentation — whether offline manual or online — is important. It will be well-organized and searchable, if online. Customer support will be available for anyone who can’t find an answer in the documentation.
The Battery Is Easy to Install
The battery of a quality moisture meter will be easy to remove and replace.
The Display Is Easy to Read
Even when working in tight spaces the meter’s display will be easy to read.
Easy Calibration Check and Re-calibration
If it’s a quality meter, you’ll know if it’s properly calibrated or not. If it’s not properly calibrated, you’ll either be able to recalibrate the meter yourself or send it to a service center.
It Comes with a Good Warranty
When it comes to warranties, the longer the better. Top-end moisture meters include warranties up to 7 years.
The best moisture meter will always be, above all else, an accurate moisture meter. Although less expensive meters might have visible features that seem attractive, the invisible features of accuracy, reliability, and consistency are what really matter.
If you’re currently in the market for a moisture meter, don’t buy one until you check out our reviews (including both pros and cons) of 9 top tested meters.