Introduction
When it comes to moisture meters for wood, accuracy is extremely important. In fact, the most important feature of a moisture meter is its ability to accurately measure moisture content.
Inaccurate moisture meter readings are completely useless.
Quality wood moisture meters provide readings that are both accurate and consistent. Cheap meters don’t. It’s really that simple.
However, the accuracy of even top-end moisture meters can be affected by the type of wood, the temperature of the wood and the ambient environment. Cheap meters are even affected by the temperature of the meter itself. Also, surface moisture on the wood can affect the accuracy of both quality pin and pinless meters.
Woo Moisture Meter Accuracy
Pin Meters
All pin moisture meters are sensitive to the temperature of the wood being tested. This is why all pin meters should come with temperature correction charts or ways to correct for wood temperature in order to ensure accurate readings. Many cheap pin meters do not come with temperature correction charts.
Cheap pin meters are also sensitive to the temperature of the meter itself. If you leave your pin meter for hours on the hot dashboard of your car and then take it into an air-conditioned environment, you’ll need to let it cool down first before taking a reading. If you don’t, your reading will almost certainly be inaccurate. (The temperature charts that come with pin meters are for wood, not the meter.)
The chemical composition of wood can also affect the accuracy of pin meters, and because of this, you’ll need to input the particular wood species before you take a reading.
If you use an external hammer probe with an attached cable for deep readings, you’ll need to be careful about how you hold the cable. The cable contains an electromagnetic field around it, and if you hold it the wrong way, the reading could be off by up to 2%. Consult the manufacturer’s instructions for more information.
Finally, pin meters are susceptible to drift. In other words, the reading can be unstable and might change as you’re looking at the meter. Some manufacturers recommend taking the reading immediately after inserting the pins before it has a chance to drift. However, this might be difficult at times.
Pinless Meters
All pinless meters are sensitive to wood density, also called ‘specific gravity.’ Because this varies from species to species, you’ll need to input the specific gravity of the wood you’re measuring in order to get an accurate reading.
Pinless meters are also sensitive to surface moisture. If there’s any moisture on the surface of the wood, wipe it off thoroughly and wait 30 minutes before taking a reading. (There are a few high-end pinless meters that are not as sensitive to surface moisture. If you’re using one of these, you’ll have much more accurate readings when measuring wood with some surface moisture.)
So, are both pin and pinless wood moisture meters accurate? Yes, if they’re quality meters.
The moisture readings provided by quality pin and pinless meters are accurate if the meters are used properly. Cheap meters aren’t accurate even when they’re used properly. So, why would anyone want a cheap moisture meter? Well…
If you’re only testing the moisture content of firewood, you can probably get away with a cheap meter because accuracy isn’t important. You’re simply trying to find out if your firewood is green or not. A cheap meter can probably do this for you. However, if you’re a woodworking professional or a serious hobbyist, you’ll need an accurate meter, which is naturally going to be more expensive.
How Moisture Meters Detect Moisture
Pin Meters
Pin-style moisture meters contain two small pins that must physically penetrate the wood. When the meter is turned on, an electrical current flows between the two pins, and the resistance is measured.
Pin meter technology takes advantage of the fact that water (with salt and impurities) conducts electricity, but wood doesn’t. This means that drier wood will show more resistance to the current.
One drawback of pin-type moisture meters is the fact that you’re only measuring the resistance between those two pins. So, if you’re trying to measure the moisture content of a sizable batch of wood, you’ll need to take a lot of readings and create many holes. This is why pin meters aren’t that useful for measuring the moisture content of furniture. However, they are sometimes used on wood flooring on the backside of the boards before installation.
Another drawback of pin meters is that the pins tend to bend and break, especially when they’re used on denser wood species. When this happens, they need to be replaced and this will cost you time and money.
Pinless Meters
Pinless moisture meters use an electromagnetic sensor to test wood moisture levels. The smooth scanning plate glides over the wood and reads the moisture content of the wood without poking holes in it.
Because a pinless moisture meter provides a non-penetrating, non-invasive method for measuring moisture content, it’s perfect for things like expensive furniture and fine hardwood flooring. It can also quickly scan large areas of wood.
Probably the biggest reason people purchase pinless meters is that they are so much easier to use. It takes a lot of work to drive pins into the wood in many places in order to get a proper amount of data to determine the suitability of the wood for use.
How to Accurately Test for Moisture Using a Moisture Meter
Pin Meters
- Turn on the meter and enter the setting for the particular species of wood you’re measuring.
- Carefully push the pins into the wood so they don’t bend or break. Keep in mind that this might be difficult with denser wood species.
- View the display and record the reading.
- Repeat if necessary. Remember, depending on the size of the piece of wood you’re measuring, you may need to take several readings (and then find the average) to get an accurate picture of the wood’s moisture content.
Pinless Meters
- Turn the pinless moisture meter on and enter the specific gravity setting for the type of wood you’re measuring.
- Press the scanning plate to the surface of the wood and take a reading.
- Move the meter to another place on the wood and take another reading.
- Continue using the pinless moisture meter in this manner until you have an accurate picture of the wood’s moisture content.
Calibration and Moisture Meter Accuracy
Moisture meters — both pin and pinless — need to be properly calibrated if they’re going to provide accurate readings. If the meter’s accuracy is off by even 2%, you could have problems. That means users need a way to determine if their moisture meter is out of calibration and if it is, they need to know how to correct it. So, let’s talk about how users can tell if their meter is properly calibrated.
The manufacturers of quality meters (both pin and pinless) offer calibration blocks that are used to determine if a meter is properly calibrated, or not. They’re easy to use and will quickly let you know if the meter needs recalibration. (Be sure to use a calibration block designed for your meter though. Calibration blocks are not one-type-fits-all.)
If your meter is out of calibration, it will probably need to be sent back to the manufacturer for recalibration. However, certain high-quality pinless wood moisture meters offer users the ability to re-calibrate the meter without sending it back to the manufacturer. In other words, you can re-calibrate the meter yourself.
So, how do we know that any particular moisture meter is accurate or not? Is there some kind of standard that they’re measured against?
Yes. It’s called the oven-dry test. Here’s how it works.
A piece of wood is weighed and then placed in an oven. The wood is then removed periodically and weighed each time. (As it’s drying, the wood will start to weigh less.) When the wood’s weight stops changing, it’s weighed and then that weight is compared to the weight before going into the kiln. The difference between those two weights is the moisture content of the wood.
But…
A meter’s accuracy cannot be determined from testing just one piece of wood. This is because even wood from the same species varies in terms of density, chemical makeup, etc. Therefore, to truly test a wood moisture meter’s accuracy requires putting hundreds of samples from different trees through the oven-dry test and comparing those results to the meter’s readings.
The closer a moisture meter gets to the results obtained from the oven-dry test, the more accurate the meter.
Accuracy is a moisture meter’s most important feature. High-quality wood moisture meters are an accurate tool to help people that need to know the moisture content of wood. Cheap meters can have significant accuracy problems. Some cheap meters can be highly inaccurate. Therefore, unless you’re only going to be measuring the moisture content of firewood to simply determine if it’s green or not, you should stay away from cheap meters. Invest in a high-quality moisture meter that’s going to provide you with accurate moisture measurement readings. It will protect the integrity of your projects and if you’re a woodworking professional, your reputation as well.