More on Moisture Meters

Split 700
Always begin by splitting your sample piece in half to reveal the center of the wood you want to measure.

Earlier in the year we published our guide on how to use an inexpensive, pin-type moisture meter to reliably measure the moisture content (MC) of firewood. (See article here)

We’ve gotten nothing but positive feedback, however some NFA members had detailed questions regarding the true accuracy of our method.

Before going into more technical detail, note that our goal is simply to provide a sort of “home study course” to learn the skill of reading firewood. Spending some time with a moisture meter can help calibrate your abilities. What many people discover is that their firewood isn’t quite as dry as they thought it was.

Anyone can benefit from this exercise. Wood that’s obviously too damp to burn is easy to spot. It’s heavy, prone to mold, hard to light and smokes while burning — producing creosote and nasty particulates (soot). It spits and hisses. No one wants that. But many people do burn wood that smokes more than it should, or builds up creosote even though it appears to be burning just fine. This wood is in the mid-20% MC range, and it’s amazing how much better a wood fire burns when 5 – 10% of that moisture is lost.

True Accuracy

In our previous article, we noted that a large margin of error was to be expected. When you consider the many variables involved, the reason for this wide margin is apparent.

Relative humidity and temperature. Both of these variables can have an impact on the accuracy of your readings, and in many environments the readings you take today will be meaningless in a week. Wood may absorb moisture from the air or release moisture into it, depending on temperature and humidity.

The meters themselves have a +/- range of accuracy. Pin-type meters are only accurate up to about 25% MC due simply to how they work. Electrical resistance in wood fiber fails to remain an accurate indicator of MC when more than 25% of the material (by weight) is water.

We’re only measuring one piece at a time. If there are 1000 sticks in a cord of firewood, we’d have to measure 50 to get a sample of 5%. A cord of firewood cut from small logs (~8”) may contain wood from several different trees. With larger trees, samples cut from the heartwood and sapwood will have a very different MC.

The methodology itself is understandably imprecise. To get a truly accurate measurement you’d have to weigh the entire cord, kiln dry it until it stopped losing weight, then compare the before and after weight to calculate the MC the wood had to begin with.

Perhaps some day there will be something similar to a thermal-imaging camera that can scan a cord for moisture and get an accurate reading. Until then, the best we can do is learn and teach others.

I Don’t Need a Meter to Know Dry Firewood!
You’re probably right. But how do you teach this skill to others? Putting numbers on it gives people a reference and allows them to learn on their own.

The Knock Test
We consider the knock test — simply knocking two pieces of firewood together — to be the easiest indicator of dryness. If they ring like a percussion instrument rather than making a dull thud, you’re probably good to go. Measure it with a meter and see if you’re right.

With experience, people also gain skill in recognizing the lighter weight of dry wood.

Other indicators to look for:

Checking on ends — small, radial cracks give some indication the wood has been drying for a while.

Color – seasoned wood has a nice, silvery sheen. Black stains are mold. Kiln dried wood retains a golden color much like lumber.

Smell – It’s hard to sniff out dry wood, but wet wood is easy.

The Most Important Factor
It doesn’t matter how expensive a stove is or who approved it, if you fill it with damp wood it will disappoint you. Damp wood also takes the fun out of campfires.

Dry wood — appropriate fuel — is the single most important aspect of burning wood. The benefits are more heat using less wood, with less work, less smoke, and less buildup in the chimney.