We do a lot of talking about the moisture content (MC) of firewood, and for good reason — but do you know the actual moisture content of the wood you’re selling — or burning? Kiln-driers are quite familiar with probes that require drilling pilot holes and specialized equipment to read the data. We can’t claim to match the accuracy of these established methods, but we can come close.
We’re using a commonly available moisture meter that costs around $25.00. While pro-grade meters can cost hundreds, the major difference is in build quality and long-term reliability. On the inside, a moisture meter is simply a specialized ohmmeter with a mathematical algorithm to make it read in percentage of moisture detected based on the electrical resistance between the pins.
One thing to know about moisture meters is that readings become inaccurate below about 9% and above 25% or so. That’s okay, because we’re mainly interested to find out if a given piece of wood is suitably dry enough to burn. By taking four readings, we can determine the actual moisture content with a fair degree of accuracy.
Moisture inside a piece of firewood exists as a gradient — if you could see it, the moisture contained would resemble the shape of a football — with the most moisture in the middle and tapering off toward each end. With this in mind, we know that we should measure near one end and then in the middle to get a good average.
The industry-established way to measure moisture is with insulated probes driven halfway through the thickness of the wood to measure the core. Here, we are simply splitting the sample to reveal the core, approximating the accepted method.
Start by measuring the freshly exposed face (from splitting) in the exact center of the sample. Since this is always going to be the dampest part of any piece of seasoned wood, a reading of 20% or less here means your wood is plenty dry enough to burn. If it reads above 20% — or you want an accurate estimate of the exact number, you will want to take more readings. Remember that numbers above 25% or so are not accurate.
The second reading is taken 1″ from one end of the piece. While you might think it should be measured at both ends, this would skew the results by over-representing the drier state of the ends when compared to the middle.
If the two readings from the fresh face average out to less than 20%, you’re good to go. Note that it is possible for very dry wood to absorb moisture from the environment, so theoretically it would be possible that the ends could be more damp than the middle.
To get an accurate estimate of the total moisture content of your sample, you will repeat the readings (center and 1″ from one end) on the weathered face, the average all four numbers by adding them together and dividing by 4.
Finally, it must be understood that this measurement only indicates the MC of one single piece of firewood. If you wanted to know the average MC of an entire wood pile, you’ll need to measure several samples from the top, the middle and the bottom of the pile — seven or eight samples per face cord.
While any one of the readings taken with a meter is expected to be accurate within +/- 1%, a 5% margin of error is considered acceptable due to the nature of wood fiber structures within, species, temperature, and relative humidity. If the temperature is much below 50F, you’ll want to correct your figure up about 1% — and if it’s much over 80F, correct it down by 1%. Similarly, some species measure a bit differently and require correction of about 1-2%. These variables illustrate the reason that a large margin of error is acceptable.
The inescapable conclusion is this — it is a lot easier to determine whether wood is dry enough to burn using your eyes and ears than it is to use a meter. Experienced wood burners immediately recognize the musical ring that two pieces of dry wood make when knocked together — and using a meter on wood that rings properly would show a reading of less than 20% MC. A visual examination of this ‘ringing’ wood would show a grayish coloration and tiny cracks (checking) in the end grain.
Then why mess with using a meter? Personal education is one reason. You can test your skill at judging dry firewood against actual numbers, and you will learn quickly how to identify it without a meter. A better reason is because this data is quantifiable. It would be hard to argue with a firewood supplier that the wood they delivered just doesn’t sound right. With actual numbers, you can prove that it either is or is not dry enough to burn. Wood over 20% MC is hard to light, and over 25% will make a sizzling sound when it burns. At 30% you will see water bubbling from the end.
Also remember that it is possible for wood to be too dry, although not likely in any typical scenario. Wood that is too dry burns too intensely, using up all the available oxygen in the stove before the gaseous and airborne byproducts of combustion can be burned in the firebox. The result is increased smoke output. If there is, by chance, enough oxygen available, burning very dry wood results in a very hot fire that’s difficult to control and can warp and crack your stove and/or chimney.
Shoot for wood that’s between 15 -20% MC for the best results.