Measure the Moisture Content of Your Firewood With a Multimeter

If you heat with wood you are probably aware that you should only burn wood that has a moisture content of 20% or less. Of course you can buy a firewood moisture meter for about 50 dollars, but why do that when you probably have one already.

This instructable will show you how to measure the moisture content of your wood and come to a decision as to whether your wood is burnable safely in your stove.

All you need is a multimeter capable of measuring 10 megaohms, higher is better. A few brad type nails and a template with holes 1.25 inches apart. I made mine from wood but paper might almost work better.

Step 1: Why You Should Care About the Moisture Content of Your Wood

Burning wet wood in your wood stove or fireplace insert can lead to creosote build up in your chimney, resulting in reduced draft and possibly a chimney fire. At best it can require more frequent cleaning.

It also reduces the heat produced from your burning wood since the heat produced is wasted vaporizing the water trapped in the wood rather than heating your home.

The image is from a YouTube video of a real chimney fire. Nothing you ever want to deal with.

Step 2: How Wood Moisture Meters Work

Most wood moisture meters work by measuring the resistance of the wood. The resistance is a measure of the conductivity of a material. Conductivity cal be thought of as the ability for electrons to flow through a material. The higher the conductivity, the lower the resistance.

The water trapped in a piece of wood will have salts and nutrients dissolved in it that will allow electrons to flow through the water and conduct electricity. Pure water has a reisistance of 18 megaohms and is a fairly good resistor.

Most wood moisture meters have sharp metal electrodes that are driven into the wood. The resistance between the electrodes will give an indication of the amount of moisture in the wood.

Commercial meters like the one pictured are calibrated to convert their resistance reading to percent moisture content but we don't have to do that. You can have the one in the picture for about 50 dollars.

Step 3: The Resistance of Wood Correlated to Moisture Content

This table is from a US forest service publication which investigated various commercial moisture meters. It is fairly informative but the important information for us is this table. It shows the resistance values of different kinds of wood at different moisture contents. I have highlighted the column for 20% moisture content.

From the table we can see that for most common North American hardwood any resistance above 5 megaohms will indicate a moisture content of 20% or below. A resistance of 10 megaohms will make this statement true for most wood.

Reference: Electric Moisture Meters for Wood, US Forest Service. Author William L. James. General Technicl Report FPL-GTR- 6, 1988.

Step 4: Measuring the Resistance of Your Firewood

The distance between the electrodes is crucial for this experiment. The distance between the electrode used in the table is 1.25 inches. I made a template out of scrap plywood, drilling two small holes this distance apart.

For electrodes I used brad nails and drove them into the wood using the template as a guide. The template lifted off the nails afterwards. Set your multimeter to the resistance setting and touch the two nails. Since we are measuring resistance in the megaohms scale the resistance of the two nails will be negligible in our result.

If you have clip on attachments for your multimeter they will help but aren't necessary

Note that the water may be in layers between the grain of the wood and as you drive the nails deeper you can be sure your wood is drier and there isn't one layer of your wood hiding moisture. You also don't want to do this if it is raining because the surface moisture will negate your results.

The article I reference suggests driving the nail in 1/5 to 1/4 of the thickness of your piece you are measuring.

Before burning you can remove the nails.

Comments

[unreal_ed]

So what do you do with the wood that IS over 20% moisture? Wait for it to dry out? throw it out?

[rrfeury]

Yes you wait for a longer period before using this wood.
If you have some dry wood you can mix in some of the damper wood and use it that way if you are short on dry wood.
Check your flue often for build up and clean as needed.
I have found that cleaning my flue twice a season keeps it clean enough to avoid a flue fire.
I can actually look at my flue cover from the ground with binoculars and see if the cover is getting build up.
I also can tap on the pipe inside the house. If I get chunks of creosote dropping down the pipe it’s time to clean the pipe ASAP.
The main thing is to periodically get the chimney hot enough to burn the creosote out. Not after it gets real thick but before it gets too built up.
Your chimney, stove, wood and other details will tell you in time and use what you need to do.
I rotate my wood by keeping two separate covered areas for wood.
Dry and green. This season I use the wood from the dry section while the green drys. At the end of the burning season I move whatever wood is left from the dry pile into the green area.
I now have some wood that is dry to state my season off letting the wood behind it dry even longer. I then use the empty area to start filling up with new green wood.
It is also helpful to find standing or fallen dead timber that you can cut up.
Many times this wood is dry or close enough to dry to use right away.
Hope this is all helpful.

[jcharleb]

Just wait for it to dry out.

[MaxwellA11]

The method shown above is accurate enough for determining if firewood is dry enough to burn, but you should calibrate your own multimeter as I describe below.

The reason I say that is that the Forest Service chart from General Technical Report FPL-GTR-6 at www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06.pdf specifies inserting "needle" electrodes 5/16 inch (8 mm) deep, but it fails to say what diameter these "needles" are, so we can't duplicate it exactly. The size of the probes has been found to determine the resistance measured. With nails, the bigger the nail and the deeper you insert it, the lower the resistance. For certain reasons, it makes almost no difference what distance apart the nails are. There is somewhat lower resistance with the grain, so it's best to insert the probes consistently in line with the grain. The contact area is critical, so to avoid opening a crack in the wood and decreasing the contact area, the end of the nails can be filed a bit flat before hammering them in. File a mark on the side of each nail as a guide so it can be inserted to the same depth each time.

To calibrate your multimeter, choose a few typical sticks of firewood that you know to have varying moisture contents. For example, one green piece and one air dried for 12 months; or pieces dried for 0, 1, 3, 6 and 12 months. Split the sticks in half to expose the inner wood. From one of these pieces split a representative slice of the wood, less than an inch thick, to get a good sample of that stick. Number them with a marker pen. Insert the nails on the flat side of these pieces, about a third of the way in from the bark and a third of the way from the ends. Measure the resistance in megohms and write it down. Remove the nails and weigh the pieces in the kitchen or at a post office. Oven dry at about 350 degrees Fahrenheit (177 Celcius) for two hours. Weigh again. The moisture content (dry weight basis) is the difference between the weights, divided by the dry weight, times 100 for percent. Compare that to your resistance measurements, and make a table or graph if you like.

From then on, you can just use the nails and your multimeter to quickly estimate the moisture content of your wood. Remember to split the wood to get at the inside, to insert the nails, because the outside may be dry but not the inside.

If you're measuring a different type of wood than you used for the calibration, you can see what difference that might make from the Forest Service table, if you know what the species of wood are.

Usually, it's recommended to dry to 20% moisture or less, which should be the case if wood is cut and split and then dried for a summer, and then kept out of the rain and dew. (Firewood suppliers usually use moisture content based on WET weight, i.e. dividing by the starting or wet weight - firewood moisture should be about 17% or less on a wet weight basis.) By coincidence, wood with 20% moisture will lose about 20% of the heat up the chimney from driving off the water. Wetter than that and it gets harder to burn and gives less heat. A modern airtight stove will give about 50% more heat than an old fashioned stove.

Now, if your multimeter is inexpensive and only goes up to 100,000 ohms and not up to 20 megohms (20,000,000) then you will need to use Matthias Wandel's trick, "Wood moisture meter" at Woodgears. Instead of measuring resistance as above, you put a 9 volt battery in series and measure the voltage across the nails, instead of the resistance. https://woodgears.ca/lumber/moisture_meter.html Again, you can't use his chart of voltage for your multimeter, you have to do your own calibration as above, otherwise you could be off by 10% moisture.

[DavidF15]

The Forest Service pub is at http://www.treesearch.fs.fed.us/pubs/9823 and the table is on page 4 (or between pages 3 & 4)

[MaxwellA11]

Yes, the PDF is given there. And is also at https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06...

[AlexandruM27]

wood humidity has to be measured in AC. In DC measurements, electrolysis does affect the measurement.

[William Keogh]

Good idea to measure wood moisture with a common multimeter, but actually using the ohms function can give wildly incorrect measurements with wood that is wet. I've found that sometimes a significant voltage is generated between the nails (I guess its some sort of battery-like electrochemical effect) and this really confuses the ohms function. Swap the probes round to check if this is happening. I've seen timber read 0.1Mohm (very wet) one way and 100Mohm (tinder dry) the other way.

If this is happening I've found a more reliable method is to use a 9V battery to drive current through the wood and measure this with the current range (microamp) on the multimeter, then calculate the resistance (R= V/I).

Incidentally the article is incorrect that probe spacing must be exactly 1.25in. Actually any spacing is fine as long as the spacing is small compared to the dimensions of the piece of wood. (why? read up on resistivity measurement. Or just try different nail spacings and you'll see you get similar resistance).

The depth the nails go in DOES matter - with wood that is in the process of drying the surface is drier than deeper in so you want to get the nails well into the timber.

[ac-dc]

I agree that wood should be dry for many reasons mentioned, but it does not waste heat vaporizing the water. The heat used to vaporize water is also released into the air to heat it.

It will heat the room slower since wet wood burns slower but it will also burn longer.

[leonardhbl]

In a closed system, you would be right -- the heat that goes into heating and then vaporizing the water would be recovered when the water condensed and then cooled, warming the air around it. But the system you are interested in (your house) is *not* closed -- the heated and then vaporized water goes up the chimney, releasing all the nice heat it absorbed from your fire outside your house. So burning wet wood transfers heat from inside your house to outside ... not exactly the desired effect, at least in my house in the winter.

[lee78]

No, the heat used to vaporize water is wasted if your purpose is to raise the air temperature. While the heat from burning wood is raising the water temperature from room temp to boiling, that heat in the water IS added to the room and not waste BUT when the water reaches boiling temperature it stops rising in temperature. Lots of heat then goes to vaporizing the water at the boiling temperature. So the room gets humid with steam, but the heat used for vaporization would have gone in to heating air to a higher temp if the wood had been drier. Wet wood wastes heat because so much of the heat does not warm air or water, it just converts water to steam at constant temperature.

[przemek]

You're right that part of the latent heat released by burning the wood goes to boiling off the water, but hopefully that water doesn't go into your room---that would be a failure of your chimney system. The fire heats your house by radiation and heat transmission from the hot fireplace and chimney. The water lowers a little bit the burning temperature, but the main effect is that it depresses the combustion of the volatile compounds which promotes creosote deposition. Another bad thing is water condensation in the chimney leading to lining damage.

[BigRob215275]

Excellent idea and comments.

[wmccarthy]

The problem I find is that the reading increases the longer you hold the electrodes on. I am assuming that is because the current generated by the meter flowing between the electrodes produses a local heating, and therefore, drying effect through the cells carrying the current.

[kudzu63]

I was ALMOST excited when I saw the table giving different reading for various wood species BUT, I looked and looked and nowhere on there could I find cedar. Eastern red cedar to be exact and that's what I use most. Good instructable though, I'm pretty sure that I can find the resistance on cedar somewhere. Thanks.

[jaswant9]

Really good, but i am looking at your multimeter reading that is 10 and 13 and I can't seem to find anything relating that in the table, can you tell how will you co-relate your multimeter reading with the ones above . Are you only going to follow the 20% table only. ? and is type of wood be taken into account ?

[jcharleb]

I'm just using it as a burn, don't burn test. Basically I consider any reading over 10 megaohms as burnable. I'm not really trying to get a quantifiable reading of the moisture content of the wood I've also noticed that day to day precision the readings can drift a bit. Depending on how much rain we've had recently and such.

[blackslax]

Instead of creating a template that has through holes that would locate the nail-electrodes for each piece of wood to be checked - do you think it would be worthwhile to create a template that has the nail-electrodes permanently embedded in the template, with the points of the nails sticking out 1/4 of an inch? This way you can just whack the template onto the piece of test wood, measure the resistance, pry the template off, and then whack it onto the next piece of wood for testing.

[MarkH442]

Very interesting. Thank you.

I was just looking at the source
document that DavidF15 gave below (thank you) and there is an
interesting paragraph about the use of permanent electrodes (page 8 or
page 10 of the PDF version):

"...This method has a serious
problem, however, in that the permanent electrodes give erratic
and unreliable data after a few hours, especially when the
moisture content of the specimen is in the upper range of readability
of the meter. However, when the moisture content of the
specimen is no greater than about 15 percent, permanent
electrodes are as reliable as freshly implanted electrodes."

I'm
not sure if this is a weakness of the meters under test, the method or
the electrodes. I'm tipping towards the electrodes. Mainly because I
would expect moisture to seek the nearest flaw to escape. That is just a
theory on my part.

It would be an interesting theory to test.

[DDW_OR]

I wonder what the resistance would be cross grain vs with grain.

the nails in your photos show them being with the wood grain