# Measure the Moisture Content of Your Firewood With a Multimeter

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## Introduction: 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.

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## Questions

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.

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.

3 replies

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.

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.

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.

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.

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

Just wait for it to dry out.

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.

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 ?

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.

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.

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

"...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.

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

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

2 replies

According to the Forest Service publication, "

The conductance of wood parallel to the grain is about

double that of conductance perpendicular to the grain.

Ratios of conductance perpendicular to the grain, in

relation to the longitudinal value, are about 0.055 for

thanks

it is nice to see a government study that is actually useful

nice idea but like JmsDwh's concerns, the process of driving nails into the wood to 20 or 25% of depth makes me wonder about the reliability of commercial meters with their inability and non-use of this process to get "good" readings. It seems that a simple comparison with a well known moisture meter would be a great adjunct experiment to publish!