Using a multimeter to measure wood moisture level

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Skier76

Minister of Fire
Apr 14, 2009
1,468
CT and SoVT
I'll have to post a pic of the multimeter I have. I can't get a reading to save my life. But the thing actually has some decent sharp probes on it. I'm probably using the wrong setting. Electrics at times confuse the you know what out of me. %-P
 

quads

Minister of Fire
Nov 19, 2005
2,744
Central Sands, Wisconsin
Skier76 said:
I'll have to post a pic of the multimeter I have. I can't get a reading to save my life. But the thing actually has some decent sharp probes on it. I'm probably using the wrong setting. Electrics at times confuse the you know what out of me. %-P
My meter has sharp probes too (so much so that I have drawn blood more than once when reaching for it in my tool pouch). But I could not get a reading by just poking them onto/into the surface of the wood. I had to drive a couple nails in, then it worked.

The other possibility is that your wood is ready to burn and in that case the resistance is out of the range of your meter.
 

Valhalla

Minister of Fire
Skier76 said:
I'll have to post a pic of the multimeter I have. I can't get a reading to save my life. But the thing actually has some decent sharp probes on it. I'm probably using the wrong setting. Electrics at times confuse the you know what out of me. %-P
The more moisture in the wood the lower the ohmic value. The lower numbers read on your scale. It is therefore more conductive because it is "wet."

The drier the wood the higher the resistance (ohmic value). The higher the numbers on your ohmmeter scale. It will be more of an insulator when seasoned properly.

Wood as a resistive insulator is inversly proportional to the amount of moisture in it. More moisture, the lower the relative resistance. Drier, better seasoned wood is our goal and identified with a high resistance.
 

precaud

Minister of Fire
Jan 20, 2006
2,307
Sunny New Mexico
An interesting update on the crab apple split that measured 80k Ohms and felt wet to the touch on Nov 1st: After 7 days sitting on top of my wood pile (sunny mid-60's days, upper 20's to low 30's evenings), it now measures open circuit (above 20M Ohms) in the same holes, has multiple deep cracks on both ends, feels dry, and has a sharp tone when struck. It's ready to burn. That's some quick dryout!
 

Graf Spee

Member
Nov 29, 2009
6
Eastern Pa
This sounds like an interesting idea. I have a craftsman dmm, what setting should I select. My initial attempt didn't get me any reading at all.
 

precaud

Minister of Fire
Jan 20, 2006
2,307
Sunny New Mexico
No reading may mean your wood is nice and dry. To get comfortable with the technique, it may be best to try it on unseasoned wood first.
 

DBoon

Minister of Fire
Jan 14, 2009
1,191
Central NY
This sounds like an interesting idea. I have a craftsman dmm, what setting should I select. My initial attempt didn’t get me any reading at all.
Hi Graf Spree, double check that you have your DMM on "ohms" or resistance mode. Put the probes together and it should read 0 ohms. That's a good check.
 

DBoon

Minister of Fire
Jan 14, 2009
1,191
Central NY
I checked my hickory in the open meadow. Wood that I had split from rounds yesterday was 16-20 kOhms of resistance, which is off the charts on the USFS PDF. I’m guessing that this is about 40% moisture (or more).

Then I checked wood that I had cut and split into ~4-6” pieces (bigger than I would burn, but small enough to dry) in the late spring, early summer. I just covered this wood 2 weeks ago, and we had a rainy summer so it got a lot of moisture. A split was measuring about 350 kOhms. That would equate to ~27% moisture content (it’s off the chart, so I extrapolated). Then, I checked a 5-1/2” round - I split it in half and checked the moisture on the inside - 20 kOhms. This is essentially as moist as wood that was split a day before. Lesson learned - split the rounds as soon as possible. Luckily, I only need half this wood for next winter, so I’ll resplit rounds in the spring and restack.
My previous post above was from November 1st. I just went up and checked the moisture content of a split about the same size as what I checked before, and on the same side of the pile. As before, I split it in half and checked it on the inside. The split was measuring 440 kOhms. That equates to 22% moisture content per the USFS document. So, in two months, moisture content has gone from 27% to 22%. This wood is definitely going to be ready to burn next winter.
 
thought id add my two sense(pun intended)
i work in the hardwood lumber industry,hence we are always strive-ing to ship lumber at the perfect moisture level(6-8 % in the the case of hardwoods for moulding,cabinets, cut to size components etc.) all this talk of moisture meters and multi meters(huh) reminds me that there is only one true way to measure accurate moisture levels in wood...the simple oven test.... green weight minus dry weight divided by dry weight multiplied by 100...moisture meters try to estimate poorly the info this technique provides...if any -one wants the low down on how to perform this test,i,ll update this post....
rod
 

Battenkiller

Minister of Fire
Nov 26, 2009
3,739
Just Outside the Blue Line
This is all very cool stuff.

As a woodworker for over 30 years, I am always a little bit embarrassed by:

A. I don't actually own a copy of Bruce Hoadley's book "Understanding Wood"

B. I don't own a moisture meter

One of the reasons I don't own a meter is that most all of the wood I use is either many years old (musical instruments) or dead green (bowl turning). With the instruments, if I even wonder if the wood is ready to use, it isn't.

So I go down to the shop to grab my multimeter and, low and behold, it ain't working. Powers up, but won't even measure the voltage of its own replacement battery. I'm going to have to get a new one and play around with this a bit. Thanks for posting this, precaud. I used to work at Woodcraft and I was always tempted to buy one of those $200 ones that read the wood without poking holes in it, but guys would return them all the time and say they had problems with them. All I need is the go/no-go info, and this will give me what I need.
 

tjsquirrel

New Member
Oct 17, 2009
3
CT
DBoon said:
Just checked some of my wood supply with a digital multi-meter to see if my moisture content is in the target range.

Cherry - 18 months old, spent half the time in a garage, half the time outside covered and in the sun half the day. DMM reading: 9 to 15 megohms, even on the inside of the split. Well under 20% moisture content

Sugar Maple - 18 months old, half in garage, half outside (like the cherry). DMM reading is 6-9 megohms, which equates to 16-17% moisture content.

Pine - 18 months old, stacked outside in sun and wind. DMM reading is 20-25 megohms, which equates to 15-16% moisture content.

Sugar Maple - 5 months old (fresh cut and split at that time). Stacked outside in the driveway, uncovered the first 3-1/2 months, covered last 1-1/2 months. DMM reading is 0.6 to 0.9 megohms, which equates to 24-25% moisture content.

Will have to check my hickory that is stacked in windrows in an open meadow, and now covered for the last 2 weeks. That was fresh cut and split this year, so it will be interested to see what that is right now.
DBoon, any idea what resistance reading you would see with Cherry around 20%? I have a bunch of cherry to check and am not sure which species to compare it to on the FS chart.
 

DBoon

Minister of Fire
Jan 14, 2009
1,191
Central NY
FYI, here is the link to the referenced forest service document http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06.pdf

As you saw, Cherry was not listed there, but based on the observation from someone else, and my own reading of the table, with few exceptions you are good to go if your resistance reading is 3 Megohms or higher. Hickory (which I have a lot of) is one of those exceptions.

In any case, my experience is that cherry seasons and dries pretty fast.
 

pyper

New Member
Jan 5, 2010
491
Deep South
My multi-meter only goes to 2 megaohms, so it's not especially useful for this test, and the wood I really want to measure (flowering pear) isn't in the list anyway, but I figured out an easy way to make the reading.

Accurate results seem to depend on accurate hole placement. The author mentions that all his tests were done with 5/16 probes, 1.25" apart. Well, at first I tried drilling holes, but then I realized it's easier to just drive two 5/16" long screws into the wood, take the measurment, and back them out.
 

gpcollen1

Minister of Fire
Oct 4, 2007
2,026
Western CT
I think that the moisture meters are a waste of time in general unless you are purchasing your wood and want to verify the moisture content on delivery. We know that much of the tree species will season pretty well in one season and oak especially takes 2 seasons to get the moisture content down. For those that process your own wood, do you really need a moisture meter??

So for me, this is a pretty good trick to show b/e there is no way I am buying a moisture meter. If I every get that curious, I can just do it this way - but I doubt it.
 

Battenkiller

Minister of Fire
Nov 26, 2009
3,739
Just Outside the Blue Line
DBoon said:
FYI, here is the link to the referenced forest service document http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06.pdf

As you saw, Cherry was not listed there, but based on the observation from someone else, and my own reading of the table, with few exceptions you are good to go if your resistance reading is 3 Megohms or higher. Hickory (which I have a lot of) is one of those exceptions.

In any case, my experience is that cherry seasons and dries pretty fast.
Me, too.

Wish I had a resistance number for the cherry.

I went out and bought an inexpensive DMM to replaced the one I fried somehow. $20 at Sears, goes up to 20 MOhm.

I've had a split of freshly cut cherry sitting on an old produce scale I have in my wood shop where the stove is. It weighed 7 lbs 12 oz two weeks ago, and now it weighs 5 lbs 11 oz - a loss of over 2 pounds of water, or about 25%. Green cherry is supposed to have about 40-42% MC when first cut, so that means it is well below 20% MC by now.

When I sank two finishing nails into it about 1/2", it didn't even register, but as I drove them in all the way (about 1 1/2 ") I got a reading of only .75 MOhm in the center. Obviously still a bit damp way inside. RH in the shop is < 10% as measured accurately with a sling psychrometer.
 

DBoon

Minister of Fire
Jan 14, 2009
1,191
Central NY
My previous post above was from January 2010. I just went up and checked the moisture content of a split about the same size as what I checked before, and on the same side of the pile. As before, I split it in half and checked it on the inside. The split was measuring 440 kOhms. That equates to 22% moisture content per the USFS document. So, in two months, moisture content has gone from 27% to 22%. This wood is definitely going to be ready to burn next winter.
In July 2010, I moved a cord of the meadow drying hickory into my garage wood crib. I didn't check it at that time, but I checked it today. I was averaging 4 MOhms when I split the wood in half and checked inside. This equates to about 16-17% moisture content. Bark is peeling off, rounds are deeply cracked on both ends - all evidence of good seasoning.

I tested some other Hickory that I left outside uncovered - it averaged 1 MOhm, or 20% moisture content. I moved this into the garage today. Good enough to burn, but not as good as the other batch that I moved into the garage earlier in the summer.

From my previous pseudo-experiment with Sugar Maple, I would conclude that drying Sugar Maple or Hickory outside in sun and wind for a full year is plenty of time when the wood is 4-6" on a side and 16" long. Putting it in a covered enclosure (garage, barn) after half this time seems to make no difference, and may actually help if the garage or barn is well-ventilated. Obviously, a wetter climate or a wetter than normal summer would probably effect these results, but you definitely don't need two full years for these woods in these sizes IF you have a good sun/wind location for the first six months of the seasoning.
 

Redcloud5400

New Member
Nov 4, 2013
21
USA
Just checked some of my wood supply with a digital multi-meter to see if my moisture content is in the target range.

Cherry - 18 months old, spent half the time in a garage, half the time outside covered and in the sun half the day. DMM reading: 9 to 15 megohms, even on the inside of the split. Well under 20% moisture content

Sugar Maple - 18 months old, half in garage, half outside (like the cherry). DMM reading is 6-9 megohms, which equates to 16-17% moisture content.

Pine - 18 months old, stacked outside in sun and wind. DMM reading is 20-25 megohms, which equates to 15-16% moisture content.

Sugar Maple - 5 months old (fresh cut and split at that time). Stacked outside in the driveway, uncovered the first 3-1/2 months, covered last 1-1/2 months. DMM reading is 0.6 to 0.9 megohms, which equates to 24-25% moisture content.

Will have to check my hickory that is stacked in windrows in an open meadow, and now covered for the last 2 weeks. That was fresh cut and split this year, so it will be interested to see what that is right now.
Great work DBoon! if more people posted their readings, species of wood, and confirmed moisture readings, and type of moisture meter, we would have a good data set to draw from. I have an idea for some test pins--will have to play with this and post up some numbers for the wood I have layin around!
 

Ralphie Boy

Minister of Fire
Feb 12, 2012
1,165
Rabbit Hash, Kentucky
It has been a while since I've said this; the best way to check the moisture content of your firewood is with a multi-year calendar.;)
 

NateH

Member
Nov 19, 2013
128
Pennsylvania
Okay, first time wood burner (patiently waiting to install). I was about to buy a moisture detector but after reading through these posts I'm wondering. I thought this was something I should have prior to purchasing wood... then there's the matter of wood - where to get it, refutable suppliers, and can I even find anything this late ready to burn this winter?
 

YkDave

New Member
Nov 24, 2013
27
Yellowknife, NT Canada
So for kicks, i gave this a shot.

Last saturday i cut down some big ol green jack pine as i am starting to stockpile for next winter (or maybe this spring if need be...)

These trees were green as can be, split and stacked them on skids sunday, and threw them up on the pallet racking in our shop just because their was some free real estate up there.

Its hotter than the gates of hell up there during the winter as the 25-30ft height mark gets all the heat while the lower half of the shop freezes, so i figured it would dry this up pretty quickly.

Well, ill be damned if i didnt check it today with the multimeter and im getting 10-12ohms. According to the chart that would be in the 18-19% range already...

Id like to have an actual moisture meter to verify this... Maybe i should get into the wood drying buiseness
 

MrNoBuddySpecial

New Member
Dec 19, 2013
22
Cincinnati, Ohio
I know this is an old thread... but, would this work?
image.jpg
this is roughly 1.25" apart at outer edges.
image.jpg
measured at inside and get a stupid low #... this was clearly wet wood after splitting the split. this is pin oak felled in Feb, left on ground chunked and split 2mo ago.

maybe will get the MM from amazon. not sure I could trust myself to do it correctly.
 

YkDave

New Member
Nov 24, 2013
27
Yellowknife, NT Canada
Not sure what your issue is?

You know it's wet, and your dmm numbers reflect that.

Resistance increases as the wood dries, and the number will increase
 

MrNoBuddySpecial

New Member
Dec 19, 2013
22
Cincinnati, Ohio
Not sure what your issue is?

You know it's wet, and your dmm numbers reflect that.

Resistance increases as the wood dries, and the number will increase
don't think you understood my post... I was asking if using a HAMMER to get the "holes" would be an effective alternative it finding nail, measuring, ensuring grain, pounding in, removing, inserting probes etc, etc.

I like the idea of using what I already have, but not sure I trust myself/meter/myself to get an accurate reading.

does it HAVE TO BE 1.25" apart EXACTLY?
does it HAVE TO BE the same grain line?
do i HAVE to get the nail depth EXACTLY right.
 

YkDave

New Member
Nov 24, 2013
27
Yellowknife, NT Canada
Now i got ya... LOL

The answer is yes, and no!

If you are going to be using someone elses resistance-Moisture% chart, then yes you have to be as exact as possible to their testing method of you skew the results.

That being said, you could do one "exact test", then on the same piece of wood, do your hammer method and see what your result is. In a sense, calibrating your results to the known resistance-moisture% charts. But, the biggest problem with your method is that its pretty hard to get repeatable results, which is pretty important here. Your method could be out by a billion OHMs compared to the charts, which is fine, but your testing method has to be repeatable or you dont know what the correction % should be
 
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