# Using a multimeter to measure wood moisture level



## precaud

After studying the old Forest Service article on electrical moisture meters, my curiosity got the best of me. Realizing that, all I really want is, maybe once or twice a year, a simple "go/no go" indication of wood's readiness to burn, I decided to try using a very generic digital multimeter (DMM) to measure it, using the technique and data in the USFS paper. Nothing exhaustive; I just wanted to see if it worked and if my measurements matched the theory. Guiding observations for the test:

: The general consensus is, you want to burn wood that is 20% or less moisture content.
: Looking at Table 1 (Electrical resistance vs Moisture content of various species) in the USFS paper, except for the few _really dense_ hardwoods out there (ash, hickory, American Elm), the DC resistance of the majority of species used for firewood are in the range of 2.0 to 4.0 MegOhms at 20% moisture, with most in the low 3.0 range. Higher moisture levels give lower resistance readings.

So if it measures 3 MegOhms or higher, it's ready. Well, even the cheapest of multimeters have a 10 MegOhm resistance range and so can easily measure 2-4 MegOhms. So a simple go/no go moisture test should be able to be done with a DMM.  And it is much more practical to have a multimeter lying around than a single-purpose moisture meter. The DMM can be used for a multitude of other things. 

Most DMM probes are too blunt to penetrate into the wood to get a good reading. One solution is to use a nail about the same diameter as the probe tips and punch a couple holes to measure in. Space them 1-1/4" apart along the grain in the fattest part of the split. You only need to go down 3/8 to 1/2 inch. Takes a few seconds to do.

So I took two pieces of Siberian elm - one that I cut and split in May, the other cut and split three weeks ago. I split each piece in half again to get to wood that hasn't been exposed to air, punched the 1-1/4" spaced holes in the fattest part, inserted the probe leads and watched the reading. If you just rest the probes in the holes, they won't make good contact and the resistance readings will jump around and steadily rise. But if you push on them to maintain pressure for better contact, the readings are very consistent.

The older split measured 3.2 Megohms, right in the 20% zone. The newer one measured around 750 kOhms. Very clear difference in obviously much wetter wood.

This impromptu test suggests this technique will easily answer the question of "is my wood moisture level at 20% or below". It may not tell you if it's 17% or 15% or lower; you might need a moisture meter to do that. But if all you need is to see if your wood is in the burnable zone, this technique should work just fine.

So the basic guidelines are:
If it measures 3 MegOhms or higher, it's good to burn.
If it measures less than 3 Megs, it needs to season longer.
If it measures open circuit (or above 10 MegOhms), then the moisture level is in the teens no matter what species it is, and it is definitely dry enough to burn.

For the super-high-density species mentioned earlier, divide those numbers by 2 or 3.


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## Dune

If you really want to get nutty with your multimeter, you could calibrate it to known moisture contents, which you could determine using the weigh the sample, bake it in an oven, weigh it again method. We do something like this in the metal business, where we calibrate a multimeter hooked up to a thermocouple and using the melting temp of various metals, construct a crude yet surprisingly acurate pyrometer.


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## precaud

Yup, that would be a way to calibrate it.


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## Jamess67

Thanks for posting this. You just saved me $20.


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## Skier76

Very interesting! Does anyone have the USFS link for how to do that? I know I saw it here, but I can't find it.


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## precaud

Here is it:

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06.pdf


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## Skier76

Thanks for posting!


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## trafick

Could you just drive the nails 1/2" into the wood 1-1/4" apart and take the reading directly off the nails?  The resistence of the nails should be minimal compared to the 3.2 megs of the wood.


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## Hurricane

That sounds like it would work fine, I agree the nails will add minimal resistance and will have better connection to the wood then the probes after the nails were pulled. 
I will try that.


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## precaud

Should be fine, I just have a habit of removing unneeded things from any measurement chain...


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## Valhalla

A sweet idea, thanks!


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## DBoon

> Could you just drive the nails 1/2” into the wood 1-1/4” apart and take the reading directly off the nails?  The resistence of the nails should be minimal compared to the 3.2 megs of the wood.



Yes.  Electrically, it will be the same.  The resistance of the metal nails will be infinitely smaller than the resistance of the wood you are measuring, regardless of moisture content.


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## madrone

You rock. Thanks!


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## Cluttermagnet

Great thread- thanks! Probably the most important thing would be to get the electrode spacing just right. Probably the electrode diameter isn't near as important. Electrode depth, I'm not so sure about, but it's probably fairly important, too. Anyway, I'd suggest that using 3d or 4d steel finish nails would make those nails pretty easy to drive with a hammer, and pretty easy to pull out afterwards.

Here, I have a pretty good quality Hewlett Packard DMM with a set of test leads that have micro hooks on them. They will clip onto the small nails, making it a hands- free measurement. It's a pain trying to hold two regular test prod tips against 2 nails. The clips make it much easier. BTW garden variety 'alligator clips' would also be great here.

Maybe this thread should be made a 'sticky'- or it might be added to that 2000+ question FAQ on site.

I'll compare this DMM method against my cheapie Harbor Freight moisture meter and see if I get good agreement.
FWIW the prong spacing on my HF 96472 meter is almost exactly 1 inch, close to the 1-1/4 inch used for the DMM method. You could calibrate for other electrode spacings as well. Changing the spacing should change every entry in the table by a proportionate amount. I think that driving the nails using a template of cardboard, plastic, Masonite, etc. would make it quick and easy to get the spacing just right every time.


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## DBoon

> Probably the most important thing would be to get the electrode spacing just right. Probably the electrode diameter isn’t near as important. Electrode depth, I’m not so sure about, but it’s probably fairly important, too. Anyway, I’d suggest that using 3d or 4d steel finish nails would make those nails pretty easy to drive with a hammer, and pretty easy to pull out afterwards.



Most certainly, the spacing of the electrodes is the most important thing.  A nail ("electrode") with a diameter big enough to hammer into a piece of wood is all you need to make this measurement accurately.  To confirm this, just measure the resistance of the length of the nail - it is a conductor, of course, and will have minimal resistance.  Varying the nail/electrode depth will allow you to check the moisture at different depths - just make sure both nails are at the same depth - otherwise, your distance between the nails will be more than 1-1/4 inches.


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## Nonprophet

DBoon said:
			
		

> Probably the most important thing would be to get the electrode spacing just right. Probably the electrode diameter isn’t near as important. Electrode depth, I’m not so sure about, but it’s probably fairly important, too. Anyway, I’d suggest that using 3d or 4d steel finish nails would make those nails pretty easy to drive with a hammer, and pretty easy to pull out afterwards.
> 
> 
> 
> 
> Most certainly, the spacing of the electrodes is the most important thing.  A nail ("electrode") with a diameter big enough to hammer into a piece of wood is all you need to make this measurement accurately.  To confirm this, just measure the resistance of the length of the nail - it is a conductor, of course, and will have minimal resistance.  Varying the nail/electrode depth will allow you to check the moisture at different depths - just make sure both nails are at the same depth - otherwise, your distance between the nails will be more than 1-1/4 inches.
Click to expand...


Or, buy a moisture meter for $29 and be done with it.........no calibrating, no pounding nails, no varying the depth of the nail penetration, no cardboard templates to maintain proper electrode spacing......

Geesh guys, I'm pretty frugal and I have a nice Fluke DMM, but even I spent the $29 bucks.........lol


NP


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## Lumber-Jack

Nonprophet said:
			
		

> DBoon said:
> 
> 
> 
> 
> 
> 
> 
> Probably the most important thing would be to get the electrode spacing just right. Probably the electrode diameter isn’t near as important. Electrode depth, I’m not so sure about, but it’s probably fairly important, too. Anyway, I’d suggest that using 3d or 4d steel finish nails would make those nails pretty easy to drive with a hammer, and pretty easy to pull out afterwards.
> 
> 
> 
> 
> Most certainly, the spacing of the electrodes is the most important thing.  A nail ("electrode") with a diameter big enough to hammer into a piece of wood is all you need to make this measurement accurately.  To confirm this, just measure the resistance of the length of the nail - it is a conductor, of course, and will have minimal resistance.  Varying the nail/electrode depth will allow you to check the moisture at different depths - just make sure both nails are at the same depth - otherwise, your distance between the nails will be more than 1-1/4 inches.
> 
> Click to expand...
> 
> 
> Or, buy a moisture meter for $29 and be done with it.........no calibrating, no pounding nails, no varying the depth of the nail penetration, no cardboard templates to maintain proper electrode spacing......
> 
> Geesh guys, I'm pretty frugal and I have a nice Fluke DMM, but even I spent the $29 bucks.........lol
> 
> 
> NP
Click to expand...

What I need to know is how to make a multimeter out of my moisture level meter.  ;-)


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## precaud

Nonprophet said:
			
		

> Or, buy a moisture meter for $29 and be done with it.........no calibrating, no pounding nails, no varying the depth of the nail penetration, no cardboard templates to maintain proper electrode spacing......



Yes, those are all terrible inconveniences...



> Geesh guys, I'm pretty frugal and I have a nice Fluke DMM, but even I spent the $29 bucks.........



You're framing it as a $$$ and convenience issue. To each their own.

For me, for something I'll do maybe twice a year, I don't want yet another special-purpose tool laying around when a general-purpose one will do just fine.

After all, a moisture meter is just a 2-digit multimeter with all of the most useful functions removed.


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## quads

I think this is a very cool idea.  

I have never, and will never, own a moisture meter.  No reason for it for me, even if they were only 59 cents.  I know how long to wait before I can burn my firewood.  But, I do have a multimeter and I think it would be interesting to play with it and see what I come up with.  Not because it's useful for me to know the moisture content of my firewood, but because of the curious experimenter in me and the fact that the multimeter is something I already have laying around.


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## Hurricane

+1

I do not care about the $29 but why spend it if not necessary? To support the chinese ?

I forgot who said it I think it was Backwoods Savage but get your wood C/S/S and let time do it's thing. I was one of those people who always cut n the spring split in the fall and burned in the winter. I have now switched so I am 2+ years ahead and shooting for 3+. I burn what I have regardless of what the meter says.


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## pgmr

From the pdf:

"Effect of Grain Angle
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
radial and 0.50 for tangential."

The table of resistance levels in the pdf is for measurements WITH the grain.  To use it with at least a modicum of accuracy, you'll need to make sure your nails/DMM probes are parallel to the grain.  If not, you'll be doubling the resistance and think your wood is dryer than it really is.


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## DBoon

> Or, buy a moisture meter for $29 and be done with it…......no calibrating, no pounding nails, no varying the depth of the nail penetration, no cardboard templates to maintain proper electrode spacing…...
> 
> Geesh guys, I’m pretty frugal and I have a nice Fluke DMM, but even I spent the $29 bucks…......lol



I don't buy a moisture meter even though that would be easier than a multimeter for the same reason I don't buy oil even though that would be easier than cutting, splitting, stacking, and loading wood.    Sometimes its more fun to "rough it".


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## 'bert

I am an electrician and have a formal education in electronics eng. but when reading this thread the only thing I could think of was *"why do it the easy way, when this works?" *


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## pgmr

After looking at the resistance table, the rhetorical question that comes to my mind is this:  How does a cheap moisture meter know what species one is testing?  

A 20% value for American Elm is .48 MO and 20% for Black Spruce is 6.3 MO - greater than 12x the resistance.  Might have to chuck that HF meter in the trash.


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## Nonprophet

pgmr said:
			
		

> After looking at the resistance table, the rhetorical question that comes to my mind is this:  How does a cheap moisture meter know what species one is testing?
> 
> A 20% value for American Elm is .48 MO and 20% for Black Spruce is 6.3 MO - greater than 12x the resistance.  Might have to chuck that HF meter in the trash.



If you read the original Forest Service article carefully, you'll see that for 99% of wood species in the US the variation is less than 1% between species. If you're making/milling thousands of dollars in wood products it might make a difference (which is why they buy $400 moisture meters and they don't fool with DMMs........) but for the average wood burner the HF $14.99 model will work just fine.........!


NP


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## Detector$

PGMR.... exactly! and why I didn't buy the HF meter. When you get up to the 70-80 dollar mark the more expensive moisture meters come with instruction manuals with specific instructions for each species of wood. The cheap meters are probably a waste of money.


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## Nonprophet

Detector$ said:
			
		

> PGMR.... exactly! and why I didn't buy the HF meter. When you get up to the 70-80 dollar mark the more expensive moisture meters come with instruction manuals with specific instructions for each species of wood. The cheap meters are probably a waste of money.



n/t


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## Gark

Excellent post, precaud. Thank you.


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## Duetech

For someone who has a HF within 25 miles, that does not stock the moisture meter and who does not want to purchase the mm without reading the pkg and handling it the multimeter approach is ideal.

Since electricity follows any path but especially the path of least resistance readings taken by an mm will be parallel to the wood grain unless the pins are situated on opposing sides of the log in a fasion to be perpendicular to the direction of the grain.

Thanks for posting this info!


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## Cluttermagnet

Yep. Well, I already had the cheapie moisture meter from Harbor Freight. With a sale plus a 15 percent coupon, I think it came in just under 10 bucks, well worth it to me. The meter is teaching me that my sense of touch is a pretty good moisture indicator- but not always! Depending on the wood, it may feel dry and yet hiss in the stove. But my hands have become surprisingly good at telling which wood is seasoned enough.

For me, this is a very interesting 'academic' exercise, to check the DMM method against a moisture meter. Heck, DMM's are getting so cheap and so common that probably millions of households have one or more. I have a bunch of them. HF has some cheapie DMMs that they often sale at 2-3 bucks! Cheap, yes, but what an amazing deal. Just a pair of test leads for a domestic instrument is going to cost more than that.

I'm with you guys, so far as laying in a wood supply for 2-3 years out. That is my goal but I'm not there yet. And yes, little doubt that properly sized and seasoned splits that old have got to be seasoned enough- no need to test.

Yes, a cheapie moisture meter should be thought of as an 'indicator' and not relied on to be an accurate measuring instrument. Still, an indication is better than a wild a**ed guess. I believe it teaches the new wood burner to have their hands better 'calibrated' to sensing moisture content. Old pros are generally *not * going to need these things much if any. 'Training wheels'? ;-)


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## Tony H

Good idea and all in all not a terrible exercise to get a good result. I do have a little cheap MM and have only used if a few times to get an idea how fast some different types of wood are seasoning. Would have worked just find using my trusty Fluke instead oh well.


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## Nonprophet

In reading through several manuals for the higher end moisture meters, it would seem that TEMPERATURE has more of an impact than SPECIES relative to getting accurate readings.

It seems that Doug fir at 70 degrees is the default setting for calibrating most MMs.  While Teak at 70 degrees would only change the reading by 1% +/- from Doug Fir, Teak at 40 degrees would change the reading by 3%.  Still fairly insignificant, but you could get as wide a variation of say 12% depending upon the temp of the wood whereas the species normally wouldn't vary by more than 1%-2% or so...........


NP


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## precaud

HP, it makes sense that temp would influence the conductivity. Just another reason to not take the absolute numbers too seriously. And to work far enough ahead to not have to worry about measuring it.


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## DBoon

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.


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## precaud

Very good results, DBoon, and very much in line with what one would hope to see.


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## Valhalla

In summary:

1. Can a wood moisture meter know the species or type of wood tested? No, of course not.

2. Is a wood moisture meter probably just a single range ohmmeter? Yes, without a doubt.

3. Can one therefore use an ohmmeter as a wood moisture meter. Yes, absolutely.

I admit to now use my ohmmeter probes by just sticking them in the cracks of the sample splits to be tested. 
Range it, or let it autorange accordingly and smile! Priceless and very logical information, thank you. 
We can all now burn a bit more wisely.


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## bren582

Great thread.. I checked some of my wood today with my Fluke. Split each split and drilled holes 1.25 inches apart with the grain on the exposed surface and stuck the probes in. nice tight fit. 

First Pic- White oak 3 years split that's been in the garage for the past year, burned some of this last year and man was it nice. 181 meg?? I guess that means its really dry.

Sec Pic - Red oak 10 months split, stacked outside with top covered. .177 meg and i can feel the dampness on the exposed wood.

third pic - Red oak 5 month split,  stacked outside with top covered.  .057 meg and its really wet on the freshly exposed surface

I have to read that article to get an idea on what these readings represent. The temperature was 70Deg F when checked. Red Oak lives up to its rep for a long seasoning. I guess one could at least come up with range of readings that represent several degrees of seasoning and go with that as a gauge. I was going to buy a meter but now maybe not. Will just use the fluke..


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## precaud

bren, with it's autoranging MegOhm range (up to 300Meg), the 8060A is a great meter for this. The readings you're getting seem exaggerated, though. Have you checked your meter for accuracy recently? Say, measure a 10Meg resistor and see what it says? The 8060A is a great little meter but most of them are getting old enough to have some of the electrolytic capacitors in them dry up, resulting in inaccurate readings. I've rebuilt several of them for exactly that.


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## bren582

That's a good point, No I haven't checked it in years..  I will have to stop by radio shack and pick up a few resistors and test it out.. Thanks..


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## DBoon

> Very good results, DBoon, and very much in line with what one would hope to see.



Thanks precaud.  I'm going to move half of that 5 month old Maple into the garage for the winter and keep half outside covered on top.  That will be my experiment to see if it dries as well inside as out.  

I was pretty surprised to see everything dry in 18 months.  I think my splits are smaller (generally 3" x 4" tops) so they dry quicker.


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## precaud

DBoon said:
			
		

> I'm going to move half of that 5 month old Maple into the garage for the winter and keep half outside covered on top.  That will be my experiment to see if it dries as well inside as out.


Yes, that will be interesting to see. What's your hunch?



> I think my splits are smaller (generally 3" x 4" tops) so they dry quicker.


No question, diameter makes a huge difference.


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## Jamess67

Tried my cheap Radio shack mm last night and couldnt get a stable reading. Guess I will be buying a moisture meter. Thats would be cheaper than a new mm.


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## precaud

Jamess67, don't sell out to the dark side! A multimeter is a far more useful tool that a moisture meter, and costs no more.

Basically only two things can cause unstable readings: a bad meter/leads or poor contact to the wood.

To see if the leads are good, short them together and make sure the reading is down near zero.

To see if the meter is measuring correctly, make sure the battery is good, then measure a high value resistor (something in the 1 MegOhm to 10 Megom range) and compare the measurement to the resistor value.

Older non-working electronic devices are a good source of components for this test. Most of us have some unrepairable electronics lying around! If you know the color code, just unsolder or snip out the resistor and then measure it.

For probes directly in the wood, add pressure to stabilize the reading. If clipping onto nails, make sure the surface is clean and rust/corrosion-free.


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## DBoon

> DBoon - 01 November 2009 12:56 AM
> I’m going to move half of that 5 month old Maple into the garage for the winter and keep half outside covered on top.  That will be my experiment to see if it dries as well inside as out.
> 
> Yes, that will be interesting to see. What’s your hunch?



The maple is CSD wood that I had dumped into my driveway, which I then resplit and stacked.  I want to understand what it does where since I don't want to move it more times than I need to, or farther than I need to.  Then, I'll know what I can get away with in the future.  This is a good test case, since the summer here was so rainy that any other summer should give me much better results.  

My hunch is that the maple might dry a little faster outside, but that once I get it from 35% to 25% by having it sit in the driveway, it will still dry to <20% in the garage in the next 12 months.  If it does, then I can move it just once, and only 25 feet from the driveway into the garage cribs.  

The hickory I have (now) covered and drying in the open meadow was split quickly into roughly 6"x6" pieces.  That gets a lot of wind and sun.  I'm expecting that two summers + one winter (18 months) will dry that out to <20%.   Then, I can bring that off the hill and straight into the garage, without resplitting until necessary.


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## quads

My oak results:

My recently processed firewood; trees were standing dead with no bark and gray surface; cut, split, and stacked within the last few months:  lowest 268k, highest 1655k

Today's supply next to the stove, 3+years old:  infinite/open circuit

After I got the infinite reading in the house, I thought I'd try some from the row in the woodpile that I will be burning within the next month, 3+ years old.  I still got an infinite/open circuit reading with every split.  I tested about a dozen from varying locations in the row.

I was a little surprised.  I never give my firewood any special treatment; stack it and forget about it for years.  It's not covered, except with snow, leaves, and pine needles.  The bottom row of splits is stacked directly on the ground (I did not unstack enough to test any of those). It has been raining often lately.  Almost 7 inches in the last 4 weeks.


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## precaud

DBoon: You'll have to report back next year and tell us which batch dried more.

Quads: That's interesting results on the stuff you cut recently. I would have thought greyed standing dead would be lower moisture. But then, you're in a wetter climate than I.

Here are a couple more of mine.

A 7" dia pinon, cut as standing dead two years ago and stacked as a log, cut and split in half yesterday, measures open circuit (greater than 20 Megs). There's no reference data on pinon pine in the USFS paper, but this is consistent with my experience with the standing dead pinon I gather - I can pretty much cut, split, and burn it right away.

On the other hand, taken from the same stack, a piece of crab apple, the 20" live tree was cut down last fall, stored as a log for one year, cut and split from the bottom trunk section today, measures 80k Ohms. It felt wet to touch after splitting it.

What I find interesting about this is that the resinous species, like the pinon, reject water even after being felled, while the others (like the apple and Siberian Elm) absorb and hold it even when dead, which causes them to start rotting quickly. I have pinon logs in my stacks that sat on the ground for years and were still fine, as long as they were in a place where water didn't pool and they had some sun exposure.


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## Valhalla

precaud said:
			
		

> Jamess67, don't sell out to the dark side! A multimeter is a far more useful tool that a moisture meter, and costs no more.
> 
> Basically only two things can cause unstable readings: a bad meter/leads or poor contact to the wood.
> 
> To see if the leads are good, short them together and make sure the reading is down near zero.
> 
> To see if the meter is measuring correctly, make sure the battery is good, then measure a high value resistor (something in the 1 MegOhm to 10 Megom range) and compare the measurement to the resistor value.
> 
> Older non-working electronic devices are a good source of components for this test. Most of us have some unrepairable electronics lying around! If you know the color code, just unsolder or snip out the resistor and then measure it.
> 
> For probes directly in the wood, add pressure to stabilize the reading. If clipping onto nails, make sure the surface is clean and rust/corrosion-free.



I second this! Here we have a great solution to a common wood burner question.


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## DBoon

> DBoon: You’ll have to report back next year and tell us which batch dried more.



I plan on it.  I just got everything moved and stacked - 50% in the garage and 50% outside.  We'll see what happens in the next few months.  

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 father-in-law is burning some of this hickory this year.  He has stacked it in his barn to dry.  Then, we bring in about 2-3 weeks supply and drop it in his basement where his woodstove is.  His belief is that the woodstove dries out the wood to "bone dry" in 2 weeks or so.  So I checked some in the barn ~200 to 300 kOhms, or nearly 30% moisture content.  And the wood in the basement next to the woodstove for 2 weeks?  Between 400 and 600 kOhms (25-20% moisture content).  So there definitely is some drying out going on there.  FYI, these splits are no bigger than 3" on a side.  His woodstove is already burning better this year than last.  I look forward to seeing how it operates a little later in the winter when his wood has had more time to dry out.


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## Jamess67

Valhalla said:
			
		

> precaud said:
> 
> 
> 
> 
> Jamess67, don't sell out to the dark side! A multimeter is a far more useful tool that a moisture meter, and costs no more.
> 
> Basically only two things can cause unstable readings: a bad meter/leads or poor contact to the wood.
> 
> To see if the leads are good, short them together and make sure the reading is down near zero.
> 
> To see if the meter is measuring correctly, make sure the battery is good, then measure a high value resistor (something in the 1 MegOhm to 10 Megom range) and compare the measurement to the resistor value.
> 
> Older non-working electronic devices are a good source of components for this test. Most of us have some unrepairable electronics lying around! If you know the color code, just unsolder or snip out the resistor and then measure it.
> 
> For probes directly in the wood, add pressure to stabilize the reading. If clipping onto nails, make sure the surface is clean and rust/corrosion-free.
> 
> 
> 
> 
> I second this! Here we have a great solution to a common wood burner question.
Click to expand...


Thanks guys. I have checked the probes and they are fine. The batteries my be an issue. If I complain enough to management (the wife) maybe I can get the Fluke of my dreams.


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## quads

precaud said:
			
		

> Quads: That's interesting results on the stuff you cut recently. I would have thought greyed standing dead would be lower moisture. But then, you're in a wetter climate than I.


Quite often it's still so wet when cut that the water practically runs out of the trunks of the standing dead oak, even after standing dead for 10 years.  The limbs are usually fairly dry.  I'm guessing the wettest readings were splits from the trunks and the driest (highest resistance) were splits from the limbs.  I did get a reading on every piece of the recently cut stuff, as opposed to not even one piece of the seasoned wood.

One thing I learned; if you are using nails to make holes in the wood, or as probes, put a little piece of pink duct tape or similar on them.  Those nails sure like to disappear in a hurry when pulling them out of the split!  As long as they don't reappear someday in my woodhauler tire.  Ha ha!


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## Skier76

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


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## quads

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.


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## Skier76

I'll try the nails and see how that works.


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## Valhalla

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.


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## precaud

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!


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## Graf Spee

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.


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## precaud

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.


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## DBoon

> 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.


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## DBoon

> 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.


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## roddy

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


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## Battenkiller

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.


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## tjsquirrel

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.


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## DBoon

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.


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## pyper

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.


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## gpcollen1

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.


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## Battenkiller

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.


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## DBoon

> 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.


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## Redcloud5400

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.


 
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!


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## Ralphie Boy

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.


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## NateH

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?


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## YkDave

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


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## MrNoBuddySpecial

I know this is an old thread... but, would this work?



this is roughly 1.25" apart at outer edges.


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.


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## YkDave

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


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## MrNoBuddySpecial

YkDave said:


> 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.


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## YkDave

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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## RobertNH

Great read! Great link!
Thank you for posting!

I now have a copy of the charts to use to check my expedited drying theories.


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## lectraplayer

Lumber-Jack said:


> What I need to know is how to make a multimeter out of my moisture level meter.  ;-)


That sounds like a challenge to me!  I'm gonna have to build me a moisture checker to check for this 3 megaohm resistance because I can.


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## Babaganoosh

Getting annoyed at this. Drove some nails, pulled, inserted probes, couldn't get a stable reading. Then drove nails and tried to read off that. Not stable either. Just slowly climbs and climbs. Tried steady pressure, same result. Tried leaving them stuck in the wood same result. 

Replaced battery, same result. Touched probes together and it immediately gives me a zero reading. I suppose it's not reading correctly but not likely. It's less than a year old and a decent auto ranging model. Klein tools. Not the 20 dollar jobber. I'm getting it to stay around 280 to 300 k ohms. But it's moving around within that range. If I remember from years ago that's 2 decimal places to mega. So around 2.85 on this maple.


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## Ashful

You need to consider how your multimeter measures resistance.  First, most output a constant current, and measure the voltage drop to calculate resistance.  On older (manual ranging) DMM's, this current level would be set by the range selection, but on newer (auto-ranging) DMM's the current just varies by load to put the voltage reading in a reasonable accuracy (100+ mV) range.  A good modern DMM like the Fluke 77 will output about 30 uA into 300 k-ohm, but nearly 1 mA into 200 ohms.  Also, into very high resistance loads (>100 k-ohm), the combination of lower current and long leads on your meter sets up the perfect scenario for induced current (eg. AM radio pickup), which causes meter wandering.

Constant climbing is usually an issue of the current output by the meter causing some heating in the device being measured, albeit it small, it is multiplied to a large change by the high resistive value.  Random wandering is usually caused by the long leads at a high resistance value.


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## begreen

For those that just want a dedicated meter, there are many on the market. Lots of recommendations in the Gear forum if you search for Moisture Meter. And here is a quick poll:





						Moisture meter
					

hey guys and gals, hoping to get a recommendation on a moisture meter to check the wood. I’ve seen some for $30 and some for $450...hoping I don’t need a fancy expensive one like the latter. Any suggestions?




					www.hearth.com


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