Moisture Meter Accuracy?

Yes...Both...Neither????

Barry Irby

I have used several types of meters over the years as a Home Inspector and to a lesser extent in my wood working. I am not an exdpert on moisture meters, but my understanding is the pin type read the resistance or conductivity between the pins. Wetter wood conducts better than dryer wood and dryer wood offers more resistance.

The Pinless meters send out a field and measure it. The moisture in the wood has a measurable effect on the field. The meters I used supposedly had a "range" of about 3/4" into the material.

So the Pin meter is telling you what the condition is between the two pins. The pinless meter may read deeper into the wood. So if your wood is dry on the surface and wet in the center (a common condition) both meters could be correct.

Also, I never use mine to determine the actual MC. If I wanted to know if a spot that looked wet was really wet, I took a reading several feet away from the wet spot on the same material and then tool a reading on the spot. If the spot read higher than the general reading, it was currently wet. If the read the same, I may have been looking at an old wet spot stain that had dried.

So, use your meters comparatively. Store some samples in your house and use those as your "Known" sample. What you want to work with is wood that has dried to the conditions it will encounter once you move it to its final destination.

maybe neither and some information

Bill Tindall, E.Tn.

Unless you live in a desert 6% is assuredly wrong. Commercial lumber is dried to 7% and in most places it gains more or less. Commercial lumber could gain up to 12% in a humid climate. Air dried could be anything.

Make note that I almost never measure the moisture content of lumber either as a user or a seller for 30+ years(now retired from this adventure). I rarely see a need to do so. I would argue that there isn't a need if you are buying commercially manufactured lumber. None of our commercial customers ever measured moisture content and I don't recall a retail customer ever doing so.

Why are you measuring? You might have a good reason. But I see no reason to know if the true value is between 6 and 10. So what?

Meters are good at high moisture content, less reliable the lower you go.

Most accurate is drying to constant weight. Weigh a test piece, dry at 220 F until it stops loosing weight. Weight loss divided by dry weight x 100 equals % moisture. This method is the Reference Standard for NHLA transactions.

a flaw in this seemingly good idea

Bill Tindall, E.Tn.

"Store some samples in your house and use those as your "Known" sample."

Meters respond to water molecules, and other stuff in the wood, which varies with species, growing soil conditions and invasions of organisms. Expensive meters have setting for different wood species but they are still fooled at low moisture content by stuff the tree takes up.

The fundamental problem is that as water decreases, the signal strength from the water gets ever smaller. At some level of water the signal from other stuff becomes a significant fraction of the water signal.

Pin meters, at least those used in commercial operations, are driven into the wood to half the depth of 4/4 lumber to achieve an average moisture content.

The fundamental problem

Bill Tindall, E.Tn.

Interference from other stuff in the lumber makes it unreliable to measure the difference between 7% and 10% with a meter. On the other hand, when would it matter? So why bother?

Re: maybe neither and some information

Randy Stukenberg

I just measured on a whim. I completed a rustic hickory kitchen earlier this year and now the panels are expanding and blowing out the frames on a few of the doors. I thought I had allowed for expansion. They were made in humid conditions so I really expected more problems (if any) with shrinking. I am making an oak door and thought I would check, just for personal info. I got the two different types of meters at an auction pretty cheap. I don't usually check.

Re:There are times when you need to know.

Hank Knight

I ordered two custom joinery saws with ebony handles. The maker didn't supply the ebony but agreed to make the saws if I provided the handle material. I ordered a chunk of 8/4 Gibon ebony for a lot of $$$. It was represented have been air dried for 16 years. When it arrived, I checked it with my Lignomat and found that it was at 18% MC. I wasn't about to send it to the maker in that condition and spend a bunch more $$$ on saws that would self destruct in two years. I built a finishing kiln and dried the handle blanks gently in my shop for a couple of months and got them down to 8%. Two years later the saws still look great. No splits, checks or shrinkages problems. If I hadn't checked my saws would have been a disaster.

Re: Moisture Meter Accuracy?

Mark Hennebury

I don't use a moisture meter;

All things being relative, you can get the information you need by observation;

When new lumber arrives at your shop,

Resaw a piece. if it cups it is not at equilibrium. return it.

if it stays flat it is at equilibrium.

Take a thin flat piece lay it on a table ( or cover one side with masking tape) and check it the next day,

if it is cupped it is not at the same moisture content as your shop.

If the edges have cupped up, your shop is dryer then the wood

If the middle is cupped up , then the wood is drier than your shop.

If it stays flat all is in harmony.

check the meters

John K Jordan

To know for sure do a test with the most accurate method to measure moisture - the oven-dry method. I wrote up this once:

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Moisture content by oven-dry method

oven dry

equillibrium

WC and SMC, 3/15/2018

I got some waxed 1" square Ebony blanks from Pete Kekel (Big Monk Lumber in KY) with unknown moisture content. Waxed wood blanks can be anywhere from green to dry and I find it difficult to guess with hard exotics. My pinless moisture meter needs a bigger contact area so I decided to use the oven-dry method which gives the absolute moisture content independent of the density of the wood. This method is always accurate and cheap if you already have the simple equipment on hand.

If not familiar with this, basically a wood sample is first weighed then gently dried in an oven and weighed repeatedly until the weight quits changing. Then subtract the dry weight from the wet weight to get the water weight and divide that by the dry weight to get the moisture content.

I cut away the surface wax and cut samples from one of the Ebony blanks. A toaster oven dried the wood. I put a thermocouple in the oven to monitor the temperature which needs to be between 212 and 220, ideally 217. The little digital scale is good to 0.1 grams (checked with my triple-beam balance).

Since I couldn't watch it all the time I put the toaster oven on the concrete floor in my shop office to keep it away from combustibles. I checked the temperature every few hours and quit when the weight was unchanged over two readings.

The moisture content of this Ebony was just over 13% which matched the expected equilibrium MC of air-dried lumber in both TN and KY. It's good to know I can turn these without further drying.

If interested in more detail: https://ir.library.oregonstate.edu/downloads/jm214q048

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BTW, I have a pinless Wagner 220. I often get different readings on the same block of wood (turning blanks) on the end grain and on areas of high density. The readings are consistent when repeated. I don't have experience with a pin type meter but I'm led to believe it checks the moisture more at the surface while the pinless reads deeper into the wood.

JKJ

not necessarily

Bill Tindall, E.Tn.

"Resaw a piece. if it cups it is not at equilibrium. return it."

If it cups immediately it has drying stress that was not relaxed in post drying phase of kiln drying. If it cups after a day or more all it means is that there was a moisture gradient. The moisture gradient could have been generated post drying in storage at the supplier or the receiving shop. You would not have gotten your money back at our business with just that data.

not if used properly

Bill Tindall, E.Tn.

" it checks the moisture more at the surface "

In use, a meter like used at a mill, has its pins driven to the center of the board. The reading is an average. A surface reading device would be useless.

Hickory

Bill Tindall, E.Tn.

Hickory has one of the highest responses to change in humidity of any of the domestic woods. Probably for this reason you rarely see it made into furniture.

I believe you have illustrated a good reason to check moisture content, at least of panels. When I make panels they are loose in the frame upon assembly. I have never had a failure. I pin the center to keep them from rattling.

Re: maybe neither and some information

Paul Leuba

I use a pin type moisture meter to test unseasoned 5/4 oak to select boards close to 19% moisture content for steam bending. I have found that relatively small (3-4%) deviations from this value has a substantial effect on the bending characteristics of the wood. If it’s much dryer than 19% it tends to split easily for example.

Paul in Hunt Valley, MD

feel the shavings

Bill Tindall, E.Tn.

I can get a useful estimate of lumber moisture content when planing it. Lumber too wet to use generates shavings that "feel wet"(to me). At 14% shavings are limp. Wood at 7% generates lots of static in my machine and shavings and dust clings. I don't like planing lumber fresh from kiln because of this static cling.

Re: not if used properly

John K Jordan

I've watched people use the pins close to the surface, probably to keep from damaging the wood they want to work with.

JKJ

Re: not if used properly

Bill Tindall, E.Tn.

In a commercial kiln test pieces are loaded along with the charge. These are measured to assess progress. The meter has different sized pins for different tasks but in any case they are buried with hammer blows. Just sticking them into the surface will be misleading. Even beyond just measuring surface moisture I am uncertain the meter will even work properly with just the tips engaged.

The measurement is conductivity which varies with water content, and other factors. I suspect the meter expects a minimum of pin exposure to get a reliable reading.

Reading between the pins

John in NM

That's what the pin type does

The deeper the pins, the deeper the read. If one wants a surface reading they can certainly get one using a pin type, but it would be pretty pointless.

I usually drive nails into the wood and touch the pins to those to get a ~1" deep reading. Or cut through and read in the newly exposed endgrain.

The other thing I remember from working in a shop that dried its own wood (mesquite) and had a nice pinless meter, is that for 4/4 and under you had to be careful that no metal was under the board - read it lying on the table saw and you got an artificially high reading. As I recall, that wasn't a problem with 8/4, so the read depth was probably 1.5" or so. Smaller units might not read so deep though.

Re: Reading between the pins

Yonak

This all makes sense and, if the reading with a pin-type meter reads between pins, why not fashion a device where the pins are set on both sides of a board so a reading can be taken through the board ? No one wants to know the moisture content of just 1/8" beneath the surface.

Re: Reading between the pins *LINK*

Sgian Dubh

As I understand it, if you're taking a reading of a board's MC with a pin type meter then ideally the pins should penetrate approximately a quarter of the way into the board's thickness.

There is almost always a moisture gradient from the surface of the wood to the core. The average moisture content is usually about halfway between the surface and the centre of the board thickness. In a board 25 mm (1”) thick this means pins roughly 7 -9 mm long (~5/16"±) when inserted fully parallel with the grain stand a reasonable chance of getting a good median reading. Similarly, with a board 100 mm (4”) thick the pins should ideally penetrate 25-30 mm (1” to 1-5/32”).

Personally, I'm rather similar to Bill because I seldom use a moisture meter - the one I have has been broken for many years now, and I've also never bothered replacing it. I can get a pretty good idea of the wood's moisture content just by the way it feels, responds to machining, or even how it reacts when it's worked with hand tools.

If I really want to know precisely the MC of a piece of wood I can always do the oven drying test using a microwave, see link below. Slainte.


Microwave oven drying for ascertaining wood MC

Re: Reading between the pins

John in NM

Probably the meter would require a more robustly built circuit to accurately measure with pins that are further apart. Larger power source and components/design to handle it. This is a type of remote sensing, and more distance is going to require more power to read accurately.

Spreading the pins will also increase the volume being averaged, which may reduce accuracy if the wood is not homogeneous, but that may not really be a detriment - the user just has to understand what goes into an average.

But this would also be something fairly easy to try out at home with some scrap wood (preferably somewhat less than fully dry, 14% or so). Need three pieces, of roughly the same size. First one is a control piece that goes with the absolute (oven) method. Second is a piece metered as normal with the pin meter. Third is a piece that gets nails driven into the edges as you suggest, use jumper wires to run to the pins. My guess is the third one will read artificially "dry", but that is only a guess, and I don't know much about how these meters work.

Re: Reading between the pins

John in NM

My meter has pins that are quite long, half an inch or so. Never can get them deep enough in a piece of hardwood anyway

I only have a meter because I occasionally get a piece of wet wood from non-industry sources - even retailers out here have been known to sell mesquite that is green. If I bought all my wood at a commercial yard I wouldn't bother with metering.

It is a conductivity measurement

Bill Tindall, E.Tn.

You are fundamentally measuring conductivity. So the meter has to know the distance between the pins. If pin distance is fixed then the meter can be calibrated to this fixed distance. Otherwise the meter is a random number generator.

The people that make meters, at least Lingomat, know how they need to be used. Follow the directions. For lumber with a moisture gradient use gets complicated. For uniform lumber Lingomat recommends a depth of 15 mm, about 1/2".

BTW, there is another variable I haven't mentioned, temperature. With my meter a reading on a cold board could be substantially different than a summer reading and the higher the moisture content the greater the difference.

Re: My reasons for using a meter

Larry Clinton At Frankfort, (Central) Indiana

I purchased much of my (now stored) lumber from Amish Estate auctions. Almost all of this was air dried, I can only remember one occasion where a gentleman had a kiln and had been selling lumber.

The Amish are excellent woodworkers in this area, apparently most select the trees they want and cut and transport the logs out individually. They also properly air dry the lumber.

My issue is that often at these auctions the lumber has been removed from its original location and stacked in an area for inspection for the sale so you cannot whether the lumber had been stickered or dry and solid stacked.

When I purchased a stack of lumber I measured the moisture to determine if I needed to sticker for a time. If so I staked and stickered the (4/4 ~ 5/4) lumber for at least a year in the loft of my shop.

It is air conditioned and heated 24 / 7 so after a year I removed the stickers and solid stacked the lumber. at that time the lumber all tested below 6 ~ 8%. Thicker stock was/is left stickered

So far I have never had any problems with post construction splitting, warpage or panel fit issues.

Re: It is a conductivity measurement

Barry Irby

At one point I had a meter intended for measuring lumber. It had a lead on it with an end that held two nails. You were supposed to drive the nails into the wood and then take a reading. I can not remember exactly how the nails were attached. It may have had a bracket that held the nails at a standard distance apart and then you clipped on to the nails for the reading. I never used it. Gave it away. There must have been a provision for pulling the nails and replacing them.

As has been mentioned, none of the meters will give an absolutely accurate reading that you can rely on. Too many variables. The classic method is the oven dry method that has been mentioned several times, but even that is not infallible. From that test what you know is the MC of the sample. A few inches from where ever that sample came from there might be a piece with a different MC. This is why the skill of the kiln operator and then the people who stacked and stored it is important to maintain reasonable but never absolute uniformity.

It seems the doors that blew up from the panels expanding indicates the panels were much dryer than you thought. If you had let them rest in the humid environment even for a few days you might have avoided the problem.

Somewhere along the way I was taught lumber equilibrates at about 20% of the RH of the surrounding air. If you live in very dry climate and buy lumber at 6% It might loose moisture, In a wet climate it would gain moisture. That's why I mentioned storing some samples in your house as something to compare too. As we all know, this equilabation is a painfully slow process.

Re: It is a conductivity measurement

TomD

I have built a few pieces with Hickory, and it is both beautiful and has not given any trouble. No closely fitted drawers, though.

I have a few meters, the good one is of the pin type, bought before you could get any other type from the retail shops. It is nice to have, but I don't really use it for furniture. My wood is on the property, and I store some of it intended for use in the living room. Where I like to look at it. So by the time I use it, it is the correct moisture content. Even though my shop is not heated, generally, I find wood stored there is pretty good also.

I few years back I had some very heavy maple and beech in the shop, and it actually raised the moisture in the shop noticeably, as it first started drying.