18 VIDEO Moisture Meter Tour

In this video I want to talk to you about wood moisture and moisture meters and everything related to that

But before I do I want to explain that you don't really need to know this stuff right now

Not to continue with the course and to tackle the next project we're going to be doing

But it is a good time to talk about wood moisture and since you're going to need to know about it later

I figured I might as well go off on a teaching tangent right now

So I've got three pieces of wood here and if you touch them they all feel dry

So that's the tricky thing about wood is that it can have quite widely varying moisture contents

but it can feel the same and so that's where a moisture meter comes in

Now why would we care about the moisture content in wood? It's because the more moisture wood has the more it swells and when wood loses its moisture it shrinks

And it can shrink quite significantly so for you as a woodworker that's pretty important

and I guess it really boils down to working with wood that is dry enough

So if we went into the forest and we cut down a tree and we took that log to the sawmill and they cut it up

it probably would have a moisture content of around 30% so really whopping wet wood for sure

30% refers to the weight so if we had a hundred pounds of this freshly cut wood

30 pounds of that would be water and that's way way too much

For indoor applications, indoor woodworking type projects, so furniture, trim, other stuff like that

in a climate where there's a heating season and I'll explain why that matters later

for that we want wood that's maybe no wetter than about 9 or 10% at the most

and a little bit drier than that is advisable. So how do we know how much moisture wood has

Especially when you can't tell by feeling it. A moisture meter takes the guesswork out of that

These are very inexpensive nowadays and as I said you don't need to buy one of these right now

Do not buy one of these right now but you need to know about it as you progress in the craft

it's going to become more and more valuable. Essentially this is an electrical resistance meter

So there's a battery inside and then these two electrodes. So the scale shows moisture content but in reality this meter is measuring

the electrical resistance of a piece of wood and the more moisture is in the piece of wood

the lower the resistance to electricity so the further over to the right hand side this meter is going to go

So can you tell which board is going to be drier? You can't, not by looking at it

These two pieces of wood have been stored outside. I just brought them in now, they've been outside forever

It hasn't been raining or anything so the surface seems dry but let's just take a look

I'll just stick these probes in here and press the button and we've got a pretty wet piece of wood here

That's over 20%. So if we decided to make some indoor project out of this stuff we would come to grief

because that wood would shrink and warp and change shape because we started to work with it when it was too wet

Now this other one, this one's also been outside but let's just take a look. I'll just stick the probes in again and now that is drier

Strange because they're both sitting on the same pile side by side and they have been for years so I'm not sure why the difference is

It's just under 12 there. That's still a little bit too wet for indoor woodworking

and then this piece of wood has been inside my heated shop all winter

It's actually been in my shop for years but it's been subjected to plenty of heat

It's fully room temperature in my shop all the time and let's see how dry it is

Whoa, that is one dry piece of wood. Actually, that might even be too dry and that gets me to the next thing

I want to explain to you about wood and moisture. So as I mentioned before, wood shrinks when it dries and it expands when it gets wet

It's what's called hygroscopic. That means wood takes moisture from its surroundings

when there's more moisture outside of the board than inside and conversely if there's more moisture in the board than outside the moisture leaves

So it's kind of always in a state of flux and that's very important to understand

because if you remember this board is super dry, actually a little bit too dry

This one is somewhere in the middle and this one is really wet. If I stored this board outside, even if it wasn't raining for two or three months

moisture content would probably rise to 12, 14, 16 percent. Something like this one here

If I took these pieces of wood, which are too wet to use for any kind of interior woodwork

and I kept them inside my heated shop here for a while, maybe a month or so

they will now be dry enough to use. So it moves either way

But there is another thing here I want to show you as well

and that's the fact that wood does not expand and contract with changes in humidity in all directions

That makes it a little more complicated. So here we have our board and the grain runs this way

When this was in the tree, it was like this. It was standing up like this

So this is what's called along the grain or parallel to the grain

And in this direction, we're across the grain. Now wood changes shape with changes in moisture content a ton in this direction

So across the grain and almost nothing at all along its length

It can be quite extreme actually. Like this piece of wood here is just under, just a slight bit under nine inches wide

Now if this piece of wood were as wet as this piece of wood

this might be at nine and a quarter inches wide. I mean really, it can, certainly it's going to get bigger by an eighth of an inch

if not a whole quarter of an inch. And of course, if you have a fairly complicated woodworking project

and that piece of wood is contained in some way and it wants to get smaller

it will get smaller. There's no stopping wood from getting smaller when it starts to dry out

And that can be a problem. It can cause cracking or warping, twisting

Many new woodworkers run into problems like this. Not with a very simple project like we're covering here in the course at the beginning

but later on when things get more complicated, loss of moisture can be a real problem

So the solution is really pretty simple, and that's just to make sure your wood's dry enough before you start

So do you remember I mentioned this wood is really kind of too dry for some things

You know, when like the moisture content on this thing is, it's really, really low

We're bordering on nothing almost. We're getting down to four percent or so

Very dry. Now, in the course we're going to be building a footstool

Very simple project. I can use wood this dry for that footstool

because all that's going to happen if it picks up more moisture

and it very well could, well, it will in the summertime when it gets more humid

all that's going to happen is those wood parts are going to get wider

And because the footstool has a top, two legs, and a cross piece, and nothing else

then those parts are free to get wider or narrower, sort of to breathe in a sense as moisture content changes

Ideally, it would have been better if this wood had been about seven to nine percent moisture content

because then it's really not going to do a whole lot of anything. It might get slightly wider in humid weather in the summer

but there's not going to be a lot of change. There's a fair amount of moisture that needs to be absorbed by this board

before it gets to kind of the normal equilibrium content for moisture in a house

So, there are projects where a piece of wood like this might be contained by something

by some other framework or some other feature of the design. So, the danger in a situation like that is that the wood is going to get bigger

and there's nothing that anybody can do to stop it. So, if it can't actually move outwards, sometimes it can break the thing that's containing it

Other times, the piece of wood can buckle upwards as it picks up moisture

because that lets it take up more room, so to speak, in that pattern

But that's the danger, that it'd be too dry. It's not usually a problem, though

It's not usually a problem because most wood that you're going to be working with

has not been sitting in a fully heated shop for months on end

Most of it's going to be maybe in an unheated warehouse somewhere or even outdoors sometimes

So, that's kind of the situation. Most of the danger is working with wood that's too wet

Very occasionally, under certain circumstances, you'll get wood that's too dry, and that's the situation we have here

This moisture meter is like quite a few of them, and as I said before, it's an electrical resistance meter

But how do you know how accurate it is? I'm sticking it in the wood here and there's differences in moisture

but how accurate are those numbers really? Well, this device comes with a calibration tool here

Sixteen percent, it says. So, when I put these things, when I put the prongs in these screw heads

that are metal contacts that go into this stuff, whatever that stuff is

there's a certain recognized electrical resistance. And when I push the button, about sixteen percent

So, pretty close. There's a little slot screwdriver that I could use to adjust the exact position of that

But this thing's accurate and quite useful too. So, that's the scoop on moisture content

I'll be talking a little bit more about that later, especially as we get into making some projects

But that's the bottom line, and you're well on your way to understanding

all of the weirdnesses of wood when it comes to moisture. Now, you might be wondering something about wood finishes

It's very reasonable to think and assume that if you put a finish

especially a thick urethane finish of some kind of varnish or something like that

thick paint maybe, you put that on wood and it's reasonable to think

that it's going to stop excess moisture from coming in and prevent moisture from coming out and drying and causing problems

The answer is no. Water is just simply too sneaky for that

Water can really get through almost anything. The only thing that your thick wood finish is going to do is slow down the changes

But ultimately, the wood is going to get as wet as the surrounding air

or as dry as the surrounding air, and it really doesn't matter at all what you put on it

There's no hope of outsmarting water like that, because it's very sneaky and very persistent too