Testing moisture content

[Chris152]

I was trying to test the moisture content of some air-dried, milled wood today. It's a damp day and the wood's still in stickers, under cover but open to plenty of moving air. It's a very damp day here in South Wales.

Putting the probes into the outside of the wood, the wood measured about 30 percent. Cutting a small piece off with a hand saw, the wood registered 18pc mc inside. A piece we planed on a planer a few days ago was 18 pc mc after a few mm from the surface had been removed, but had gone back up to 25 pc after being stored in the same damp air, under cover.

I guess the action of sawing/ planing can raise the temperature some and maybe dry the surface revealed, and assume the outside of the wood (usually drier than the inside) is holding some of the moisture from the air. Both of which would give a reading that's wrong in relation to the inside of the wood?
How I'm supposed to know the true moisture content of the wood?!

Thanks, Chris

 

[dzj]

Try the oven drying method. Use a few samples that you think represent the whole amount of your timber. It's a pretty precise method and can be done at home if you have a decent weighing balance/scale.

 

[Beau]

25% is a tad high but not far off what it could go up to in this weather. This table is pretty handy https://www.woodworkerssource.com/shop/mois.html

 

[Sgian Dubh]

Chris, air dried, under cover, stickered up, yet very open to external moving air and RH conditions, in winter, and having perhaps sat there for many months or years, then 20% MC and above isn't too much of a surprise.

There is always a moisture gradient in wood in normal circumstances: abnormal circumstances would be a strictly climate controlled atmosphere where temperature and RH vary minimally allowing the wood to reach EMC. Such circumstances might be museum archives or laboratory conditions of some sort. Outside of that, wood is always adjusting its MC to come into line with whatever RH and temperature conditions surround it, either getting wetter, or drier, so wood is not "normally" drier at the core than it is at the shell. The reality is that sometime it is, and sometimes it's the reverse, in normal situations, e.g., in a habitable building, a workshop, a draughty shed, externally, and so on.

If you really need to know precisely the average moisture content of your wood then, as others have said, do the oven drying test. Below is a link to a method using a Microwave oven, which works very well, and would only take about an hour to accomplish: http://www.richardjonesfurniture.com/Ar ... -wood.html

If you feel the need to bone up a bit (a lot maybe?) on your knowledge of wood, I'm pretty sure there's a half decent book out there that might help you … but the name of the book's author, and its title seems to have completely slipped my mind for the moment, ha, ha. Slainte.

 

[Chris152]

Thanks all for the replies. I'll have a go at the oven drying method over the weekend - sounds like it's something I need to learn. That's a great write-up, Richard - and I've had that book by an unknown author in my sights for a while, saving the pennies...

Out of interest, is the problem I've identified (with using a meter) common, or is it just the odd conditions I described? Does cutting or machine planing the wood dry the surface enough to give a false reading?

 

[worn thumbs]

You have found the reality of using a meter;it only measures the moisture content in the region of the probes.If you try the oven method be aware that you should only go a little above 100 deg C,too much heat will drive out the resins and oils in the wood and the additional weight loss will mislead you with regard to the moisture content.

 

[profchris]

Out of interest, is the problem I've identified (with using a meter) common, or is it just the odd conditions I described? Does cutting or machine planing the wood dry the surface enough to give a false reading?

I'd say it's the conditions.

If the wood is stored in a place where humidity is fairly constant, then after time it should have (at least approximately) the same moisture content throughout.

But if humidity is changing (outdoors for example) then the wood is constantly either absorbing water from the atmosphere, or losing moisture to it. This will always give different readings at different points, because only the surface is exposed to the atmosphere. So in those conditions, all a moisture meter can give you is a minimum and maximum value. Thus the average moisture content of your wood was between 18 and 30 percent, that's all you know.

 

[Chris152]

Couldn't wait til the weekend!
My cheap microwave oven doesn't have a lower setting than Defrost so I went with that for 45-second cooking, with a minute or so between. I measured them on a digital, Salter kitchen scale, with 1g increments.
The readings I got were: 40, 40, 39, 37, 36, 35, 34, 35, 34, 34, 34, 34.
So, using Richard's equation, that works out to 16.6 pc mc. Brilliant!
I'd conclude that the sawing/ planing made no significant difference to the mc at the surfaces of the cuts (/reading on a meter), and that the differences on the surface were indeed due to atmospheric conditions - as was suggested above.

Definitely going to get saving harder for that book.

Many thanks! C

 

[Sgian Dubh]

Couldn't wait til the weekend!
So, using Richard's equation, that works out to 16.6 pc mc. Brilliant!
Definitely going to get saving harder for that book. Many thanks! C

Many thanks for the feedback Chris. I'm pleased the method I described worked well.

Just one thing to bear in mind: although you came up with 16.6% MC that's the average MC of the wood because there will almost certainly be a moisture gradient within it. For example, the shell might be something like 18% MC, the intermediate zone, say 16% MC and at the core, perhaps 14% MC, or the position of those MC figures reversed, if you see what I mean.

If you manage to save the pennies enough for a copy of Cut & Dried: A Woodworker's Guide to Timber Technology, I truly hope you find it informative and helpful in your woodworking. After all, as I've said consistently from the moment I started writing it nearly thirteen years ago, "I'm writing as a woodworker for other woodworkers." In other words, I wrote about all the stuff I felt it was really important to write about from a woodworker's perspective rather than as a wood scientist for other wood scientists.

And thanks again for the feedback. Slainte.

PS, as others have said, localised conditions can mess up the readings of both pin type and pinless moisture meters, e.g., a particularly localised damp spot can skew the readings.

 

[Chris152]

Thanks Richard - I liked the fact a novice (me) could follow your writing and be confident to get it right.
How come they're not selling through Amazon UK?

 

[Sgian Dubh]

Thanks Richard - I liked the fact a novice (me) could follow your writing and be confident to get it right.
How come they're not selling through Amazon UK?

Lost Art Press have a policy of not working with the big online sellers, such as Amazon. I'm fairly sure their reasoning is along the lines that Amazon (for example) would want such a large wholesale discount it would make publishing their specialist niche market books economically non-viable. I'm guessing a bit, but if an author sells thousands of copies of a book through Amazon thus netting a small margin, then the earnings are acceptable. On the other hand, a title such as mine from a small publisher, is only going to sell in relatively small numbers, leaving little or no profit for anyone.

So, the only outlets here in the UK at the moment I think are Classic Hand Tools in the south, and The Woodsmith Experience in Tyne & Wear. Slainte.

PS. I've just noticed that Classic Hand Tools seem to be "Awaiting Stock" of Cut & Dried again, after recently receiving, and as I recall only as recently as late November, a new batch of books from Lost Art Press's shipper.

 

[Chris152]

PS. I've just noticed that Classic Hand Tools seem to be "Awaiting Stock" of Cut & Dried again, after recently receiving, and as I recall only as recently as late November, a new batch of books from Lost Art Press's shipper.

That's a shame, but good news for the book. Thanks again, C