Thread: Oven testing timber for MC

Hi Nifty!
This may shed some light??


Moisture content of wood
The moisture content of wood is calculated by the formula (Siau, 1984):
moisture content = (1.1)Here, is the green mass of the wood, is its oven-dry mass (the attainment of constant mass generally after drying in an oven set at 103 +/- 2 °C for 24 hours as mentioned by Walker et al., 1993). The equation can also be expressed as a fraction of the mass of the water and the mass of the oven-dry wood rather than a percentage. For example, 0.59 kg/kg (oven dry basis) expresses the same moisture content as 59% (oven dry basis).

Students in the United Kingdom would recognise this formula written as
moisture content = x100%Where the wet weight is the weight of the original 'wet' sample and the dry weight being the weight of the sample after drying in an oven. Moisture contents being expressed as a percentage.

Cheer's

Has anyone out there done it and how do you do it. A local flooring guy does it in a microwave I would prefer to know better methods

I have also used a microwave but you have to make sure you don't burn the wood. You only want to cook off the remaining moisture, not the carbon. You also need very accurate scales. Weigh it, cook it, weigh it, cook it and so on until the weight doesn't change. The difference in weight is the remaining moisture content.

You can get special ovens but the microwave method is used widely. An I reckon it's pretty accurate as long as you don't burn the wood.

Steve

Here is a good discussion regarding use of microwave ovens for testing MC

Moisture content oven tests

And here are some interesting discussions on kiln drying
Kiln Operation at WOODWEB's Knowledge Base

Thanks a bunch for the replies fellas, starting to make more sense

If you are going to do it in a microwave make sure that it isn't the one your missus uses in the kitchen!!!! Or you might find yourself in the dog house.


I have also heard of a similar thing being done using a fridge. weigh it put it in the fridge weigh it a few days later til the weight doesnt change. the difference in weight is how much moisture it has lost. I do not know how well this works but I have heard it does!

Nifty

Some very general drying times for ovens:

Coventional oven 24hrs
Fan forced oven 12 hrs
Infra red lamps 4 hrs
Microwave <1 hr

It would also depend on the size of your sample. The aim is to end up with something weighing around 50grams after drying. This is clearly quite small. Around six samples would be taken in a commercial situation.

A set of digital scales or a beam balance are obviously a must to achieve an accurate result. Fortunately nowadays digital kitchen scales are quite cheap if a more sophisticated option is unavailable.

Regards
Paul

Originally Posted by Bushmiller

Nifty
:

.

A set of digital scales or a beam balance are obviously a must to achieve an accurate result. Fortunately nowadays digital kitchen scales are quite cheap if a more sophisticated option is unavailable.

Regards
Paul

By memory.
I think I payed around $800 for a set of digital scale's, some year's ago, And somewhere around $1000 dollar's for a moisture meter
cheer's

Thanks again guys, i do have 3 moisture meters on the go but its good to make sure. And now with the microwave in the shed its a good thing

Hi Nifty,
I just remembered a couple of other things to watch out for.
1. When you take your sample out of a board be careful to do a clean cut that doesn't burn or heat up the timber with the saw blade.
2. Remove any splinters or dust from the sample before the first weigh.

Helps with the accuracy side of things.

regards
Steve

Gday

Just any crappy old electric oven will do I spent two years running the second largest Mountain Ash kiln operation in Victoria 100 to 150 cube on any given day and thats all we had to do our oven drys in it worked and did the bloody job well

Regards Chris

Originally Posted by gumred

By memory.
I think I payed around $800 for a set of digital scale's, some year's ago, And somewhere around $1000 dollar's for a moisture meter
cheer's

I suppose that is the sort of expense I wasn't recommending. My wife's kitchen scales will weigh to 1 gram although I have never verified their accuracy. It would be quite easy to verify their accuracy from a known weight. She tells me they were $95 from Tupperware, but added they would be cheaper now.

My own moisture meter is an old fashioned timber box affair with half a dozen batteries inside ( actually probably less than that).I bought it second hand for less than $100, but it would not surprise me to learn that it was $1000 25 years ago.

I think "AWR" did a review on moisture meters a while back, but can't be sure.

Regards
Paul

Originally Posted by Stopper

Hi Nifty,
I just remembered a couple of other things to watch out for.
1. When you take your sample out of a board be careful to do a clean cut that doesn't burn or heat up the timber with the saw blade.
2. Remove any splinters or dust from the sample before the first weigh.

Helps with the accuracy side of things.

regards
Steve

Absolutely. Sharp blade and go slow.

Also, remember correction charts for temperature and species. I have just remembered that this is only for electrical resistance moisture meters so disregard this paragraph.

Regards
Paul

Pual thats spot on with the scales they do not need to cost an arm and a leg as long as they are consistant Mate

I use to have a metal calibration bar and recalibrate the scales every time I did an oven dry sample batch .

Regards Chris

If you're going to do it with a microwave your scales will need to weigh down to a tenth of a gram. As you said your sample will be 50 to 100 grams so a set of scales accurate to one gram will not be good enough. (Won't stand up in court when the plaintiff's lawyer questions how you determined the moisture content of the said timber sold to my client) Sorry, shouldn't have brought that up. Don't want to stifle anyone's enthusiasm. But $300 will seem cheap in that situation.

As Chris said if you use an oven then you can test a bigger sample so the scales don't need to have the fine tolerance.

cheers
Steve