Assessing moisture content via weight

[woodbe]

This is for *cough* firewood :)

I chopped up some council gum prunings at the beginning of summer and have left them on the old trampoline in the sun. While cutting them, I marked a few pieces with their green weight, and I'm wondering if I can calculate the moisture content now, and if we've reached anywhere near optimum for firewood.

As the photo shows, this piece weighed 1409g green. It now weighs 1084g

Are we there yet? :D

woodbe

[woodbe]

Thinking about this, I think it all comes down to what the green moisture content was. I did some sums, and came up with these results:

60% moisture content when green:
Dry Timber Weight: 563g
Moisture Weight Now: 520.4g
Moisture Content Now: 48%

50% moisture content when green:
Dry Timber Weight: 704g
Moisture Weight Now: 380g
Moisture Content Now: 35%

40% moisture content when green:
Dry Timber Weight: 845.4g
Moisture Weight Now: 238.6g
Moisture Content Now: 22%

Unless someone has more clue than this, my approach will be to measure the same block after another couple of weeks. If it is still changing rapidly, I'll have to assume green moisture content was higher than 40% because 22% must be pretty low (but about ideal) based on comments found on Google.

woodbe

[Toymaker Len]

Optimum for firewood would be twenty years drying in the paddock. Thenyou take a sliding scale back to green wood. Rule of thumb 'If it burns OK without much smoke it is dry enough for firewood.' Moisure content is for cabinet timbers.

[woodbe]

Moisure content is for cabinet timbers.

Firewood - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Firewood#Heating_value_of_firewood)

Cheers

Woodbe

[jmaxwell]

you work the other way for MC you weigh it then dry the timber in an oven until there is no change in weight (no change in weight no moisturein timber sample)The difference in weight expressed as a % will give you the MC of the timber or get a moisture meter

[pjt]

I did something similliar a while back, I measured weight every month for over a year and saw the mass dropping until it stopped dropping or had slowed, without further forced drying the MC is about where it will stay, not sure of the numbers right now but a piece that was ~3000g lost over a 1000g, was an interesting exercise, I should test it with the meter and see where it is at:?


Pete

[old pete]

Hi Woodbe,

As usual there isn't a definitive answer to your question. The MC% of wood is the green mass minus the dry mass divided by the dry mass and then multiplied by 100 to give a %.

The problem is that even in a given species or indeed in a given flitch moisture content and dry density can vary enormously. It's common in Tassie messmate for example to get a MC% varying from 80% to 160% measured on a dry mass basis in the same tree. The good news is that the wetter wood dries much more quickly and as the mix of MC% classes approaches the commercial dry target of say 12% the whole batch is at this MC% give or take a % or 2. The wetter wood is invariably of lower basic (dry)density. You can do the calculations the way you have done which assumes the dry density to be a constant so long as you qualify the limitations of the method

The dry mass is determined by oven drying small samples at 110 oC for 24 hours when it is assumed that the wood is then "bone dry" ie a MC% of zero.

Just to make life more difficult there is generally a MC% gradient from the core of timber to the case whether kiln dried or air dried. The limits to which this is acceptable is set down in AS 1080 and AS 2796 depending on the end use of the dried product. There is also an AS Drying Quality Standard the number of which escapes me just now.

If you get your firewood in SE Oz to an average 20% MC you will have a top product. That's about the lower limit you can hope to achieve. That figure is not an assumption of mine I've done many hundreds of measurements as part of a project to attempt to frame regulations aimed to control the MC% of firewood for sale in Tassie. The concept of regulating this product for MC% was ultimately dropped due to the difficulties of measuring the variables themselves.A legal nightmare but quite good for the pocket of consultants if you don't mind taking your pay in the form of semi dry wood!!

By contrast I'd expect the MC% of dry red gum in the centre or southern parts of NT to closer to 5%MC judging by the way it burns and how difficult it is to extinguish but I have not made those measurements.

MC% of timber is a many splendored thing. It's a good idea to be careful in ones use of the term.

Cheers Old Pete

[woodbe]

Thanks for the replies, and especially old pete. You're a wealth of information :)

Next time I do this, I'll weigh and mark more pieces, I only did a couple in this batch. This one is the biggest piece and it's also from the heartwood of a larger branch so I'd expect it to be drying the slowest.

The context is we don't actually have a wood fire at home, but we do go camping. Last year at the end of a trip we found ourselves up at the Flinders and out of firewood. In the outback, we often pick timber from beside the road, but as this is a no-no in the Flinders, we do try and observe the park management's wishes. So we bought a bag of firewood from the Wilpena Resort store. Big mistake - the timber was some sort of redwood, difficult to light and steamy/smoky as all heck. In the end we threw some heatbeads in to get things going but even then the firewood was really only decoration.

Hopefully, this wood will be dry enough to use come winter. I've cut it small so that it will dry quickly and also stack neatly. It will be used for small cooking fires, not bonfires like some of our fellow campers seem to prefer :)

woodbe.

[BobL]

Using one piece of wood to determine MC can lead to an MC result that is not representative of the bulk of the wood.

As soon as the wood is cut, cut your self about a dozen test pieces of wood from different parts of the log and weigh them immediately (WT1g, WT2g, WT3g, . . . .). Then dry them to constant weight in an oven, and then weight them again (WT1d, WT2d, WT3d, . . . )

Their starting moisture contents (MC1, MC2, . . . ) can be then determined using
MC1 = (WT1g - WT1d)/WT1g x 100
MC2 = (WT2g - WT2d)/WT2g x 100
MC3 = (WT3g - WT3d)/WT3g x 100
etc
Averaging the these MCs will give the starting %MC of the wood.

At the same time as you cut the test pieces, cut a second set of other test pieces and weigh them (W1g, W2g, W3g, . . etc) and then place them back into the wood pile.

The "dry to constant weight" of these test pieces can now be calculated from
W1d = W1g - (MC x W1g)/100
W2d = W2g - (MC x W2)g/100
etc

At any time you like after that you can collect up these test pieces and weigh them (W1, W2, W3 . . . ) and determine their current %MC using
(W1 - W1d)/W1 x 100
(W2 - W2d)/W2 x 100
etc

Clear as mud?

[woodbe]

Clear as mud?

Good info, thanks BobL !

woodbe.

[woodbe]

Checked that piece again today, roughly 4 weeks since last weigh.

No change. (well, 1g less) We had a little bit of rain last week, but the wood isn't wet.

Found a couple of other smaller pieces I wrote weights on at time of cut.

That 1st piece is heartwood and weighs about 300g more than the other two pieces which are outer pieces including some bark.

#1 has lost 23% of it's weight
#2 and #3 have lost 33-34% of their weight.

Fire restrictions are over at the end of this month...

woodbe.