Funny, I was driving by the local HF yesterday and decided to drop in to get a cheap moisture meter to compare with the microwave oven-dry method I developed. I knew about the palm test because I used to sell moisture meters when I worked for Woodcraft. I insisted on checking the meter in the store, and it read exactly 35% in my palm, so I bought it. Then I got home and started to spear various bits of wood. All of my firewood read from 0-10% on the outside. I split a piece of cherry and it read 16% on the inside, same piece read 6% on the outside. Every piece of woodworking lumber in my shop read 0%, so I was ready to toss the thing because there's no way wood gets down to 0% outside of an oven.
Fast forward to this morning. I grabbed a big split of cherry. It measured 0% on the outside. I split in down the middle and remeasured the inside... 24% MC! You could pick up a hint of cherry smell and the re-split face actually felt slightly moist and cool to the back of my hand, so it was obviously wetter on the inside (outside felt dry as dirt).
So.....
I split off about 1/2" from the inside of the re-split face and measured it. 24% MC on both faces now. So I cut segment out of the middle and used the oven-dry method to determine the MC. Original piece was 71 grams, final dry weight was 57 grams. 71-57=14. 14/57=.246, or 24.6%! That's just about exactly what the cheap meter said it was.
Then I split off about 1/4" from the outer face and measured that. It still measured 0% on the outside face but 10% just 1/4" in from the outside. I weighed and oven-dried this small piece. Original weight was 19.6 grams, oven-dry weight was 17.8 grams. 19.6-17.8=1.8. 1.8/17.8=.10, or 10%. The cheap little meter was correct again.
What I figure is that the meter has a range 6-42% MC, so anything below 6% is outside the low range and will just register as 0% on the readout. So most of my woodworking stock that has been in the shop for years and has been exposed to the dryness of the heated space is probably not 0%, but lies just outside the range the meter can read. Given the low RH in my shop, this makes total sense since I would expect this wood to be slightly under or over 6% MC. It may be 5.9%, but the meter can't display that so it just displays 0%.
So how dry in my wood? Probably about 6-10% on the outside, 20-25% on the inside (depending on how thick it was split), so that averages about 15% or so for the entire piece. That's just a few points lower than when I did a similar oven-dry test about a month ago (18% then). That makes sense as well, since the wood has been sitting in the dry basement for an extra month.
Take home message? I'm not sure. Clearly, though, the cheap ($12.99) meter was highly accurate... at least on cherry. These things works by measuring the electrical resistance, and that
varies among species at the same MC. My ash, oak, maple, box elder and hickory may not give the same degree of accuracy. I'll have to run some more comparisons in the near future.
As far as the problem presented by the OP? Yes, like others here I fully expect that 18 month oak might still be 35% MC on the inside and read only 15% on the outside. Wood dries a lot from the ends, but it dries from the faces as well.
The general rule of thumb for woodworkers air-drying hardwood planks is that it takes 1 year per inch of thickness to completely dry to the equilibrium moisture content of your storage area. So a 4" thick board would take 4 years to completely dry. Firewood is shorter than a board, so the end drying process accelerates the drying time. Still, don't expect the very center of a split to be the same as the outside is, even with a wood like ash. Much more likely, you will find a
moisture gradient across the thickness of the split, just like I found in my tests. That will probably be present for at least a couple of years, maybe three or four in the case of oak.
Will it burn OK? About the same as anybody's 18 month old wood, I'd guess. Still, better to season it outside and move it into the shed after a year, methinks. And leave the oak for twice as long, that's just something everyone comes to the same conclusion about with seasoning oak.
Oh... I'm keeping the little Harbor Freight meter. I have to say I am pleasantly surprised with its performance, so I guess I have to eat some crow for the disparaging remarks I've made here about these things. Besides, there are charts available on the Interweb that give the resistance of various wood species that I can use to develop a list of factors that can be multiplied by the meter readings. That's all the $200+ meters do anyway, only it's programmed into the unit so you don't have to make the calculations. Still, you have to be able to identify the firewood you are testing. Ironically, those new burners who will most benefit from the use of a moisture meter will be the least likely to be able to ID their firewood (as is obvious by the plethora of "ID This Wood" posts). Without knowing the correct species, you may get an incorrect idea of your moisture content. But it seems to work for cherry just the way it is at any rate.
BTW, don't try the palm test if the sales clerk happens to be an extremely foxy babe. The sweaty palms will surely throw the test off. I pinned the thing, then had to dry off and go find the ugliest dude in the place to try it again.
What the hell??? Some of the wood I measured has been sitting in my living room for 18 months. So, what is the correct way to measure wood moisture content? Do I rely on the exposed surface readings of previously split logs, or do I have to re-split the wood and use that measurement? My wood looks great to the casual observer (and burns just fine) but I would like to know for future deliveries of wood.
