Question about moisture meters

chris-mcpherson said:

First... what moisture content should I be looking at for well seasoned hardwood? 20 percent and under.I wanna buy one without breaking the bank. Can a decent one be had for under $50? I see a few at Amazon around the $20 point... are they any good? I have the four pin from amazon ( wanted the two pin) but it should give you a good idea on the moisture content of the wood. zaphttp://www.amazon.com/HQRP-Moisture-Analizer-Drywall-Inspection/dp/B000XUQNLM/ref=sr_1_16?s=power-hand-tools&ie=UTF8&qid=1289237380&sr=1-16
Please notice the four other models shown under "What Do Customers Ultimately Buy After Viewing This Item".
What would you recommend?

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Chris this is the moisture meter I use.


zap

This $12.99 instrument from Harbor Freight will do you fine for firewood.

http://www.harborfreight.com/catalogsearch/result?category=&q=moisture+meter

I do disagree about the moisture content, however. The meter will give your the MC expressed the way the lumber industry expresses it, but the wood burning industry expresses it differently. I won't keep repeating the reason why because nobody but a few on this board believe it. Regardless, when your electric resistance meter says your wood is 25% MC, it is really only 20% water by weight. Therefore, anything under 25% as read by a moisture meter is good to go since it is within the moisture range that these stoves are tested at.

Here is the one I have from Lowes. It was $30. Does just fine for what I ask of it.....which is- just a geneal idea of my borderline, 16-18 month oak and locust.....and for s**ts and giggles testing other stuff. I love learning.
http://www.lowes.com/pd_78059-56005...pl=1&currentURL;=/pl__0__s?Ntt=moisture+meter

Call ahead. I called and nobody in the store knew where to look for it. They finally found it in the 'plumbing' section. It took a while, though.

chris-mcpherson said:

First... what moisture content should I be looking at for well seasoned hardwood?
I wanna buy one without breaking the bank. Can a decent one be had for under $50? I see a few at Amazon around the $20 point... are they any good?

http://www.amazon.com/HQRP-Moisture...wer-hand-tools&ie=UTF8&qid=1289237380&sr=1-16
Please notice the four other models shown under "What Do Customers Ultimately Buy After Viewing This Item".
What would you recommend?

Click to expand...

If you have to buy wood I wouldn't count on finding any one selling it dry enough to burn. I would take responsibility for drying my wood and build a woodshed so when it comes time to burn I know the exact condition of the wood. If you lack either the space, the means, or the ambition to crib up a sufficient volume of wood for a sufficient length of time then you should consider switching to pellets or propane.

My HF meter seems to work OK.

chris-mcpherson said:

Battenkiller... thanks for the heads up real MC, I'll look for 25% or less.
wood fan... glad to know I can get one at Lowes, I don't have a HF near me and I'd rather not pay shipping charges... off to Lowes I go.

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Chris, here are the formulas used for each method, and two that you can use to convert back and forth between wet and dry basis moisture content. As I said, the meters are calibrated for dry basis because that is what the timber industry uses, while the wood burning/testing industry uses the wet basis calculation because they are interested in the heat lost to evaporation of water. Mathematically, the difference between the two becomes less and less as the MC gets lower, but at 50% MC dry basis, the actual water by weight is only 33%. Ironically, at 33% MC on the meter (above the upper useful limit of most meters), you are almost there at 25% water by weight. I think this is such an essential point to understand that you might as well not even own one if you disregard it. Folks are giving themselves enough stress with these things as it is.

Moisture Content: Web Basis and Dry Basis

Moisture content in wood fuel is calculated as the percentage difference between the wet weight of the wood and the dry weight of the wood. If the percentage is calculated relative to the wet weight, it is called the wet basis moisture content.
MCwb=(Ww-Wd)/Ww ×100

If the percentage is calculated relative to the dry weight, it is called the dry basis moisture content.
MCdb=(Ww-Wd)/Wd ×100

The two methods of reporting moisture content are related by the equations:

MCdb = MCwb/(100-MCwb)x 100 and MCwb = MCdb/(100+MCdb) x 100

For Stove testing, the CSA testing procedure requires that the moisture content of the wood be between 16 and 20% (wet basis), for an average of 18%.

See: B415.1-00 Performance Testing of Solid-Fuel-Burning Heating Appliances, Canadian Standards Association International, December 2000.

From: Residential Wood Burning Emissions in British Columbia Environment Protection Division, Ministry of Environment, British Columbia April 1, 2004, Revised May 17, 2005

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Green wood is like a soaked sponge. A sponge can hold a lot of water, but if you squeeze it as hard as you can, most of the water will be removed. This is equivalent to what is called "free water" in the wood. It moves relatively easily through the wood structure and is the first part to evaporate from the wood. But at this point, a sponge is still quite damp with what is termed "bound water". In wood, this state is called the "fiber saturation point"(FSP), and is about 28% MCdb in most species of wood. Below the FSP, wood will begin to shrink and warp, and it will start to check at the ends. Above the FSP, resistance meters aren't really accurate at all, so by the time your wood is dry enough to get an accurate reading on a resistance-type moisture meter (28% MCdb)...

MCwb = MCdb/(100+MCdb) x 100 = 28/(100=28) x 100 = 22% MCwb

... for all intents and purposes, already dry enough to burn. ;-)

Generally speaking green wood cut down, should be 35% reading? Close to it. Oak is forever(2 yrs) drying to . I could be wrong, but somethings not right. I've got 2 yr old wood(maple/beech) that just hit 15%.

Rule of thumb, no wood is dry unless its split and stacked close to 12 months. Really dry is going to be 18 plus months in the making.

Trust your gut, you got wet wood. Ever noticed it, while it's burning, p!ss!ng out water on the ends?

For me it was, but more curious than anything. Make sure you put the probes with the grain of the wood. Split a piece and measure the inside.

chris-mcpherson said:

I got the meter yesterday and am a little surprised. I tested some wood that I thought would be borderline and it's showing 10%-15%. I then tested Red Oak that was cut down less than3 months ago and split less than 1 month ago and it's showing 20%-25%.
The whole reason I got the meter was because I assumed the wood I have wasn't seasoned enough because of the tough time I'm having keeping a good fire going without the draft controls wide open. The next morning I have chunks of wood mixed in the ash... this tells me I have wet wood. Am I getting inaccurate readings because of the FSP your describing? Is there a standard that can be used to test the accuracy of my meter?

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I only brought up the FSP because the meters some folks have (mine included) give a reading of up to 42% MC, but that is so far above the FSP that these readings can't be trusted. In your case, the spurious readings have nothing to do with the FSP as you are getting readings way below that point. I simply find it very hard to accept that you could cut red oak and have it burnable in 3 months, no matter when you split it or where in the country you seasoned it. Most others on this board would agree. Based on what you are telling us about the quality of your burn, I really think your wood may not be ready to burn. Therefore, there may be something wrong with your meter.

But first...

- How big are your splits on average? If you split them really thin, they may in fact already be seasoned.
- Did you re-split the wood and take a reading from the fresh faces? This is absolutely essential to do, wood dries very rapidly on the outside.
- Did you bring the wood inside to get it up to temperature? Otherwise, you need to make a temperature correction.
- Did you push the prongs into the sides (not the ends) with the prongs running in the same direction as the long grain? Failure to do this can skew your results by a couple of points.
- Did you hold the prongs in firmly, and long enough to get a steady reading?
- You may need to make a few points correction one way or the other depending on what species you are measuring. Red oak, however, should need no correction.

As far as a standard goes, if you push the prongs into your palm, you are supposed to get a reading of about 35% MC. Of course, that depends on what species you are. :cheese:

But the plot thickens...

Calibration
Most resistance moisture meters use Douglas-Fir (Pseudotsuga menziesii) as their default species calibration. Table 2 shows the Australian resistance values used as the Douglas-Fir standard (Edwards, 1974 and AS/NZS 1080.1: 1997). Note, there is a slight discrepancy between the figures shown for meter readings of 6 and 7 % between Edwards (1974) and AS/NZS 1080.1 (1997).
Confounding the issue of Douglas-Fir calibration, is that different world regions and different meter manufacturers can, and do, use different standard resistances for Douglas-Fir. To illustrate this point, resistance values for Douglas-Fir (Coastal) from James (1988) are included in Table 2. Skaar (1988) confirms that these are the resistance values used by most US made resistance moisture meters. Many meter manufacturers do not actually provide or specify the standard resistances they use as their Douglas-Fir calibration.

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The table below contains the resistance data for Doug Fir from three different sources. As you can see, two of them concur with each other, the third is way different. According to the text quoted, the third column (James [1988]) contains the resistance figures that are used to calibrate most American made meters (i.e. Delhorst). Where was yours made? :shut:

At any rate, if you accept that yours was probably calibrated to the U.S. standard, you can go to Radio Shack and get 2 or 3 resistors that correspond to the values of interest on the tables and see if they agree with your meter. For example, a 600 KOhm resistor should give a reading of 24% MC on your meter, 2.14 MOhm should read 20%. etc. But I bet you're not likely to go to all that trouble. ;-)

I wish it could be easier. People want to just go out and purchase a cheap device that takes the doubt out of their efforts and they end up having more doubt. Or else they just blindly trust the device. That's why many here (including myself) don't think using meters is the best way to go. I bought one, found the results interesting, checked the calibration using the oven-dry method and find that it's close enough for government work. Beyond that, the tried and true method of storing your wood for a long time and testing it by burning some in your stove works best.

My guess is that you did not split your wood and take the readings from the heart of the fresh split. That is where you MUST insert the pins of the MM. I mess with mine from time to time ( from lowes ) , and on fresh oak splits -its rings 50% (highest reading). Thin splits (3") at 12 months - 20% ......a little thicker(4 1/2-5") are 23-25%.

chris-mcpherson said:

I mistakenly thought it was as easy as "purchasing a cheap device that takes the doubt out of my efforts" Mainly, I hoped to keep the wood purvayers I need to rely on this year honest. What is the oven dry method?

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Your meter giving a reading of 37% in your palm sounds close enough to me.

The oven-dry method is where you weigh the wood, slowly dry it in an oven set for about 215ºF to drive out all the moisture, re-weigh it to get the amount of water lost and then divide the water lost by the oven-dried weight to get the MC (dry basis). To get the wet basis MC (as is used for wood burning), divide the water lost by the original weight instead of the oven-dried weight. This number will always be lower, but it won't correspond to the meter readings (which, as you now know, are computed using the dry basis formula). Regardless of the formula you use, this method is 100% accurate if done correctly, and it gives the average for the entire split, no matter what temperature or species it is. No corrections need be applied.

Let's hold off on that for now. I don't think your wood dealers will have the patience to wait for the results. ;-)

You already know the answer - you didn't re-split. Go out now and pick out a couple average size splits and re-split them right down the middle. Check the freshly split faces immediately, lining up the two prongs with the lengthwise direction of the grain. Don't worry about the temperature for now, it's not all that critical. I'll bet it's way higher than you thought it was. That would make sense given the description of your burn.

Good luck, hope this helps.