Measuring the water/moisture content of wood electrically

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Measuring the water/moisture content of wood electrically

Postby ToneWood » Wed Jun 19, 2013 10:57 am

For selecting firewood & when finishing greenwood bowls, chair legs, etc. it is sometimes useful to know how wet the wood is. Inexpensive moisture meters are now readily available (e.g. on ebay & from Stove/wood-burner dealers), even so, I have not yet felt sufficient need to justify such a purchase. However, I recently purchase an inexpensive but rather nice digital multi-meter (DMM, a V97) and it occurred to me that I could probably use this to get an indication of moisture content.

So what/how do moisture meters measure water content? My initial guess was probably resistance or, failing that, possibly capacitance. I did some quick googling: (as of 1963) it was typically determined either by resistance,capacitance, a combination of both or radio frequency (capacitance or power-loss). But it appears** most cheap, portable, modern meters measure electrical resistance :). Resistance is measure in Ohms but moisture meter typically instead display percentage water content (%MC). The first reference below contains tables (e.g. table 1) and graphs mapping the resistance of different woods to %MC. Some key points:
1. typical resistances of wood are measured in Mega Ohms (unsurprisingly)
2. resistance increases as moisture content drops (as might be expected).
3. Inexpensive DMMs typically "top-out" at around 40-60 Mega Ohms or less, which corresponds to about 14-15%MC - although there can be considerable difference between wood species.

* Electric moisture meters for wood - By William L. James, Forest Products Laboratory (U.S.)
** ... /articles/
Last edited by ToneWood on Thu Jun 20, 2013 8:46 pm, edited 2 times in total.
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Using a DMM to measure moisture content of wood

Postby ToneWood » Wed Jun 19, 2013 11:16 am

This is just my own rough interpretation of the data in Mr. James's table 1, an initial working draft but might be a useful starting point/guide/rule-of-thumb:

    Resistance v. %MC
  • <1MOhm = > 23%MC+
  • 2-4MOhm => 20%MC
  • 4-6MOhm => 18-19%MC
  • 40-60MOhm => 14-15%MC
  • >1GOhms = > <10%MC

Mr. James points out that temperature and wood species are also factors that will affect the resistance of wood (but cheap handheld meters would have to ignore the latter and will probably ignore the former too - they might include written guidance on adjusting for these factors though). Also, the species differences are not as straightforward as softwoods v. hardwoods.
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Re: Measuring the water/moisture content of wood electricall

Postby gavin » Wed Jun 19, 2013 10:14 pm

To use one of these meters reliably, you must split your wood in half and measure the moisture content (MC) in the midde of the split billet. Measuring MC on the surface of a bowl blank I think pointless. But such destructive testing of your bowl blank is not practical. Practise and experience will soon enough tell you how mellow a bowl blank is and suitable for any carving or turning purpose.
I would not bother with such a meter for aught but firewood - and only if I was given one and also was very bored; looking at the end grain for cracks is a better test IMO for firewood. Of course, if you were buying firewood, then they would be useful for testing the vendors claims re MC - but only if you split first and poke your probe in the mid-split billet. Topical testing will always underestimate MC.
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