After studying the old Forest Service article on electrical moisture meters, my curiosity got the best of me. Realizing that, all I really want is, maybe once or twice a year, a simple "go/no go" indication of wood's readiness to burn, I decided to try using a very generic digital multimeter (DMM) to measure it, using the technique and data in the USFS paper. Nothing exhaustive; I just wanted to see if it worked and if my measurements matched the theory. Guiding observations for the test:
: The general consensus is, you want to burn wood that is 20% or less moisture content.
: Looking at Table 1 (Electrical resistance vs Moisture content of various species) in the USFS paper, except for the few really dense hardwoods out there (ash, hickory, American Elm), the DC resistance of the majority of species used for firewood are in the range of 2.0 to 4.0 MegOhms at 20% moisture, with most in the low 3.0 range. Higher moisture levels give lower resistance readings.
So if it measures 3 MegOhms or higher, it's ready. Well, even the cheapest of multimeters have a 10 MegOhm resistance range and so can easily measure 2-4 MegOhms. So a simple go/no go moisture test should be able to be done with a DMM. And it is much more practical to have a multimeter lying around than a single-purpose moisture meter. The DMM can be used for a multitude of other things.
Most DMM probes are too blunt to penetrate into the wood to get a good reading. One solution is to use a nail about the same diameter as the probe tips and punch a couple holes to measure in. Space them 1-1/4" apart along the grain in the fattest part of the split. You only need to go down 3/8 to 1/2 inch. Takes a few seconds to do.
So I took two pieces of Siberian elm - one that I cut and split in May, the other cut and split three weeks ago. I split each piece in half again to get to wood that hasn't been exposed to air, punched the 1-1/4" spaced holes in the fattest part, inserted the probe leads and watched the reading. If you just rest the probes in the holes, they won't make good contact and the resistance readings will jump around and steadily rise. But if you push on them to maintain pressure for better contact, the readings are very consistent.
The older split measured 3.2 Megohms, right in the 20% zone. The newer one measured around 750 kOhms. Very clear difference in obviously much wetter wood.
This impromptu test suggests this technique will easily answer the question of "is my wood moisture level at 20% or below". It may not tell you if it's 17% or 15% or lower; you might need a moisture meter to do that. But if all you need is to see if your wood is in the burnable zone, this technique should work just fine.
So the basic guidelines are:
If it measures 3 MegOhms or higher, it's good to burn.
If it measures less than 3 Megs, it needs to season longer.
If it measures open circuit (or above 10 MegOhms), then the moisture level is in the teens no matter what species it is, and it is definitely dry enough to burn.
For the super-high-density species mentioned earlier, divide those numbers by 2 or 3.
: The general consensus is, you want to burn wood that is 20% or less moisture content.
: Looking at Table 1 (Electrical resistance vs Moisture content of various species) in the USFS paper, except for the few really dense hardwoods out there (ash, hickory, American Elm), the DC resistance of the majority of species used for firewood are in the range of 2.0 to 4.0 MegOhms at 20% moisture, with most in the low 3.0 range. Higher moisture levels give lower resistance readings.
So if it measures 3 MegOhms or higher, it's ready. Well, even the cheapest of multimeters have a 10 MegOhm resistance range and so can easily measure 2-4 MegOhms. So a simple go/no go moisture test should be able to be done with a DMM. And it is much more practical to have a multimeter lying around than a single-purpose moisture meter. The DMM can be used for a multitude of other things.
Most DMM probes are too blunt to penetrate into the wood to get a good reading. One solution is to use a nail about the same diameter as the probe tips and punch a couple holes to measure in. Space them 1-1/4" apart along the grain in the fattest part of the split. You only need to go down 3/8 to 1/2 inch. Takes a few seconds to do.
So I took two pieces of Siberian elm - one that I cut and split in May, the other cut and split three weeks ago. I split each piece in half again to get to wood that hasn't been exposed to air, punched the 1-1/4" spaced holes in the fattest part, inserted the probe leads and watched the reading. If you just rest the probes in the holes, they won't make good contact and the resistance readings will jump around and steadily rise. But if you push on them to maintain pressure for better contact, the readings are very consistent.
The older split measured 3.2 Megohms, right in the 20% zone. The newer one measured around 750 kOhms. Very clear difference in obviously much wetter wood.
This impromptu test suggests this technique will easily answer the question of "is my wood moisture level at 20% or below". It may not tell you if it's 17% or 15% or lower; you might need a moisture meter to do that. But if all you need is to see if your wood is in the burnable zone, this technique should work just fine.
So the basic guidelines are:
If it measures 3 MegOhms or higher, it's good to burn.
If it measures less than 3 Megs, it needs to season longer.
If it measures open circuit (or above 10 MegOhms), then the moisture level is in the teens no matter what species it is, and it is definitely dry enough to burn.
For the super-high-density species mentioned earlier, divide those numbers by 2 or 3.