I just dug out a PDF from a very comprehensive Aussie study on several pro-quality meters. My take on the conclusions is that, even with the very best pinless meters out there, the expected accuracy is only about half of that attained with a good resistance meter. Furthermore, the pinless meters use species corrections based entirely on density, creating a higher degree of uncertainty due to variations in density among species grown in different locales, trees within the same local that grow more or less dense because of growing conditions, and even variations in density between upper and lower regions of the same tree. The species corrections used for resistance-type (pin) meters are not based on density at all, but on the amount and type of extractives found in that species. Basically, if you are sure of your species, the pin-type meter is at least twice as accurate, and will remain so for any density variations within that species.
Also, note the comment at the bottom about extrapolating species corrections on MC readings beyond 26% MC. That's why most correction tables only go to 24% or so. In general, the wetter the wood, the less confidence you should have in your reading, and that goes for both types of meters. The most amusing and frustrating thing about all of this for me has been seeing folks all excited about their new meter and then going out and cracking open a randomly selected piece of wood and finding out it is over 25% MC. Then the universal cry from the membership is, "Not ready. Wait at least another year"
Quite contrarily, I feel that because these meters are only truly as accurate as these studies show them to be, and because they use dry-basis calculations,
any reading that can be trusted (24% or below) means the wood is totally ready to burn.
I think I'm going to invent a firewood-specific resistance-type meter. It will have built-in species correction and a simple "go/no-go" display. Scroll down to where it says (red oak), hit enter, jam the prongs into the wood and if it's ready it will say "BURN". That will happen at the point where the actual MC of the wood is at or below 25% dry-basis. For red oak, that will occur about 17 years after you first stacked the splits in an ideal location.
CONCLUSIONS AND RECOMMENDATIONS
This project has generated new species corrections for a range of moisture meters for 10 of the most commercially important eucalypt species in Australia. When used on timber with a MC between 7 and 22%, the 95% confidence intervals, or expected levels of accuracy, of the corrected meter readings are generally between ±1.5% and ±3.0% for resistance type moisture meters and between ±3.0% and ±6.0% for dielectric or capacitance type moisture meters. The main reason for the poor confidence intervals, with the dielectric type meters is the effect of within species density variation on meter readings. For most species and most meters, with boards equalized under the same or similar conditions, the effect of density on meter readings is as great if not greater than that of OD MC%.
The corrections provided here for both types of meters are only for timber with a moisture content between 7 and 22% as that is the range used to generate the regression equations. The corrections can probably be reasonably safely extrapolated out to about 24-26%. However, the wide spread of data points shown in Appendix C (for the resistance type meters) with measurements taken on boards with MCs above 30%, clearly shows the danger and inaccuracy of extrapolating the corrections much beyond the 24-26% range.
