Magic‼ Friggin' magic, I tell ya'‼
Your moisture meter measures the electrical conductance between the two pins... more moisture, more conductance.
First of all, the fiber saturation point of wood is approximately 30%. As moisture content in wood approaches fiber saturation point the relationship between electrical conductance and moisture content correlates poorly. Meaning any readings over about 25% from an inexpensive moisture meter are flat meaningless. Even readings from the best and most sophisticated meters are questionable at moisture levels much over 30%.
Second, temperature has a significant effect on electrical conductance in wood... your inexpensive moisture meter does not do temperature correction calculations.
Third, the electrical conductance of wood changes with species... at any given moisture level the conductance will be different in Hackberry and White Oak. For example, at 80°, if the measured resistance is 1 million ohms in Hackberry that's around 18% moisture... but 1 million ohms in White Oak is closer to 21% moisture. Your moisture meter does not know what species of wood it's testing, and it don't know what the temperature of the wood is, it only know that it's seeing 1 million ohms... so what value do you suppose it displays??
I'll tell you what it displays... it displays the moisture content of the species it's calibrated for, at the temperature it's calibrated for...
And if I had to guess how it's calibrated... pine and fir construction lumber at 70°... so 1 million ohms will cause it to display 23% moisture. And the error is not linear... meaning as the wood gets dryer the the error factor increases. At 15% moisture the Hackberry value would be about 4 million ohms... but White Oak at 15% moisture would be nearly 13 million ohms, over 3 times higher‼
There ain't no friggin' magic... your 18-20% Hackberry reading is wrong, your 30% White Oak reading was wrong (likely way, way, way wrong), and your 50% reading is just flat bogus.
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