This is a pin-type meter designed for measuring relative wetness in drywall, concrete, and wood.  Regardless what the sales literature says, it's really not designed for measuring absolute lumber moisture content.  For the latter, one needs a good wood moisture meter, which can be quite pricey.  I own a Wagner MMC-220 that costs $270 that I use to measure lumber moisture content.  The readings given by the Wagner and this Sonin are drastically different when they are used to measure wood moisture content.  I suspect the problem is it's almost impossible to push those thick pins of the Sonin-Rapidtest meter into lumber to get a good reading.  If the pins are not pushed in deep enough into lumber or drywall, the reading will be off.  It's not too difficult to push enough of the pins into drywall to get a meaningful reading, but it's almost impossible to push them into most woods.  However, that said, it's important to keep in mind that this Sonin meter is intended primarily for home inspectors rather than woodworkers.  For woodworkers and lumber yard owners the absolute moisture content is very important, but for home inspectors a general relative wetness indication (i.e. damp vs. dry) is good enough.  Even though if used on wood, the moisture content will be way off, it is still useful as a general wet wood/dry wood indicator.  Just don't rely on the absolute % of water content on the scale.  Surprisingly it's not hard at all to get meaningful readings on concrete because you just have to push the pins hard against concrete without having to penetrate the concrete.