I use a pinless moisture meter that I got from Amazon (Dr. Meter Pinless Wood Moisture Meter) to measure moisture content in wood resawn for use in guitars, which is thin stock – about 3/16″ thick as resawn. These use an electromagnetic field that penetrates the wood and is affected by the wood’s moisture content. I wondered whether this would be accurate on thin stock, since it seemed like the field could penetrate right through the stock and be looking partially at air on the other side. It seems like this is in fact happening: measurements on thin stock show lower moisture content than on thick stock, up to around 9/16″ thickness; at this thickness and beyond, the readings stay consistent. But the depth of penetration of the field depends on the moisture content of the wood – the wetter the wood, the less the depth of penetration, so the readings are accurate even for thinner stock. The following describes some experiments I did to establish this.

Methodology

I took some sets of resawn plates, each 3/16″ thick (and quartersawn, though I wouldn’t expect that to make a difference in moisture measurements), stacked up 5 of them to make a thicker plate (about 1″ thick), and took a moisture measurement. I then removed one 3/16″ plate at a time and re-did the measurement. The pictures below show the process.

Results

The minimum thickness to get an accurate measurement depends on the moisture content of the wood: for woods in the typical range of air-dried wood (5-10% moisture content), the stock needs to be about 1/2″ thick to get an accurate reading. For green stock, the reading seems accurate even down to a thickness of 3/16″ (the minimum I measured). Specifically, down to a thickness of 3 plates (9/16″), the readings were consistent regardless of the moisture content – thickness didn’t matter. At 2 plates (3/8″ thick), the moisture content reading decreased by about 20% for the completely air-dried stock (moisture content 5.5%) and 10% for the partially air-dried stock (moisture content 11%), and not at all for very green wood (35% moisture content); at 1 plate (3/16″ thickness), the moisture content reading was about half the “thick” reading for the two drier sets, but still read approximately correctly for the green wood. The tables below shows the measurements.

Hackberry (air-dried over 20 years)

Maple (air-dried for about 3 weeks)

Beech (just resawn):

Conclusion

The measurements show that the accuracy of the readings given by a pinless moisture meter depend on the thickness of the stock being measured – if the stock is too thin, the meter penetrates right through the stock and treats the essentially moisture-less air as if it’s part of the wood being measured. For the meter I’m using, a stock thickness of about 9/16″ is sufficient to make sure you don’t experience this “through penetration” effect. This minimum thickness for accurate measurements will likely depend on the specific meter you’re using, as its field’s depth of penetration will depend on the strength of the field generated by the meter, so you’ll want to do some experiments to determine this thickness if it’s not stated by the manufacturer. If the stock is considerably wetter than normal dried stock, even thinner pieces will still give accurate measurements. But to be safe, it’s best to stack enough pieces together to reach the default thickness or greater to ensure accurate measurement.