Moisture Meter Technology
Capacitance and Resistance
What’s the difference between pin meters and pinless meters?
Wood moisture meters use two primary methods of determining moisture content: Capacitance and resistance.
Resistance Meters (Pin Meters)
Commonly known as a pin meter, this type used to be the most widely used meter throughout the world. Two or more pins are pushed directly into the wood in two different spots, and a direct current travels from one pin to the other. This process measures the resistance to the current between pins and correlates that resistance to wood moisture content.
Electromagnetic Field Technology (EMF, or Pinless Meters)
Often called a pinless meter, this type measures the moisture content of wood without piercing the wood with pins. An electrical wave is emitted through a sensor pressed against the wood. This creates an electromagnetic field (EMF) the size of the sensor to a depth of .75” to 1.0”, depending on which model is specified. The field behaves differently depending on the amount of moisture present in the wood.
EMF meters are based on the capacitance method, but a properly designed meter will take many more factors into consideration. These meters measure the capacity of wood to store energy (capacitance), the amount of power the wood absorbs from the field (power loss), or the wood’s resistance to the field (impedance). They translate this electrical information to a percentage of moisture content.
Here’s a short list of factors that affect each type of meter:
|EMW Meter||Pin Meter|
**Some pin meters may correct this using a lookup table.
Some factors on this list may be familiar. Since EMW meters read a large 3D-volume average of wood MC, the orientation of the wood won’t affect their readings. Pin meters are affected by wet pockets, even if the surrounding wood is significantly drier, and may give a higher MC reading due to the smaller path of the current traveling through it. Temperature can also affect the resistance of wood measured by pin meters.
From the above list, you may have already developed a healthy skepticism regarding the accuracy of pin meter readings. Although some woodworkers may rarely have to deal with chemicals, the factors affecting pin meters are the most difficult to assess.
Surface texture, density, and species may either be assessed visually or will already be known. Wet pockets, however, are impossible to correct with a pin meter. On the other hand, some EMW meters will read up to 1-2% lower on extremely rough surfaces as you begin to measure the air trapped their instead of the wood itself. On most boards, simply pressing the meter firmly against the wood negates this factor. But on extremely rough wood, you may want to add 1% to your reading.
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Additionally, keep in mind this little-known yet significant factor —Many pin moisture meters have a cord connecting the pins that are hammered into the wood (the models with pins and meter connected can’t be driving far enough into the wood to get an accurate reading). On these particular meters, try simply changing the position of the cord and watch the readings change! It would be almost amusing if the consequences of inaccurate readings weren’t so serious.