There are two main methods of MC measurement – Capacitance and Resistance.

Resistance Meters (Pin Meters)

These are commonly known as Pin Meters and used to be the most commonly used meters throughout the world. They have two or more pins that are pushed into the wood. Direct current travels out one pin into the moist wood and is picked up by another pin. These meters measure the resistance to a current. Dry wood allows little current to pass, damper wood permits more. The meter reads how much resistance there is to the current and correlates the resistance to wood moisture content.

Electromagnetic Wave Technology (EMW Meters)

Sometimes called pinless meters, these meters measure the moisture content of wood without piercing the wood with pins. These meters emit electrical waves through a sensor that is pressed against the wood. The waves create an electromagnetic field (EMF) the size of the sensor to a depth up up to either .75″ to 1.0″ depending on which model is specified. The field behaves differently depending on how much moisture is in the wood. EMF meters are based on the Capacitance method, but a properly designed meter will take many more factors into consideration. EMF meters measure the capacity of wood to store energy (capacitance), the amount of power the wood absorbs from the field (power loss) or the woods resistance to the field (impedance). The meters translate this electrical information to percent moisture content (%MC). The first practical and portable EMW moisture meter was invented by Delmer Wagner in the 90′s.

Here’s a short list of the factors affecting each type of meter.

EMW Meter Pin Meter
Temperature No Yes
Chemicals No Yes
Wood Orientation No Yes
Moisture Gradient No Yes
Wet Pockets No Yes
Wood Species Yes* Yes**
Wood Density Yes* No
Surface Texture Yes No

*Certain EMW Moisture Meters correct this either digitally or using a lookup table.

**Some Pin Meters may correct this using a lookup table.

Some factors on this list you may intuitively understand. Since EMW meters create a 3D average of the wood moisture content, the orientation of the wood won’t affect their readings. Pin meters are affected by wet pockets when the small path current travels through it, even if the rest of the wood is significantly drier. Even temperature will affect the resistance of wood measured by pin meters.

From this list, you probably have gained a healthy skepticism regarding the accuracy of pin meter readings. Although some woodworkers will almost never have to deal with factors like 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 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 there instead of the wood. On most boards, simply pressing the meter firmly against the wood negates this factor; on extremely rough wood, you may want to add 1% onto your reading.

Little-known (yet significant) factor: Many pin-type moisture meters have a cord connecting the pins that are hammered into the wood to the reading unit. (The models with pins and meter connected can’t be driven into the wood enough to get a significant reading.) On these meters, try simply changing the position of the cord and watch the readings change. It would be almost amusing if the consequences of incorrect readings weren’t so serious.

Next, learn Are All Pinless Moisture Meters Created Equal?