Air Drying

Air drying is the traditional method of drying timber. Once the log has been sawn, the boards are stacked on battens or ‘stickers’ These should be made of softwood to prevent marking the boards. Stacks are built up which should be protected from rain and sunlight. If green wood is exposed to the elements the heart doesn’t dry and the outside keeps cycling from wet (when it rains) to dry (when the sun shines) this is one cause of surface checks.

The exposed end grain is often treated to prevent the ends of the board drying out too quickly, which would cause the timber to split. A disadvantage with air drying is that it can leave the timber open to attack from wood boring beetles while it’s drying, often for several years.

Kiln Drying

Kiln drying dramatically reduces the drying time of timber, from years and months to weeks; temperature, humidity and ventilation must be carefully controlled in order for the process to work effectively. Some kilns run an extra hot cycle after the timber has been dried, to kill off any wood boring beetle larva present in the timber.

Hopefully I’ve convinced you that knowing the moisture content of the timber you are about to use is important, so how do we go about measuring it? There are three practical methods of measuring moisture in wood.

1. Physically weighing the timber before and after drying it and calculating the MC.
2. Using a pin type meter which measures the electrical resistance of the timber.
3. Using a meter that utilises electro-magnetic field technology.

Method 1: The first method involves a fair bit of time and effort but little cost. Cut a small sample of the timber, about an inch long. Accurately measure the weight of the wood in grams to determine its ‘wet weight’. Completely dry the sample by placing it in an oven set to 100degreesC until it reaches its "dry weight"; this may take a couple of days.

To work out the moisture content;
MC% = wet weight - dry weight / dry weight x 100

An advantage of this method is that it will measure the moisture content even if it is very high; most moisture meters have a maximum reading of about 30%MC.

Method 2: The second method measures the electrical resistance of the timber, two pins are applied to the wood and a small current is passed through the timber; the reading gives the moisture content. This gives an instant reading but unless care is taken the machine will not produce accurate results. Just pressing the pins into the surface of the timber will give a consistently low reading. A more accurate reading is obtained by cutting a section off the timber, ideally at least eight inches from the end, then measuring the heart of the wood on the cut surface. This method does give a more accurate reading.

Temperature is usually a factor with pin type meters; the meter is normally set to measure moisture at 70 degrees Fahrenheit (this is the temperature of the timber, not the air temperature). If the wood is at a different temperature, the actual moisture content needs to be read on a temperature adjustment table. Chemical treatment of the timber, wood preservatives for instance, can affect the reading as can surface moisture which may not necessarily be visible. Pin meters come into their own with particularly rough timber and timber which doesn’t have a flat face.

Temperature is usually a factor with pin type meters; the meter is normally set to measure moisture at 70 degrees Fahrenheit (this is the temperature of the timber, not the air temperature). If the wood is at a different temperature, the actual moisture content needs to be read on a temperature adjustment table. Chemical treatment of the timber, wood preservatives for instance, can affect the reading as can surface moisture which may not necessarily be visible. Pin meters come into their own with particularly rough timber and timber which doesn’t have a flat face.

TIP I’ve found the most precise reading is made by driving two 50mm pins 25mm into the timber at about 15mm apart. The pins of the machine can now be pressed onto the protruding pins extending the meter pins 25mm into the timber; this method gave me consistently accurate readings.

Meter Comparisons

I used a block of European oak for this meter test. It had been in my workshop for several years. Firstly I used the Wagner meter. After setting the species to 55, which corresponds to the specific gravity of European oak, (which is 0.55), the Wagner gave a reading of 16.2% MC.

Pressing the Timber Check pins into the timber gave me a reading of 12%. I then drove a couple of pins into the block and the measurement increased to 14%. This reading needed converting into the actual moisture content by looking up the adjustment tables. Looking at the species adjustment tables there is no adjustment to be made when measuring European oak. However the temperature of the timber was 10 degrees Centrigrade, so I compensated for the low temperature by using the wood temperature adjustment tables. The 14% meter reading thus converts to 15.9% MC.

I tried the same procedure on a block of beech. The Wagner measured 13.1%MC whilst the Timber Check gave a measurement of 13.7% MC, once corrected for temperature and species. As you can see, both meters gave very similar measurements.