What are the best methods to measure moisture content in wood structures?

1 Why measure MC?

Measuring MC helps you avoid problems caused by wood movement, such as cracks, gaps, warping, buckling, and splitting. Wood movement occurs when wood gains or loses moisture due to changes in temperature and humidity. Wood movement can compromise the structural integrity, appearance, and performance of wood structures. By measuring MC, you can determine if wood is at its equilibrium moisture content (EMC), which is the MC at which wood is in balance with its surrounding environment and does not change dimension.

2 How to measure MC?

There are two main methods to measure MC in wood structures: destructive and non-destructive. Destructive methods involve cutting or drilling a sample of wood and weighing it before and after oven-drying. This gives the most accurate MC value, but it damages the wood and requires time and equipment. Non-destructive methods use devices that estimate MC based on the electrical properties of wood, such as its resistance or capacitance. These methods are fast, easy, and non-invasive, but they are less precise and can be affected by factors such as wood species, temperature, density, and surface condition.

3 What are the types of non-destructive devices?

There are two types of non-destructive devices to measure MC in wood structures: pin-type and pinless. Pin-type devices use two metal probes that penetrate the wood surface and measure the electrical resistance between them. The higher the resistance, the lower the MC. Pin-type devices can measure MC at different depths and locations, but they leave small holes in the wood and can cause corrosion or staining. Pinless devices use an electromagnetic sensor that scans the wood surface and measures the electrical capacitance of the wood. The higher the capacitance, the higher the MC. Pinless devices can measure MC over a large area and without damaging the wood, but they are more sensitive to surface moisture and cannot measure MC below the surface.

4 How to calibrate and use non-destructive devices?

Non-destructive devices need to be calibrated before use to ensure accurate readings. Calibration involves adjusting the device settings according to the wood species, temperature, and moisture range of the wood. Some devices have built-in calibration tables or functions, while others require manual calibration with known MC samples or reference scales. To use non-destructive devices, follow the manufacturer's instructions and recommendations. For pin-type devices, insert the probes into the wood at the desired depth and angle, and read the display or dial. For pinless devices, place the sensor flat on the wood surface and press the button or trigger, and read the display or dial.

5 How to interpret and use MC readings?

MC readings from non-destructive devices are only estimates and should be verified with destructive methods if possible. MC readings can vary depending on the device type, calibration, wood species, temperature, density, grain direction, and moisture distribution. To get reliable and representative MC readings, take multiple measurements at different locations and depths, and average them. Compare the MC readings with the EMC of the wood environment to determine if wood is at equilibrium or not. Use the MC readings to guide your decisions on wood selection, installation, maintenance, and repair. For example, you can use MC readings to check if wood is dry enough for painting or staining, or if wood needs to be acclimated or conditioned before installation.

6 Here’s what else to consider

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