What Is a Wood Moisture Meter?
A wood moisture tester is an instrument that measures the moisture content in wood.
Accurate determination of moisture content is important because it greatly affects the shrinkage, swelling, weight, strength, and durability of wood. Also, when wood with a high moisture content is processed, it can warp, bend, or crack during the drying process.
There are various methods for measuring moisture content in wood, including electrical methods that measure electrical resistance, electrical capacity, absorption of radio waves, etc., the total drying method that measures weight before and after drying, and chemical analysis methods.
Uses of Wood Moisture Meters
Wood moisture meters are used to measure moisture in wood drying and preservative treatment plants, sawmills, wood chip mills, plywood mills, wood furniture mills, housing manufacturers, and construction companies.
For example, building materials for housing are heavier and less strong when moisture content is high, so drying makes them ready for use. Also, high moisture content can cause discoloration and decay fungi, so correct construction management is necessary.
Wood moisture meters are also used to control the moisture content of fuel wood chips for power generation. Moisture in wood is an important control item because it has a significant impact on power generation efficiency.
Principle of Wood Moisture Meters
Wood moisture can be measured by electrical resistance, electrical capacitance, microwave, infrared, or dry weight methods.
1. Electrical Resistance Type
Electricity is applied to the wood to be measured, and the electrical resistance is measured. The resistance value is converted to a moisture value. Since the resistance is about 100 MΩ, a high voltage of several tens of volts is applied.
This method is suitable for measuring objects with a large change in specific gravity, but its disadvantage is that it damages the object to be measured since a needle with a diameter of 2 to 3 mm is inserted.
2. Electric Capacitance Type
High-frequency electricity is applied to the measured object, and the change in electric capacitance is measured, converted to a moisture value, and displayed. A transmitter circuit is used for detection, and the change in capacitance is measured as a change in the transmitter frequency or period. The most commonly used oscillation frequencies are 100 KHz to 20 MHz.
The advantage is that it is easy to miniaturize as a handy type. It can measure from the surface of the wood to several centimeters inside. This type is also called a high-frequency moisture meter.
The disadvantage of the electric capacitance type moisture tester is that it is affected by the specific gravity and thickness of the measured object, so it is necessary to set the specific gravity and thickness. Also, since the measurement is made by contact, the sensor may be affected by the adhesion of tar and other substances when measuring large quantities.
3. Microwave Type
This is a moisture meter that measures the amount of microwaves attenuated by moisture, converts it to a moisture value, and displays the value. The microwaves used in moisture meters are in the vicinity of 10 GHz. Microwaves are easily absorbed by moisture and are easily converted to heat.
The microwave moisture meter consists of a transmitter and receiver. The transmitter emits radio waves to one side of the wood, and the receiver is placed on the opposite side of the wood to receive the transmitted radio waves. The more moisture there is in the wood, the more radio waves are absorbed, and the moisture value is displayed by measuring the strength of the received radio waves.
One of the advantages of microwave moisture meters is that they can measure internal moisture content down to the center of the wood. Moisture can be measured without damaging the measured object. In addition, measurements can be taken continuously. The disadvantage is that the device is large. It is installed in lumber mills and other facilities.
4. Near-Infrared Ray Type
Near-infrared rays are invisible light outside of red. Since it is well absorbed by moisture, this method irradiates light containing near-infrared rays, measures its reflectance, and converts it to a moisture value.
Near-infrared moisture meters have the advantage of being easy to install because they measure only the surface of the object to be measured. It is also possible to measure even thin objects without damaging them. The disadvantage is that it can only measure surface moisture of less than 1 mm from the surface. Therefore, it is affected by surface water. Also, it is affected by changes in the color of the measured object.
5. Dry Weight Method
This is a method in which the measured material is dried in a convection dryer and the moisture content is measured from the weight before and after drying. This method is called the total dry method. The weight of wood dried at 105°C until there is no change in weight is called the total dry weight, which can be calculated using the weight of the wood before drying to obtain the moisture content.
The disadvantage of the dry weight method is that it is time consuming to measure and cannot be used on a production line. Also, if the sample is cut, the product may be damaged. Manpower is another disadvantage.
6. Others
There are other methods, such as the neutron method, which irradiates wood with fast neutrons and measures the amount of neutrons converted to thermal neutrons, and the chemical analysis method, which uses Karl Fischer reagents.
Types of Wood Moisture Meters
1. Handy Type
The handy type can measure moisture content simply by placing the sensor on the surface of the wood, and is applicable to around 20 types of wood. In addition to moisture content, some products have functions such as average value, temperature compensation, and upper limit alarm.
Wood moisture meters are specially designed to measure the moisture content in wood chips. The measurement can be made by inserting the moisture meter’s sensor into a pile of wood chips.
2. Stationary Type
Stationary types are installed in lumber mills and other facilities and can continuously measure moisture content. Most are microwave wood moisture meters, with the transmitter and receiver installed separately. The wood will move between the transmitter and receiver to measure the moisture content.