Wood Moisture Meter

Wood moisture meters make it possible to determine the moisture content in wood and wood products. When the percentage of wood moisture is given it means the water content is related to the dry matter.

Wet weight - dry weight
----------------------------------------- X 100 = wood moisture in%
Dry weight

Wood is a material with high swelling and shrinkage behavior. It expands when water is absorbed and contracts when it dries, which often leads to shrinkage cracks, especially when it goes about larger cross-sections. In order to limit the damage caused by this material-specific behavior, the maximum values were set. These values can be easily determined with the help of the wood moisture meters and compared with the specifications.

Below you may find more information about various measuring methods and then the applications of the wood moisture measuring devices.

A Measuring method in the wood moisture measuring devices
A1 Basic information on the wood moisture measuring devices

When speaking about the wood moisture meters, a distinction can be made between measuring methods, when
- samples must be taken -> kiln / Darr method
- puncture holes or drill-holes in the material are necessary -> electrical resistance measurement
- the measuring device is only put on -> dielectric or capacitive measurement.
When choosing the wood moisture meter, you should pay attention to the measurement accuracy. A high resolution of the display is not identical to the high measurement accuracy of the wood moisture measuring device. Electrical wood moisture measurement depends on the temperature. That is why good devices have temperature compensation. If different types of wood / groups of wood are stored, you should always select the correct wood type curve. If possible, check the measured values by carrying out the repeated measurements at different points.

A2 Wood moisture measuring devices with kiln / Darr method

According to the standardized Darr method, the wood moisture content is determined by means of drying at 103 ° C up to the constant weight. Until 2007, it was the only standardized, exact method of determination in the valid at that time DIN 52183 and also serves as the calibration method for the other methods in the current standard DIN EN 13183. Current wood moisture meters in which the Darr method is used, offer drying chamber and precision balances in one device. Most of these devices are designed for very small amounts of sample, which significantly shortens the drying time.
Attention, not all devices can display the wood moisture directly as a result. Instead, the material moisture content is related on the total weight of the examined sample, i.e. the wet weight.

Example:
A sample with 100g wet weight and 80g dry weight has 20% material moisture but 25% wood moisture.

A3 Wood moisture meters with resistance measurement method

The highest measurement accuracy of the wood moisture meters with resistance measurement is in the wood moisture range between 5 and 25%. Measurements should be taken across the fiber. Use the appropriate pin length of the impact electrodes for your measurement. With partially insulated pins, the moisture can be determined separately in different cross-sectional depths.

A4 Wood moisture meters with capacitive measurement method

The measurements carried out with the capacitive wood moisture meters, are non-destructive and can be repeated as often as required. The wood moisture can also be determined above the fiber saturation. The capacitive measurement also works through the layers of paint. In cases of inhomogeneous moisture distribution, differences in bulk density and large cross sections, however, the measurement accuracy decreases. For the wood moisture meters with surface sensors, larger, flat contact surfaces are necessary, ball sensors require little space.

A5 wood moisture meters with microwaves or VHF band waves

For the measurement in the production, drying or filling process, the wood moisture meters with microwave or VHF band sensors can be used, which continuously record and transmit the measurement data. The process can then be controlled via switch contacts or control technology.

B Application areas of the wood moisture measuring devices B1 Wood moisture measuring devices for timber, construction timber

There are several regulations concerning the permissible wood moisture to avoid the damage caused when the construction timber is too damp. Compliance with these requirements can be checked with the help of the wood moisture meters.
DIN 18334 from January 2005 specifies a maximum installation moisture:
Unless otherwise specified below, construction timber is to be made of softwood with a wood moisture content of at most 20%.
Other maximum wood moisture levels are:
- wooden house construction maximum 18%
- floors maximum 12%
- sub-floors maximum 15%
- stairs 9 +/- 3%

Section 6.2 of DIN 1052 states

(3) To reduce the shrinkage cracks and dimensional changes, the timber with installation moisture of at most 20% must be taken for the usage classes 1 and 2, for the usage class 3 the installation moisture should be at most 25%.
Procurement and contract regulations for construction work (VOB, DIN 18355, 2.1.3) provide the framework for the "correct wood moisture content" for the delivery of interior components. It stipulates: “The moisture content of the fully assembled wooden parts for the fittings that are not in contact with the outside air, may be up to 10%, based on the kiln weight, when leaving the manufacturing company. For the components that are constantly in contact with the outside air, - up to 15%. This moisture content must be verified at the request of the client. "
The use of the dry wood is also an important requirement for being able refuse from chemical wood protection. "A wood moisture content of more than 20% is required for the development of wood-destroying fungi." (DIN EN 335-1, Appendix A, 2.19)
Which wood moisture values usually occur after a long period of time under normal conditions is specified in DIN 1052 as equilibrium moisture.

Area of application	wood moisture u in mass %
buildings closed on all sides
-with heating	9 ± 3
-without a heater	12 ± 3
covered, open structures	15 ± 3
constructions fully exposed to the weather	18 ± 6

Woodworking companies can avoid the possible future defects or complaints caused by the wood that has been installed too damp if they determine the wood moisture with wood moisture meters and document the measurement data at the stages of wood delivery, processing and handing over the service. In their own interest, the construction contractors and site managers should also use the wood moisture meters in order to check the wood moisture of the delivered or installed timber material as soon as possible.

B2 Wood moisture meters for firewood

Firewood has a moisture content of more than 50% immediately after being cut. Before it can be heated, it must be stored for about 2 years or dried technically. If the wood moisture value is too high, not only the pollutant emissions increase due to incomplete combustion, there is also the risk that the stove and chimney will be damaged because of the lower combustion temperatures. The ideal for heating wood moisture is 15%. Wood with the wood moisture above 20% wood moisture should not be burned. For the measurement of the wood moisture, e.g. the wood moisture meters with penetration sensors are well suited.

B3 Wood moisture measuring devices for wood pellets, wood chips

Wood pellets consist mostly of sawmill by-products and forest residues. Shredded wood with the high initial moisture must be dried to about 12-14% moisture before being pressed into pellets. The wood moisture meters with inline sensors (e.g. with microwaves or VHF band waves) are recommended for continuous measurement in the drying process. Also, the finished pellets must not have high moisture content, as this negatively affects the combustion. The moisture of the pellets stored in bulk can be checked with special wood moisture meters. The measurements are taken then at the tip of the long penetration probe. There are also special moisture meters for sawdust, chips and wood chips. For example, there are devices in which a sample of the material is pressed in the measuring chamber and then its moisture content is measured.

Timber industry is one of the leading branches in many countries and wood as material, in different shapes, is widely used for various applications. One of the main conditions for the production of high-quality products is the compliance with all the requirements during the processing and manufacturing process and application of the correct raw material. What is important here, it does not matter, whether it goes about paper or house, since both need wood to be produced. Wood itself is liked by many, since it is a pleasant to work with material, warm, natural and even quite resistant. Nevertheless, those who choose wood need to be ready for some challenges!

Wood is one of those materials that is very dependent on the environment and is sensitive to the amount of moisture present in the air. Between the stage, when the wood is freshly cut to the moment when the finished product is ready to be delivered to the customer or for some particular application there are many stages, at which continuous monitoring of the moisture parameter is required. With the help of the moisture measuring equipment designed for measurement on wood, this task becomes quite feasible. On the contrary, if the manufacturer decides to save at that point and relies on just visual inspection, the damage may turn out devastating and the correction of the mistakes after the outcome of the product, will be no longer possible.

Moisture should be measured in the raw material, since only after that it becomes clear whether it needs additional drying, or on the contrary, needs to be moisturized before the processing – a lot depends on the application. When it goes about industrial sphere, where different technological processes are used, the moisture meters are indispensable devices for the determination of the relative moisture in the material. When the total weight of the wood and the water/moisture part in this weight are known, the ration between these two values will indicate the relative moisture. Sometimes it is often compared with the absolutely dry sample of the same material to determine the level of moisture volume in the present material. Too much moisture is the wood, which is usually detected in the wood after being for a long time in water or immediately after being cut, rarely makes it appropriate for the processing. Even every person who has ever made a fire or tried to start the fire in the fireplace at home knows that wet wood pieces won’t give the desired flames and amount of warmth, but too much smoke. The moisture amount in the freshly-cut wood or wet wood may reach 40, 50 and even 60%. That is why the next stage is usually drying in the open air. Private users say, it should be stored for at least two years to become good material for the fireplace in winter. In the timber industry, wood is usually piled in the open air so that a certain amount of moisture present in the growing trees evaporated naturally. The moisture meter is applied for monitoring the changes in the moisture value and provides the operators with the information when the wood may be moved to another stage, for example, drying chamber, if the application requires absolutely dry wood. When it goes about wood products for heating purposes, then it is necessary in order to achieve the highest energy release level and the highest efficiency. In addition, the excessive moisture in the wood products (saw dust, chopped wood, whole stems) leads to development of many natural bacteria, fungus and insects which may be devastating. If this happens that, for example, the pieces of furniture or wood used in the construction, contain the elements and spores of the harmful substances, the product may decay over some time and lose its optical looks as well as functionality.

Application of stationary and portable moisture measuring devices allows the producer carry out continuous and numerous measurements, which ensures stable and reliable results. Thus, any objects made of wood, windows, doors, parquet, furniture as well as heating materials may be produced correctly and have not only appropriate optical looks, but also long service-life.