With a wood moisture meter, the moisture level of firewood can be determined in a few seconds. Also, the device is suitable for measuring residual moisture in logs or sawed wood, building materials and cardboard – even in gypsum, cement and hardened mortar. The moisture content of the piece of wood is displayed in percent and ranges from 6 to 42% residual moisture.
The firewood is considered to be dry and ready to fire if it has a residual moisture content below 18%, but the optimum residual moisture content is 10%. In this case, the calorific value is approx. 4.2 kWh / kg, whereas raw wood only achieves 2.5 kWh / kg. The knowledge about the wood moisture is essential in the commercial area because every artisan knows the problem when the moist forest is processed – mold and warped workpieces are then not uncommon.
Even in the private sector, it is essential to know the residual moisture content of firewood because too damp wood burns only a little efficiently and produces a lot of soot and smoke.
Methods for determining wood moisture
The wood absorbs the moisture from its surrounding environment. As the relative humidity increases or decreases, so does the wood in or out more moisture. It is necessary to use individual moisture meters for wood to measure the moisture content by continuous changes that occur in the relative humidity in the surrounding environment of timber.
To determine the residual moisture in the wood, there are different methods of measurement – most often, the analysis of the capacitance and the resistance is practiced. In practice, the following two types of devices or techniques are usually used for this:
- Measurement of electrical resistance (with penetration probes)
- Measurement of electrical impedance (non-destructive humidity meters )
Resistance meters are commonly known as pin gauges and are the most widely used gauges for determining firewood moisture. Although these devices are relatively inexpensive, they still deliver excellent results. They usually have two needle-like pins that are pressed into the wood. DC goes from one nail to the damp wood and is picked up by another pin.
These meters measure resistance to current. Dry wood allows little power to flow, while wet wood allows much higher current flow. The meter shows how much strength the wood gives to the stream and correlates this resistance to wood moisture. Since puncture holes remain visible from the donors with this method of measurement, this method is less suitable for workpieces, or the analysis should at least be made in an invisible area. However, this does not matter in firewood.
Electromagnetic Wave Technology (EMW / EMF meters)
So-called penless measuring devices measure the moisture content of the wood without penetrating the wood with pins. These gauges emit electrical waves through a sensor that is pressed against the wood.
The waves generate an electromagnetic field (EMF) in the size of the sensor, up to a depth of about 2.5 meters, depending on the model. The area behaves differently depending on how much moisture is present in the wood. EMF meters are designed based on the capacity method, but an adequately designated meter will also consider many other factors.
Among other things, EMF meters measure the ability of wood to store energy (capacity), the amount of energy that the wood absorbs from the field (loss of power), or the resistance of the forest to the ground (impedance). The meters ultimately translate this electrical information to the percentage of moisture content.
This method is very accurate and trouble-free, but the corresponding devices are a bit more expensive because their design is more complicated. Depending on the type and type of wood, each wood can absorb different amounts of water. For accurate measurements, such data must be stored as a parameter in the meter.
Thus, individual wood moisture meters differ mainly in the described measurement technique – i.e., with or without a puncture probe. But there are also multifunction measuring instruments that combine these two measuring methods described. Of course, the accuracy of the measurement is another critical point.
Thus, a reasonable device should achieve an accuracy of at least 1% and operate in a measuring range of 10-40% residual moisture. With many tools, the determined values can be displayed in an LCD, saved, or even transferred to a computer, to be able to be evaluated there.
How to use a wood moisture meter
The measurement with the wood moisture meter should always be carried out in several places of the piece of wood, in case the wood is differently damp at different locations. You should not measure where cracks, knuckles, or bark are present, as this may lead to a false result.
To use a wood moisture meter, press the tips of the device firmly against the wood. If the probes do not immediately push into the stove fuelwood, then do it gently and protect the equipment. Usually, softwood measurement is much more comfortable, while dry hardwoods become even harder when dried.
Therefore, it is best to split the piece of wood and perform the measurements in the middle of the freshly cut side. Measures on the upper layer do not necessarily provide correct results, because the surface can be dried out by the action of the sun or, on the contrary, also have excessive moisture, e.g., due to rain. The arithmetic mean of all measurements then provides the final and actual measurement results.
With the help of a wood moisture meter, you can ensure that your wood does not exceed the recommended residual moisture level.
How high should the residual moisture in firewood be?
Too wet wood gives off little heat, develops a lot of smoke and can leave residues in the hearth. For proper heat development, the residual moisture should not exceed 22%, better still less than 17%.
Good firewood should:
- be at least two years old.
- Be stored outside protected from the weather.
- Be stacked so that proper ventilation is guaranteed.
- Not covered.
Measure wood moisture in firewood
The wood should be measured at several points from storage, as within a stack, different moisture levels may be present. The wood log to be measured is split and tested with the measuring device for residual moisture at two points across the fiber direction. The measurement should be made at the core of the log, not on the front sides.
Measure wood moisture in lumber (e.g., roof truss)
If timber can not dry on all sides after installation, e.g., B. because it is used as a roof joist, it can cause cracks and deformation, mold or insect infestation. Therefore, it is essential that even here, a moisture limit of 20% is not exceeded. In interior design, a value of 18% has been proven.
Due to the broad cross-section, timber should not be measured on the surface. Several measurements across the fiber direction at different depths and different locations provide the best information about the moisture course within the wood.
How to measure the wood moisture in the parquet?
When creating joints on the parquet floor, you can check the wood moisture to prevent misalignment.
To do this, measure with a moisture meter at different points in the room. Measured in the middle of the room, but at least 10 cm away from the walls. Radiators should be switched off during the measurement.
The parquet floor is laid with wood moisture of 9 +/- 2%. If the humidity in the parquet is too low, additional room humidification can help to reduce joint formation.
Conclusion: Cheap entry-level models are sufficient
If you heat your apartment with firewood in winter, you can not do without a wood moisture meter. According to the legislator, the fuel must be checked for residual moisture so that it can be incinerated in an environmentally friendly way and ultimately also achieves a higher calorific value. Above all, measuring devices for measuring conductivity have prevailed for determining the residual moisture.
This method is often cheaper because the tolerances are slightly larger, but they can be kept within a range of up to 2%. Such meters are already under $30 to have and are perfectly adequate for the private sector because, after all, the device is used only a few times a year. In professional and permanent use, however, you should resort to a high-quality and professional moisture meter, for example, as a renovator, woodworker or timber dealer, so that extreme values are determined.