Why measure
wood moisture?
Knowing the moisture of wood is a very important need in all stages of processing, from the purchase of the sawn wood to the painting. During the purchase phase it is necessary to verify that the proposed material has the declared and necessary characteristics. During processing it is essential to ensure that moisture is uniform between the various elements and conforms to the intended use of the product. For painting it is necessary to ensure the correct level for the process used. Even for firewood, knowing the moisture allows you to calculate the caloric yield (and the right value) of the material supplied.
How to measure it?
Although the weighing method (difference between weight before and after drying process) is the only worldwide recognized reference method for the exact measurement of moisture, many good instruments (but even more easy-to-use, faster and less intrusive) are today available on the market for most of applications.
The most important thing is, first of all, to choose the right instrument, according to customer's needs, as different needs can only be satisfied with different instruments. Buying an instrument without having analyzed the pros and the cons may often lead to the wrong purchase and to customer's disappointment.
Many meters are available today on the market and can be divided into two main groups: contact meters (which measure the dielectric characteristics of wood) and pin type meters (which measure the resistance between two electrodes).
Contact or Pin-type, what's the difference?
Contact meters allow the measurement even in depth, thanks to the electric field generated. The depth of detection depends on the characteristics of the instrument; the measured value is an average of the measured humidity value, calculated with decreasing sensitivity as the depth increases.
Pin Type meters measure the electrical resistance between two electrodes; the measurement is carried out only in the space between the electrodes, at their insertion depth. The measured value corresponds to the maximum humidity found between the electrodes, whether on the surface or in depth. If the wettest area is the central one, it is easy to obtain an indication of the humidity distribution inside the axis, observing the variation in the indication of the instrument during the insertion of the electrodes; otherwise, the only way to obtain this information is to use insulated electrodes, with only the tips uncovered, in order to exclude the influence of the surface layers.
From the operating principle it is evident that the only condition in which the measurements obtained with the two types of instrument are comparable is with seasoned or dried timber, having a uniform distribution of the humidity inside.
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Which instrument is more accurate?
Both the instrument types may be affected by particular characteristics of the sample under test, but the different measuring methods make every instrument more sensitive to specific critical factors. The pin-type meter is more affected by saline preservatives and it may give not reliable results when measuring timber with a thin wet layer on the surface (e.g. wood exposed to dew), if standard electrodes are used. In addition, it could be difficult to measure very low moisture contents (using pin-type meters). The contact type meters, on the opposite, are less affected by saline preservatives and by very thin moisture layers, and are more suitable to measure very low moisture contents, but are not suitable to measure timber having free water among fibres (a condition which occurs at moisture contents above 20-30%, depending on the wood species). In this situation, the measured value should be taken as a reference, useful only for comparison. Using a contact type meter, the minimum thickness and surface of the sample must always be considered. If the sample is too small or too thin, several samples can be positioned side by side or one above the other, avoiding any air gap, in order to achieve at least 2 centimetres of thickness and 2 cm all around the electrodes, otherwise, the meter will return a value lower than real. Special care requires also the surface where the samples are laid for measurement: if the sample is not very thick, it is recommended to lay it over a polystyrene foam support or to keep it suspended in air (keeping the hands far from the electrodes) in order to avoid to measure also the support. Due to the particular in-depth penetration of the electric field, it is not possible to estimate the distribution of the moisture inside the sample; the value returned by the instrument could represent the absolute measure of a perfectly conditioned timber as well as the average measure of timber drier in the surface and wetter in the core. This is why contact type meters are not suggested to measure timber where high gradients of moisture could be present; they are suitable to measure only well-conditioned timber, while the measures on timber not conditioned should be taken as references for comparisons only.
Which one measures deeper?
Contact moisture meters measure the moisture at a given depth, thanks to the electric field that penetrates the sample under test. The measurement indicated is an average of the moisture content all around the electrodes, with sensitivity decreasing as depth increases. Using a pin-type meter it is possible to measure the moisture of the wood at the desired depth by simply inserting the electrodes at the required depth. By observing the indication of the instrument while inserting the electrodes, it is possible to get an idea of the distribution of moisture in the wood, which is only reliable enough when the surface layers are drier than the inner layers (most common situation). When the surface layer is more humid than the internal layers (it can happen in case of dew or rain on the surface), to measure the internal moisture it is necessary to use special insulated electrodes to exclude the influence of surface humidity. All this because the pin-type meters always return the value of the wetter layers between the pins, while the influence of the drier layers is negligible.
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Which to choose?
When to choose a pin-type meter?
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If moisture values in material to be measured are above 20-30%
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If you suspect that moisture is not evenly distributed
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If the measurement speed and material damage are secondary to the required accuracy
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