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 meters or pin type meters?
Contact meters can make readings in depth, due to the generated electric field. The reading depth depends on the characteristics of the instrument. The obtained value is an average of measured moisture value, which is calculated with decreasing sensitivity at the increase of depth.
Pin type meters are used to measure the electric resistance between two electrodes. The reading is made only on the space between the two electrodes at the depth where they have been inserted. The measured value corresponds to the maximum moisture between the electrodes, both on surface and in depth. If the highest moisture is in the middle, you can easily know moisture distribution into the board simply by noticing the reading changes on the instrument when electrodes are placed. Otherwise, the only other way to get this information is by using insulated electrodes, with unshielded ends, to avoid the influence of surface layers on readings.
Contact meters are better...
- If moisture on tested material is lower than 8-10%
- If it's essential not to cause any damage on tested material
- If wood surface is wet with dew
- For a great number of readings in a short time
Pin type meters are better...
- If moisture on tested material is higher than 20-30%
- If material is less than 2 cm thick and you can't put one layer on other without air gaps
- If you think that moisture isn't uniformly distributed into the wood
- If testing speed and damage on material are less important than required accuracy
From this difference between the two meters, it becomes clear that the only condition in which we can compare the measurements obtained with the two instruments is with seasoned or dried wood, in which moisture is uniformly distributed into the wood..
The instruments produced by LogicaH&S have been calibrated on standard reference samples, which are based on measurements taken on many timber species, where moisture have been tested according to the regulation UNI 9091-2/87 (weighing method).
Other timbers have been classified in analogy with tested samples.
As wood is not a homogeneous material, the instruments' performance cannot be automatically guaranteed on samples which are different from tested ones, even if of the same wood species.
For this reason, on customer's request, LogicaH&S can supply all instruments with the Certificate of Inspection which is made directly on customer's wood, released by CATAS S.p.A. (Authorized testing laboratory).