GANN Hydromette RTU 600 Betriebsanleitung Seite 17

6.1 Handling of electrodes for wood moisture measurement
Connection of the electrodes
The meter can be used with different types of measuring electrodes according to the individual application.
The electrodes M 6, M 20 are connected to the meter socket (1) by means of the measuring cable MK 8. On
the meter side, this cable is fitted with a BNC plug. Turn clockwise to lock it. To disconnect, turn notched
fastening ring anti-clockwise. Do not use force and do not pull on the cable.
Grain Direction
GANN wood moisture meters have been calibrated for taking readings with electrode pins driven into the test
sample across the grain. As the electrical resistance is greater across the grain than parallel to the grain, too
high a reading will be obtained if the electrode pins of GANN meters are applied parallel to the grain. The
effect can be neglected at readings below 10 % m.c., whereas around 20 % m.c. the meter will read about 2
% m.c. higher.
Thickness of Wood
Electrodes with pins having a penetration of 10 mm can be used on wood up to 30 to 40 mm thick, whereas
pins with a penetration of 17 mm are designed for wood thicknesses up to 50 to 65 mm. For thicker boards
or planks, the ram-in electrode M 18 should be used which permits the use of pins with a penetration depth
of up to 54 mm. For stock with equilibrated moisture content, non-insulated pins can be
used, whereas for all other applications insulated pins making contact only with their uncoated tip having a
uniform contact area with the wood, regardless of the penetration depth, should be used. Any change in meter
readings taken with insulated pins at different penetration depths clearly reflect an actual change in moisture
content representing the existing moisture gradient.
Drive-in Electrode M 20
Drive the electrode into wood with the needles across the grain (the electrode body is made of impact-resistant
plastic). When withdrawing the electrode, the pins can be loosened by slight sideways rocking movements
across the grain.
For determining the average moisture content, the pins have to be driven to a depth of approx. 1/4 to 1/3 of
wood thickness.
If thicker boards or planks are to be measured, the needles can be replaced by longer ones. Naturally, the
liability to breakage and/or bending increases with the length of the pins, especially when withdrawing them.
Therefore, it is recommended to use the ram-in electrode M 18 for testing thicker wood.
The cap nuts should be tightened by means of a spanner. Loose needles may easily break.
6.2 General Information on Wood Moisture Measurement
The measuring principle of most GANN Hydromette moisture meters is based on the electrical resistance or
conductivity measuring method well known for many years. This method is based on the fact that the electrical
resistance is dictated to a large extent by the wood moisture content.
The conductivity of bone-dry timber is very poor and its resistance very high so that no current worth mentioning
can flow. The conductivity of wood increases with increasing moisture content whereas its electrical resistance
decreases.
In the range above the fibre saturation point (about 30 % m.c.) readings become progressively less accurate
depending on the species of wood, its specific weight and temperature. With European conifers and some
exotic woods such as Meranti/ Lauan, greater measuring errors must be expected in the range above 40 % m.c.
where relatively accurate readings can be obtained with oak, white afara, etc. up to a range of 60 – 80 % m.c.
To achieve as accurate readings as possible, the samples selected should be measured at several spots. It
should always be observed that the minimum penetration depth of the electrode pins, driven into the wood
across the grain, is 1/4 and the maximum depth 1/3 of wood thickness. Testing frozen wood with a moisture
content is excess of 20 % is not possible.
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15.11.2006 Hydromette RTU 600 #013629