H60.0.13.6C-17 Operating Manual GMH 3851 Page 7 of 22
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5 Some basics of precision material moisture measuring
5.1 Measuring method
The electrical resistance depends on the material moisture in many cases. Therefore the device measures the (possibly
extremely high) values of resistance and converts them to the displayed value by means of integrated characteristic
curves. The temperature has to be compensated especially at the measurement of wood –please refer to chapter 5.5.
The contact is realised e.g. by nails that are driven into the material or by injection probes.
Frozen material cannot be measured!
5.2 Moisture content u and wet-basis moisture content w
Depending on the Application one of the two units is necessary.
Carpenters, joiners and the like commonly use the moisture content u (sometimes referred to as MC).
When evaluating firewood, wood chips etc., the wet basis moisture content w is needed.
The instrument can be configured to both of the values. Please refer to chapter “configuration“.
Moisture content u or MC (relative to dry weight) = dry basis moisture content (mind the arrow at left bottom!)
The unit is %, sometimes used: % MC.
The unit expresses the moisture content like calculated below:
Moisture content u [%] = (weight
wet
- weight
dry
) / weight
dry
*100
Or: Moisture content u [%] = (weight
water
) / (weight
dry
) *100
weight
wet
: weight of the wet material
weight
water
: weight of water in the wet material
weight
dry
: oven-dry weight of material
Example: 1kg of wet wood, which contains 500g of water has a moisture content u of 100%
Wet-Basis Moisture Content w (relative to total weight, mind the arrow at left bottom!)
The wet-basis moisture content expresses the ratio of the mass of water to the total mass of the substance. The ratio is
represented by the following equation (the unit is % as well):
wet-basis moisture w[%] = (weight
wet
- weight
dry
) / weight
wet
*100
Or: wet-basis moisture w[%] = (weight
water
) / weight
wet
*100
Example: 1kg of wet wood, which contains 500g of water has a moisture content u of 50%
5.3 Special features of the device
466 wood specimens and 28 building materials are stored directly in the memory of the device:
Thus more exact measurements could be reached than with common devices with group selections would ever reach.
Even the usage of complex conversion tables for building materials won’t be necessary any more!
Example: Common wood-moisture-measuring-devices use one single group for spruce and oak, in reality the deviation
of these characteristic curves is more than 3%! (Base for this statement are complex statistical surveys, considered
measuring range 7-25%). This random error will not occur for the whole GMH38xx series, with the help of individual
characteristic curves highest resolution is achieved.
Extreme wide measuring range: 0-100% (depending on characteristic curve) percent moisture content in wood.
Moisture evaluation: Additionally to the measuring value, an individual moisture evaluation will be displayed
simultaneously.
5.4 Auto-Hold function
Particularly when measuring dry wood, electrostatic charges and other similar noise could dither the measuring value.
With activated auto-hold function the device will acquire an exact measuring value automatically. During that, the device
could be put down to avoid noise through discharge of the clothing etc. After having acquired the measuring value, the
display will change to ‘HLD’: The value will be frozen as long as a new measuring is initiated by pressing key 6 (store).
If the logger is switched on (‘Func CYCL’ or ‘Func Stor’), the auto-hold function can not be used. The device
works like it is set to Auto-HLD = off.
5.5 Automatic temperature-compensation ('Atc')
An exact temperature compensation is important for a reliable wood-moisture-measuring. These devices feature a high
quality thermocouple-input for type k thermocouples. Thus you could connect common surface-temperature-probes –
The needed measuring-time ‘afield’ will be drastically lowered compared to common (non-surface-) temperature-
probes. The used temperature-value therefore is:
Temperature-probe connected
Temperature-measuring through connected probe
No temperature-probe connected
Device-internal temperature-measuring
Independent from temperature-
probe
Manual input of temperature: shortly press Temp-Button
then use (key 2) or (key 5) to input the
temperature confirm selection with ‘Store‘(key 6)
When connecting a probe that is not insulated you must have to observe not touching the wood or
the electrodes nearby the unshielded electrode. We suggest using our insulated probe
GTF38 (already included in standard case sets SET38HF and SET38BF).
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