The Resistograph drill drives a fine needle into wood at a constant feed rate (Drilling hole diameter: 3mm). The power consumption of the motor is recorded as a measure for the mechanical drill resistance. The mean level of the obtained charts correlates with the density of the penetrated wood. Variations in density caused by decay, compartmentalization zones, compression wood and different tree-ring zones are revealed. Resistograph systems are applied to study wood anatomy, to assess wood quality, to calculate timber-construction stability, to evaluate growth rates of forest trees and traffic safety of standing trees.
Radial wood density profiles and derived time series of tree-ring density parameters characterize wood quality aspects, tree growth and growth-influencing factors, such as site conditions and climate (Schweingruber 1986). The corresponding measurements are commonly done by X-Ray densitometry.
In 1985, two German engineers invented a method for measuring drill resistance of a needle to find decay in utility poles (Kamm und Voss, 1985). Further developments, carried out in Heidelberg by the author, led to several different prototypes of drilling devices, originally named DENSITOMAT, in 1993 renamed to RESISTOGRAPH (Rinn 1988, Brandt und Rinn 1989; Rinn 1989a-c; 1993). Accuracy and reproductivity have been increased up to a linear resolution of 100 measurement points per mm which makes tree ring density structures visible in the RESISTOGRAPH profiles of dry wood: visualization of radial wood density profiles. Consequently, graphics dominate the following text.
Description of the Resistograph-Method
The RESISTOGRAPH tool drives a fine needle into the wood and measures the drill resistance as it rotates continuously with ca. 1500 rpm (Fig. 1, 2). The drill resistance is concentrated at the tip of the needle because its width is double the width of the shaft (Rinn et al. 1990 and 1992). An electronic regulation of the motor guarantees a constant feed rate of the needle adapted individually to the wood density: 500mm/min for extremely dense wood like Lophira alata, 700mm/min for very soft wood like Populus nigra. Maximum drilling depth of the shortest RESISTOGRAPH version for timber inspection is 280mm. The standard version for tree inspection provides 410mm and the longest version allows for 950mm. The ordinate of the RESISTOGRAPH-charts represents a relative measure for the power consumption. Electronic resolution is 12Bit, the ordinate values therefore differ from 0 to 4095, representing a relative scale of drill resistance.
More than 30,000 experimental measurements, carried out by the author since 1986, and several other scientific institutions have proved the method’s suitability for different fields of scientific and practical application. The bore chips remain in the 3mm drilling hole after measurement. As compared to core sampling with bore-hole diameters of 10 to 40mm, the resistographic drilling is much less destructive.
Uses of the RESISTOGRAPH method
For dry wood, the drill resistance correlates to the gross density (r2-0.8; Görlacher und Hättich 1990; Büchele 1995; Winnistorfer et al. 1995). For calibration of drilling in standing trees, the influence of the moisture content has to be taken into account (Eckstein und Sass 1994). However, in most of the practical applications relative evaluations of the charts (intact-decay or earlywood-latewood) are sufficient. For wood quality evaluation, grading according high, middle or low mean level of density seems to be sufficient.
Tree-ring density variations
The RESISTOGRAPH-charts reveal variations in density of earlywood and latewood areas (fig. 3-13).
So far, RESISTOGRAPH analysis has been limited to the detection and evaluation of relative changes in the drill resistance, providing sufficient information for decay detection and growth rate assessment. The influence of the wood moisture content on drill resistance values is currently being studied in order to assess absolute values of wood density of standing trees.