USING HIGH-TECHNOLOGY INSTRUMENTS TO ASSESS DEFECTS IN TREES

The technical features of instruments used to assess wood defects are described and opinions are expressed on their reliability, ease of use, and interpretation. The instruments include penetrometers, electrical conduc- tivity meters, sonic and ultrasonic detectors, thermography, radar, and X-ray tomography. Keywords. Tree stability; VTA; penetrometers; electri- cal conductivity; sonic and ultrasonic detectors; thermogra- phy; radar; X-ray tomography; hazardous trees, tree structure analysis. Assessment of tree stability according to Mattheck and Breloer (1994) consists of 3 stages: visual inspection of the entire tree to look for ex- ternal evidence of internal defects; detailed ex- amination to see whether such defects exist and their extent; and instrumental measurement of internal defects and calculation of the residual strength. Elaboration of this method of analysis by Claus Mattheck has undoubtedly led to clarity by introducing scientifically demonstrated rules in a sector in which evaluation had always been based on personal experience. It has also en- couraged the resumption of comparison and dia- logue between researchers for the development of new instrumental investigation methods. The Pressler increment sampling borer/core extrac- tor had been the only tool to assess wood decay for many years, but recently many new technolo- gies have been developed. This paper offers an overview of the devices currently on sale or under development. Their technical features are individually described and the results of our tests and those conducted by other researchers are presented. Penetrometers Three similar instruments originally designed for evaluation of the soundness of wooden structures can be found on the market: the Densitomat-400, the Resistograph, and the Sibert DDD 200. These are self-powered drills that insert a very thin probe at a constant, electronically controlled speed. The power demand of the motor during penetration expresses the density of the wood and is con- verted into graphic signals (dendrograms) that both reveal and quantify decay in the wood. Densitomat-400 and Resistograph. These instruments are both commercially available. They consist of a sophisticated recording system that gathers up to 50 data per mm of forward travel. The geometry of the probe tip enables it to cut a hole wider than the 1.5 mm (0.06 inches) diameter of the shaft, which means that penetra- tion is not influenced by resistance imposed by the walls of the hole. The bit does not extract the shavings, and the hole is left packed with com- pacted wood dust. Both devices are interfaceable with printers or personal computers and can carry drill bits from 30 to 150 cm (11.8 to 59 inches) long. These devices are also able to provide in- formation on single growth rings and are com- monly used in dendrocronology. Sibert DDD 200. This commercially available instrument penetrates the wood more quickly than the Densitomat-400 or the Resistograph, but it provides fewer data per mm. It carries bits 20, 31, and 40 cm (7.9,12.2, and 15.7 in.) long, with a diameter of 1.27 mm (0.05 in.). It, too, provides a plot of the penetrability of the wood on a co- axial roll of millimeter-scale graph paper. Analy- sis is quicker with the DDD, and decayed areas within a tree can be reliably quantified, although the instrument cannot provide very detailed in- formation about a single growth ring. The above-described penetrometers are quite similar to each other; for this reason, advantages and disadvantages can be considered valid for all of them. The advantages of the various types of penetrometers are • They furnish objective data that can be used to make uniform and standard quantitative assessments of decay.