2 losses in density caused by deterioration. Several studies have focused on evaluating the correlations between the amplitude of the drilling resistance obtained by the equipment and the wood density (Botelho 2006) or its effectiveness in detecting dete- riorations in wood structures (Eckstein 1994; Brashaw et al. 2011; Rinn 2012; Tannert et al. 2013; Rinn, 2016), logs (Brashaw et al. 2011), trees (Johnstone et al. 2007; Wang et al. 2008; Kubus 2009; Johnstone et al. 2010), and damaged wooden poles (Jenkins 2011; Reinprecht and Supina 2015). Density is a wood material property that describes wood condition, and it differentiates between intact and decayed parts (Means et al. 1985). How import- ant it is to differentiate between the various stages of decomposition becomes clear when realizing that, depending on the type of decay, a loss in density by only 10% can lead to a loss in strength of 90% or even more (Wilcox 1978). Structural stability of wood is thus highly dependent on even slight changes in wood density. Therefore, many research projects have focused on developing a method for non-destructive assessment of the density profiles of standing trees and timber (Rinn 2016). Shortly after the first interna- tional press releases described how a German physi- cist (F. Rinn) drilled thin needles into trees in order to inspect for defects by measuring density (Schubert 1989), many companies worked to develop similar devices. For a few electronically recording resistance drills, a high correlation (r2 > 0.9) of the profiles to wood density was found, even in standing trees (Brashaw et al. 2013); and because density is the most important material parameter for this technique, there is a need for studies to evaluate the accuracy and reliability of the profiles obtained by non- calibrated resistance drills. For resistance drills using mechanical spring-driven recording, the correlation to wood density was found to be weak (r2 ~0.5; John- stone et al. 2011), making it correspondingly difficult to evaluate wood condition based on such profiles. Due to this weak correlation, it is impossible to iden- tify and differentiate various stages of decomposition in these profiles (Johnstone et al. 2011). Conse- quently, only big defects with advanced stages of decay or voids can be identified reliably. For other resistance drills, it is not yet known how well the mea- sured values correlate to wood density, although these drills are frequently used on the market, and have been for several years. Thus, it is not yet clear how to ©2019 International Society of Arboriculture Reis et al: Profiles of a Non-Calibrated Resistance Drill interpret these profiles correctly in terms of wood condition. To fill this knowledge gap, researchers compared profiles of a non-calibrated resistance drill with pic- tures of the surface of the drilled stem cross-sections, with a special focus on how the different stages of decomposition are revealed. MATERIALS AND METHODS Sampling was composed of logs collected from six tree species: Centrolobium sp., Tabebuia ochracea, Liquidambar styraciflua, Platanus spp., Caesalpinia pluviosa, and Copaifera sp., widely spread in the urban arborization of São Paulo State, Brazil. Six discs approximately 200 mm high were removed from these logs. The discs were conditioned in a place with temperature and relative humidity (RH) controlled (temperature around 25°C and RH around 65%) until reaching equilibrium moisture. When they reached the equilibrium, species heartwood and sapwood pre- sented moisture content around 10% and 11%, respectively. It is important to note that even in green condition, hardwoods tend to exhibit smaller differ- ences in moisture between heartwood and sapwood than softwoods, and in some species, heartwood moisture is higher than sapwood moisture (Haygreen and Bower 1989). Eight equidistant points were marked on each disc perimeter for measurements with the drilling resistance equipment (IML PD 400, Wiesloch, Germany) (Figure 1). The measurements were performed in the perpendicular direction to the grain in the 8 marked points (measurement routes), obtaining 48 graphs of amplitude of drilling resis- tance. To help with the test, the discs were fixed to a concrete table with sergeants (Figure 1). The species used in the research had different den- sities and deterioration conditions (Table 1). The model of equipment used needs the manual choice of Figure 1. Drilling-resistance test on discs.
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