In everyday circumstances trees break or fall because defects or decay are present and compromise the structural integrity of the tree. Tools are now available to aid in the detection of decay in trees. These tools can also help determine the nature and extent of cracks and cavities that are the most common cause of tree failure.

The Resistograph is one such tool. The resistograph is an instrument, or rather, a family of instruments, for detecting decay and defects in trees and timber. The instrument measures the resistance to a needle inserted into the wood under constant drive. The constant drive is provided by a battery driven electric motor. The resistance of the needle tip is transferred through an "intelligent" satellite gearbox to a pointer on the top of the instrument that maps the results on a waterproof wax paper printout.

Drilling resistance correlates to the mechanical properties of the wood. Defects such as cracks, areas of decay, hollows and to a certain extent, tree ring structures can be detected and mapped. The resistance is mapped on a 1:1 scale on the wax paper, giving a clear graphical representation of the mechanical properties of the wood.

Sample of the Resistograph's Printout see examples for field study

The resistograph utilizes a 3mm needle tip and a 1.5mm flexible needle that tends to "squeeze" between the fibres of the wood causing very little wounding. The resistograph is a relatively new instrument and research continues. The high level of accuracy and readily readable results mean that the limitations are only in our ability to interpret the information offered by the resistograph.

I have been using the resistograph in decay and defect detection for the last eight years and have found it to be the best way to find the most information with the least damage to the living tree.

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