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Decay Detection Techniques


PICUS survey in progress

PICUS survey in progress.

Visual inspection will reveal much about a tree. However, occasionally, a tree will show symptoms that suggest problems which cannot be seen externally. Consequently, an internal inspection is needed to assess the tree’s structure. JCA offers three such methods; the Picus Sonic Tomograph, the Resistograph and the Fractometer.

Picus Sonic Tomograph

The Picus Sonic Tomograph measures the speed at which a sonic pulse travels through a tree from various points (based on the principle that sound travels faster through good wood than decayed wood). The sonic pulse is generated by contact with pins with an digital hammer, around the tree’s circumference. Sensors, attached to the pins, measure the time sound takes to travel through the wood.

Once all the pins have been tapped, the information is sent to a laptop where the speed variations are displayed using a range of colours. The images below illustrate this:

Sample output from PICUS 1 Sample output from PICUS 2Brown indicates sound wood with significant strength. Purple, Blue and White indicate decayed wood or possibly hollows. Green indicates transitional wood.

This system is an essential tool in modern arboricultural consultancy due to its relatively non-invasive method. For further information, visit Argus Electronic GmbH who designed the equipment.


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Simon Horne, Principal Consultant, SRH Design Solutions


The Resistograph is a device used to detect decay in trees. It works by driving a probe into the tree at a constant speed.

As the probe enters the tree, the timber’s resistance is recorded. The results are printed onto a scaled strip of paper which can then be analysed (the lower the resistance, the softer or more decayed is the wood). This device determines at what point the wood becomes decayed and how much sound wood remains.


The Fractometer is a device for measuring the strength of timber by assessing its bending angle to the point at which it snaps. The sample is obtained by first taking a core from the tree by using an increment borer. The strength of the wood is measured along the sample so an idea of the state of the tree stem, at this point, can be determined.

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