The Whole Tree Company regularly carries out Internal decay detection on tree stems and limbs utilising an IML Resi PD400 drilling resistance device; as a very fine 40cm long drill bit progresses into the subject timber, two measurements are electronically recorded – drilling resistance and feed force. These measurements are overlaid on the same graph to give an accurate visual representation of wood structure in that one plane. A number of tests around the circumference of the stem, limb or in the area of a feature of interest allow a picture to develop of the internal condition.

Results are read from right to left along the x axis, 0 being the point the bit exits the device with the first resistance (usually softer bark and cambium) showing soon after, to the maximum reach of the device at far left (40cm). The sharp peaks and troughs represent annual growth rings. Varying stages of decay or cavities manifest as breaks in this pattern with potential drops in amplitude, smooth featureless areas, irregular profiles all potentially indicative of a breakdown of wood structure. In the example the drilling curve (green) does not drop significantly until 28.5cm and indicates the presence of an internal defect. The more sensitive feed curve (blue), falls off at 18cm indicating areas of early decay either side of the same defect, thus detecting a significantly larger proportion of affected wood.