The Resistograph measures the amount of damaged wood at one point on the tree, by drilling. It is much more invasive than the Picus.

The Resistograph

 This tool consists of a drill with a 300mm long drill bit which is driven into the tree at a specified point.  The Resistograph registers the resistance to the rotation of the drill as a line on a strip of pre-carbonated paper 300mm long, marked in 10mm squares. On the original paper strip each centimetre represents a centimetre's progress into the tree but the printouts below have been reduced in size to fit on the page.

On a uniformly healthy tree the line would be fairly straight while showing oscillations of the scribing needle as summer wood and winter wood are encountered. As the drill penetrates further into the tree the height of the line increases gradually. 

If there is decay the needle scribes a fall in the resistance of the wood by tracing a falling line. A sudden drop usually indicates a cavity or very badly decayed wood. 

The entry point through the bark is on the right and the drill progresses from right to left across the print-out to a depth up to 300mm. Entry is not always at the edge of the paper where the machine cannot fit tight against the bark e.g. because of a groove or flute in the stem.

 

 Example drilling traces

 The first and third of these three drillings show decay once the drill has penetrated 18cm.
The middle trace shows solid wood.
(Apologies for the poor quality of the image)



Charges

For using the IML Resistograph to make test drillings (usually four per tree) and present the results with interpretation: £100 per tree plus travel costs as for the Picus.  Resistograph is quicker and clearly cheaper but only tests at one point while the Picus scans a slice through the tree.  Resistograph is best used as a confirmation of Picus findings.