I am also studying for the tech cert but have little experience with decay detection. I'm going to add my bit though because it is good for a bit of exam preparation.
There are limitations with all devices marketed as decay detection devices. It is important to understand that most do not actually detect decay.
Decay detection devices are mostly used to make an estimate of the dimensions and quality of residual wood where a cavity is suspected. Many other factors must be considered to determine acceptable residual wall dimensions for tree retention. The limitations of the internal investigation techniques must be fully understood by the user to allow an accurate report to be completed. To overcome the problems of individual decay detection devices two investigations using different methods are often used.
A core sample can give an accurate measurement of the residual wall at the point it is taken but it is necessary to take multiple samples to get an estimation of the shape and size of the decay pocket. Core samples are invasive but allow for inspection of reaction zones and growth rates. A fractometer can be used to measure the bending angle and force required to snap the core sample but to expand on this measurement to give an idea of overall tree stability requires consideration of so many other factors that it is not practical.
Drilling with a 4mm bit is invasive but can yield a range of information. Resistance to progress of the drill bit is felt and interpreted by the user. Shavings can be collected at measured depths and can be investigated further.
Acoustic testing (mallet) can indicate the presence of a cavity and its location but further investigation is often required to justify retention of a tree with potential targets.
Shigometers (and Vitalometers) measure electrical resistance across two points very close to each other. Resistance is higher in healthy cells, decayed cells have a higher concentration of inorganic salts. The probe can be inserted into a drilled hole and the resistance measured at intervals. Changes in readings may indicate decayed wood. Dry decays can give false readings. The Shigometer is not widely used.
Digital microprobes measure the resistance to penetration of a 1.7mm blunt probe. The resistance is recorded graphically and digitally. The resistance to penetration is measured very accurately and growth rings can be detected. The problem is that the probe does not always go in straight and may bend. Multiple measurements must be taken.
Resistographs measure resistance to Torque of a 3mm probe with a cutting tip. Early detection of Ketzshmaria deusta is often missed. Widening of growth rings provides less resistance to the cutting bit and this can be misinterpreted as decay. Deflection of the probe can also occur. Fungal colonisation through the breached reaction zone has been shown to occur so this should be considered an invasive technique.
The Picus measures the speed of the leading edge of a sound wave as it travels through the tree. By using multiple sensors a matrix of sound attenuation is recorded. Using the speeds relatively computer software is used to create a map where colours are used to show areas of different sound attenuation. If a stem is evenly decayed the software may show the whole of the cross section as healthy wood. Ring shakes can also distort results.
Thermography uses an infared camera to detect the surface temperature of the tree. Uneven heat distributions are associated with degraded tissue or cavities. This system does not provide an estimation of residual wall thickness, but may help in targeting other investigations.
Would you include anything else in the decay detection topic? Tree statics, VTA ?
Please feel free to point out any false statements I have made and any criticisms.