Scraping and Pick Devices. The depth of surface decay can be detected by
the pick test. The pick test (see Chapter 4) will detect infected wood where strength loss of at least
10% has occurred. Scraping using a shovel or triangular blade is often used to detect below g-round
decay on the pole surface. The decayed wood is simply scraped or cut away until sound wood is
Moisture Meter. Resistant type meters (Figure 7-5) can detect the moisture
content of wood to a depth of about 2 1/2 inches. Because the high moisture content of decaying
wood--above 25 percent--causes a steeper than normal moisture gradient, the meter can be useful
for determining the extent of decay in poles and other timbers. Meter readings above 20 percent
and steep moisture gradients indicate the height of decaying wood in Douglas-fir poles with rot
below, but not above the ground line. Moisture readings below 20 percent indicate the absence of
active decay to the depth of the electrode.
The batteries and calibration of the meter should be checked frequently. For the meter to read from
the tips of the electrodes, the coating on the electrode shanks must be intact. When necessary, meter
adjustments for ambient temperatures and wood species must be made. Oil borne preservatives nor-
mally do not affect meter readings, but inorganic salts may. Comparative tests for treated and un-
treated wood should be conducted to determine the effect of preservatives.
Shigometer. The shigometer measures the pulsed electrical resistance of
wood and was developed for detecting decay in trees. To conduct the test, a 3/32 inch diameter hole
is bored into the wood. The probe, which consists of two twisted, insulated wires with the insula-
tion removed from the tip, is inserted to various depths in the hole. A marked change in electrical
resistance indicates rot or a defect. The moisture content must be at the fiber saturation point (about
28 percent) or higher for the shigometer to work. Decaying wood or that in ground contact is at the
fiber saturation point or higher.
Attempts to apply this technology to the detection of decay in utility poles have revealed that many
factors, in addition to decay, influence the electrical resistance of wood. This has contributed to
some confusion in the use of this methodology. Therefore, when the device indicates decay in
poles, the poles should also be bored to verify the presence and extent of decay.
Pilodyn. The pilodyn is an instrument that drives a spring-loaded pin into
wood and indicates the depth of penetration. The instrument has been used to measure tree density,
sorting wood and detecting surface decay and soft rot in poles. It can generally characterize the
outer two inches of a pole or that zone which accounts for 70 to 90 percent of the load-carrying
capacity. The pilodyn can be used in combination with a density determination to predict breaking
strength of ponderosa pine poles. Pilodyn measurements must be corrected for moisture content
since moisture affects wood strength and because it varies substantially in outdoor wood structures.
For quantitative measurements and maximum benefit, the instrument should be calibrated for
species, moisture content, and the amount and type of preservative. Pilodyn readings on Douglas-
fir can be adjusted for moisture content by using a resistance moisture meter.