FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

Results are obtained within about 10 to 20 minutes for dead woody
fuels, or about one hour for live fuels. Fuels can be dried at 95 C. (203
F.), minimizing any loss of volatiles.
While quite accurate, a major disadvantage of the Computrac is the
very small size of the sample which can be processed, approximately 3
to 10 grams of material. In order to obtain a representative sample,
many samples must be subsequently processed. A second major
disadvantage of the Computrac is its high purchase price.
(d) Drying ovens. Detailed procedures for use of a scale and drying
oven can be found in Countryman and Dean (1979) and Norum and
Miller (1984). Processing of fuel moisture samples in a drying oven has
long been the standard for measurement of fuel moisture content.
Samples are weighed on a scale to the nearest 0.1 gram, dried in the
oven, and then weighed again to determine the amount of water lost.
Ovens are customarily set to 100 C. (212 F.) for dead woody fuels, and
80 C. (176 F.) for live fuels. The standard drying time is 24 hours. Major
advantages of a drying oven are that many samples can be processed
simultaneously, and accurate values are obtained if proper procedures
are followed. The disadvantage is the 24 hour delay in arriving at the
values for moisture content.
e. Fuel moisture meters. Several brands of fuel moisture meters are
presently available that provide a direct measurement of fuel moisture.
Most meters work by measuring the electrical resistance between two
probes which are inserted into a piece of wood. Most of these meters
were developed for testing the moisture content of kiln dried lumber and
are most accurate at lower moisture values. Some of these probes are
calibrated on a wet weight basis, not a dry weight basis, and will not
give answers that can be used as input to fire behavior prescriptions.
Most of the probes are less than an inch in length and cannot penetrate
deeply enough into large diameter wood to measure its moisture
content. However, a fairly accurate measurement of large fuel moisture
content can be made by cutting across the diameter of a large piece of
woody fuel and inserting the probe into the freshly exposed surface.
Because meters were developed to measure moisture content of wood,
a fairly dense substance, they cannot give an accurate reading of
moisture content within litter or duff layers, or of soil. These meters are
not suitable for live fuel moisture estimation because the probes cannot
be adequately inserted into the live fuels, and most meters do not
operate at high moisture contents.