Fundamentals - Wildland Fuel Fundamentals and Applications - page 25

than 1.0 m −1 for large logs), whereas thin particles that are long and wide, such as
leaves, have high values (over 2000 m −1 for grass blades and pine needles). SAVR
is a fuel property that indirectly characterizes particle geometry (shape), and this
corresponds to the particle's importance in fire science. Particles with high SAVR
(e.g., foliage) are more flammable and easier to ignite than low SAVR particles
(e.g., logs; Pyne et al. 1996 ). SAVR indirectly represents the effect of fuel size on
combustion processes. It also represents the rate of response of fuel particles to
temperature and moisture fluctuations; particles with high SAVR values lose heat
and moisture more quickly than particles with lower SAVR values (Brown 1970b ).
SAVR is extremely difficult to measure accurately because most fuel particles
are complex in geometry. The most common way to measure SAVR is to use the
simple formula developed by Brown ( 1970b ) where the fraction of particle perim-
eter divided by the average cross-sectional area taken for cross sections along the
length of the particle. This technique requires an assumption of a geometric shape of
the cross section, and most efforts assume a circle to represent the fuel particle vol-
ume, although many have used other shapes for needles, leaves, and grass blades.
However, most fuel particle cross sections are difficult to describe with any general
geometric shape, rather, they are complex amorphous forms. Another method is to
estimate volume by submerging the particle in a liquid and measuring the displace-
ment of the liquid, and measuring the surface area by assuming some geometric
shape and measuring various dimensions to estimate area. A more complex tech-
nique would be to measure the rate of drying of the fuel particle and correlating that
rate to surface area. The problem with all of these techniques to estimate SAVR or
density is that particles are constantly changing in response to endogenous and ex-
ogenous biophysical processes. All fuel particles are in some state of decay, and the
degree of decay and its distribution across a particle can affect SAVR. Moreover,
fuel particles are constant changing shapes in response to fluctuations in moisture
content, temperature, and relative humidity as mentioned above. These responses
sometime result in the fragmentation of the particle, which then increases surface
area and SAVR. This dynamic quality of fuel particles results in greater variability
in the estimation of particle SAVR.

2.3.1.3 Particle Density ( ρ p )

Particle density is the dry weight of the particle per unit volume (kg m −3 ). The term
specific gravity is also used to represent particle density; specific gravity is the
density of a substance relative to the density of water at a specific temperature and
pressure. One needs to multiply specific gravity by 1000 to convert to density (e.g.,
0.42 specific gravity is 420 kg m −3 ).
Particle density is measured using variations of two techniques. The particle is
always oven-dried and weighed to determine mass. Then there are two techniques
for measuring volume. The first technique calculates volume by assuming various
geometric shapes and using diameters and lengths to define shape dimensions (see