Campbell Scientific CS616 and CS625 Water Content Reflectometers - Manual (Page 43)

CS616 and CS625 Water Content Reflectometers
with θ
v
, the volumetric water content (m 3 m -3 ); τ ,
the CS616/CS625 period
(microseconds); C
o
, the intercept; and C
1
, the slope.
The calibration coefficients are derived from a curve fit of known water
content and CS616/CS625 period.
The number of data sets needed to derive a calibration depends on whether the
linear or quadratic form is being used and the accuracy requirement. Consider
the expected range of soil water content while viewing FIGURE 7-1 and
FIGURE 7-2. If the expected response is nearly linear, fewer laboratory
measurements are needed to derive the calibration. A linear response is best
described by data taken near the driest and wettest expected water contents.
Collecting measurements of CS616/CS625 period and core samples from the
location where the probe is to be used will provide the best soil-specific
calibration. However, intentionally changing water content in soil profiles can
be difficult.
A vertical face of soil can be formed with a shovel. If the CS616/CS625 is to
be used within about 0.5 meters of the surface, the probe can be inserted into
the face and water added to the surface with percolation. After adding water,
monitor the CS616/CS625 output period to determine if the soil around the
rods is at equilibrium.
With soil at equilibrium, record the CS616/CS625 period value.
Soil hydraulic properties are spatially variable. Obtaining measurements that
are representative of the soil on a large scale requires multiple readings and
sampling. The average of several core samples should be used to calculate
volumetric water content. Likewise, the CS616/CS625 should be inserted at
least 3 times into the soil recording the period values following each insertion
and using the average.
Remove the CS616/CS625 and take core samples of the soil where the probe
rods were inserted. This is necessary for measurement of bulk density. Copper
tubing of diameter ≥ 1” and length about 2” works well. The tubes can be
pressed into the soil surface.
It is good to take replicate samples at locations around the tray surface. Three
carefully handled samples will provide good results.
The sample tubes should be pushed evenly into the soil surface. Remove the
tube and sample and gently trim the ends of excess soil. Remove excess soil
from outside of tube.
Remove all the soil from tube to a tray or container of known weight that can
be put in oven or microwave. Weigh and record the wet soil weight.
Water is removed from the sample by heating with oven or microwave. Oven
drying requires 24 hours at 105 C. Microwave drying typically takes 20
minutes depending on microwave power and sample water content. ASTM
Method D4643-93 requires heating in microwave for 3 minutes, cooling in
desiccator then weighing and repeating this process until weigh is constant.
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