Campbell Scientific CS616 and CS625 Water Content Reflectometers - Manual (Page 42)

CS616 and CS625 Water Content Reflectometers
Method D4643-93 requires heating in microwave for 3 minutes, cooling in
desiccator then weighing and repeating this process until weigh is constant.
Gravimetric water content is calculated after the container weight is accounted
for.
θ
g
wet dry
dry
m m
m
=
−
For the bulk density
ρ
bulk
dry
cylinder
m
volume
=
the dry weigh of the sample is divided by the sample tube volume.
The volumetric water content is the product of the gravimetric water content
and the bulk density
θ θ ρ
v g bulk
= *
The average water content for the replicates and the recorded CS616/CS625
period are one datum pair to be used for the calibration curve fit.
8.4 Collecting Field Data for Calibration
Required equipment
1. CS616/CS625 connected to datalogger programmed to measure probe
output period
2. Cylindrical sampling devices to determine sample volume for bulk density,
e.g. copper tubing of diameter ≥ 1” and length about 2”
3. Containers and scale to measure soil sample weight
4. Oven to dry samples (microwave oven can also be used)
Data needed for CS616/CS625 calibration are the CS616/CS625 output period
(microseconds) and an independently determined volumetric water content.
From this data, the probe response to changing water content can be described
by a quadratic calibration equation of the form
( )
θ τ τ τ
v
C C C = + +
0 1 2
2 * *
with θ
v
being the volumetric water content (m 3 m -3 ); τ , the CS616/CS625
period (microseconds); and C
n
, the calibration coefficient (n = 0..2).
The linear form is
( )
θ τ τ
v
C C = +
0 1
*
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