Campbell Scientific CS616 and CS625 Water Content Reflectometers - Manual (Page 40)

CS616 and CS625 Water Content Reflectometers
8.3 Collecting Laboratory Data for Calibration
Water content reflectometer 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 period
(microseconds); and C
n
, the calibration coefficient (n = 0..2).
The linear form is
( )
θ τ τ
v
C C = +
0 1
*
with θ
v
, the volumetric water content (m 3 m -3 ); τ , the CS616 period
(microseconds); C
o
, the intercept; and C
1
, the slope.
Required equipment:
1. CS616/CS625 connected to datalogger programmed to measure output
period
2. Cylindrical sampling devices to determine sample volume for bulk density;
for example, copper tubing of diameter ≥ 1 in. and length about 2 in.
3. Containers and scale to measure soil sample weight
4. Oven to dry samples (microwave oven can also be used)
The calibration coefficients are derived from a curve fit of known water
content and probe output 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.
The measurement sensitive volume around the probe rods must be completely
occupied by the calibration soil. Only soil should be in the region within 4
inches of the rod surface. The probe rods can be buried in a tray of soil that is
dry or nearly dry. The soil will be homogeneous around the probe rods if it is
poured around the rods while dry. Also, a 10 cm diameter PVC pipe with
length about 35 cm can be closed at one end and used as the container.
It is important that the bulk density of the soil used for calibration be similar to
the bulk density of the undisturbed soil. Using dry soil without compaction will
give a typical bulk density, 1.1 - 1.4 g cm -3 . This is especially important when
bulk density is greater than 1.55 g cm -3 . Compaction of the calibration soil to
similar bulk density may be necessary.
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