Campbell Scientific CS615 Water Content Reflectometer - Manual (Page 9)

CS615 WATER CONTENT REFLECTOMETER
5
The Pulse Count instruction uses accumulators
to monitor pulses on the datalogger Pulse
Count Channels. At the beginning of each
execution interval for the table containing the
Pulse Count instruction, the accumulator count
is dumped to a section in datalogger RAM, the
accumulator is then reset to zero and begins
accumulating counts again. When the Pulse
Count instruction is reached, the value in RAM
is modified by the multiplier and offset in the
Pulse Count instruction and the result written to
input storage. The RAM is then reset to zero.
Consider the simple 21X program in Table 1.
Additional instructions which might be needed for
multiplexer control or other functions have been
omitted for simplicity. This program is written to
obtain a CS615 reading every 15 minutes.
When the program is compiled by the
datalogger, the accumulators begin monitoring
the Pulse Count Channels. Immediately
following compilation by the datalogger, there is
no signal on the input channels because the
CS615 is not enabled until the 15 minute interval
specified in the Instruction 92 is reached. When
the 15 minute interval is reached, Instruction 86
is executed which sets the enable of the CS615
high and the probe outputs a signal which is
detected by the pulse counters.
The Loop Instruction in this application uses a
Delay of 1 and a Loop Count of 2. Program
execution pauses during the first loop count
until the execution interval of 2 seconds is
complete. This delay is necessary because the
probe has not been enabled for the entire
execution interval which means a complete
count is not obtained. More complex programs
will contain additional instructions prior to the
instruction to enable the CS615, and these
instructions can take a significant amount of
time to execute. During the second time
through the Loop Instruction the pulse counters
see the CS615 output for the full execution
interval. At the end of this interval the
accumulators transfer to RAM and are reset.
When the program execution then moves to the
Pulse Count Instruction (P3) the value in RAM
is converted to kHz and written to input storage.
7.3 PERIOD MEASUREMENT
Instruction 27, Period Measurement is available
only on the CR10, CR10X, or CR500. For
convenience, the following tables from the
datalogger manual are presented here. See the
datalogger manual for detailed description of
the instruction and the example programs for
typical values.
8. MAINTENANCE
The CS615 does not require periodic
maintenance.
9. CALIBRATION
9.1 GENERAL
The information in this calibration section
applies only to CS615 version 8221-07. The
version number is listed on a cable label near
the end of the probe cable.
The CS615 provides an indirect measurement
of soil water content by using the effect of
changing dielectric constant on applied
electromagnetic waves. The probe rods act as
a wave guide and the material surrounding the
rods (soil) varies in dielectric constant with the
amount of water in the material.
The dielectric constant of the soil is a weighted
summation of the dielectric constants of the soil
constituents. The dielectric constant for water
is significantly higher than that of other
constituents. Changes in the dielectric constant
of the soil system can be attributed to changes
in water content. This is the basis for the
measurement technique.
There are two soil properties which affect the
response of the CS615 to changes in water
content. High clay contents (greater than 30%)
or high electrical conductivity (greater than
1 dS m
-1
) may require that the calibration be
adjusted or generated for the specific soil.