Sentek RT6, Hardware Manual

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CHAPTER 3: TECHNICAL DESCRIPTION
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counts the pulses over 0.5 seconds to provide a
"count" (hence equal to half the frequency). For
example if the sensor is oscillating at 100 MHz the
output of the sensor would be 48.828 kHz
(100,000,000/2048) so the logger would record a
count (equal to half frequency) of 24,414 (48,828/2).
These counts are recorded by the logger and
downloaded to a computer without being processed.
Software in the computer passes the counts through a
calibration equation to provide values in soil water
content for graphing. The default calibration equation
provided in software was independently established
(by CSIRO, USDA, etc) on various soil types using
gravimetric sampling which is the international
standard used to calibrate soil moisture sensors and
soil moisture instruments such as the Neutron Probe.

Caution: The default calibration equation in the IrriMAX
software is provided for those customers who
require only soil moisture dynamics for irrigation
scheduling purposes. Customers who require
accurate soil moisture data for their particular soil
type must calibrate the sensors to that soil.
Gravimetric sampling is the recognized method for
determining calibration equation (and constants)
for a particular soil type.
Following the calibration process on a specific
soil/medium the constants of the calibration equation
can be changed in the software to provide more
accurate data.
Power Supply
SLA battery The Sentek RT6 has an internal 12 volt 7A/hr SLA
battery which provides power during night periods (if
solar powered) or in the event of external AC/DC
power failure. This battery has its charge maintained
by applying a power source to the Main Board which
incorporates a charger specifically designed to charge
SLA batteries.