Instruction manual

CS650 and CS655 Water Content Reflectometers
8.2.1 Use of Multiplexers
Multiplexers such as Campbell Scientific’s AM16/32B may be used to connect
up to 32 CS650 probes to a single control port. When using multiplexers, the
simplest configuration is for all probes to have the same SDI-12 address.
When multiplexing CS650 probes, the switched 12V channel should be used so
that power to the sensor may be turned off under program control before the
multiplexer switches to the next channel.
Failure to turn off the switched 12 volt channel before
clocking the multiplexer will result in damage to the
multiplexer relays.
The proper sequence in the datalogger program for measuring CS650 probes
on a multiplexer is:
1. Set RES control port high to enable multiplexer
2. Pulse CLK control port to advance to next multiplexer channel
3. Set switched 12 volt channel high to supply power to CS650
4. Send SDI-12 command(s) to CS650
5. Set switched 12 volt channel low to remove power from CS650
6. Repeat steps 2 through 5 for each CS650 connected to the multiplexer
7. Set RES control port low to disable multiplexer
Program examples in Appendix B, Example Programs
(p. B-1), show the
commands used in CRBasic.
8.3 Water Content Reflectometer Method for Measuring
Volumetric Water Content
8.3.1 Description of Measurement Method
For the water content measurement, a differential emitter-coupled logic (ECL)
oscillator on the circuit board is connected to the two parallel stainless steel
rods. The differentially driven rods form an open-ended transmission line in
which the wave propagation velocity is dependent upon the dielectric
permittivity of the media surrounding the rods. An ECL oscillator state change
is triggered by the return of a reflected signal from the end of one of the rods.
The fundamental principle for CS650 water content measurement is that the
velocity of electromagnetic wave propagation along the probe rods is
dependent on the dielectric permittivity of the material surrounding the rods.
As water content increases, the propagation velocity decreases because of
increasing dielectric permittivity. Therefore, the two-way travel time of the rod
signal is dependent upon water content, hence the name water content
reflectometer. Digital circuitry scales the high-speed oscillator output to an
appropriate frequency for measurement by an onboard microprocessor.
Increases in oscillation period resulting from signal attenuation are corrected
CAUTION
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