Campbell CS650, Руководство пользователя

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CS650 and CS655 Water Content Reflectometers
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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 on-board microprocessor. Increases in
oscillation period resulting from signal attenuation are corrected using an
electrical conductivity measurement. A calibration equation converts period and
electrical conductivity to bulk dielectric permittivity. The Topp equation is used
to convert from permittivity to volumetric water content.
8.3.2 The Topp Equation
The relationship between dielectric permittivity and volumetric water content in
mineral soils has been described by Topp et al. (1980) in an empirical fashion
using a 3
rd
degree polynomial. With

v
the volumetric water content and K
a
the
bulk dielectric permittivity of the soil, the equation presented by Topp et al. is
CAUTION