Campbell TDR200, Инструкция по эксплуатации

Страница: 30 / 53

TDR200-Based Time-Domain Reflectometry System
19
Minimizing cable lengths should always be considered in the design of a
measurement system using TDR. If long cable lengths are necessary, the
adverse effects can be minimized by using low attenuation cable such as RG8.
Careful probe design ensures correct probe impedance giving robust
reflections. All TDR probes offered by Campbell Scientific are designed to
optimize accuracy when longer cable lengths are used.
8.2.2 Soil Electrical Conductivity Effect on Water Content Measurement
The signal at the probe is attenuated when ionic conduction occurs in the soil
solution. This inherent attenuation is used in TDR measurements to determine
soil electrical conductivity as described by equation [5]. The presence of ions
in the soil solution provides a path for electrical conduction between TDR
probe rods. The attenuation of the signal can affect the accuracy and resolution
of water content measurements.
FIGURE 8-2 presents a series of waveforms when a solution with an electrical
conductivity of 1.0 dS m –1 is added to a soil which has essentially no salt
present. FIGURE 8-3 shows data for solution with high electrical conductivity.
FIGURE 8-2. Waveforms collected in a sandy loam using CS610 probe
with RG8 connecting cable. Volumetric water content values are
0.10, 0.16, 0.18, 0.21, and 0.25. Solution electrical conductivity is
1.0 dS m
–1
.
water content = 0.095
water content = 0.25