Campbell Scientific TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 - Manual (Page 15)

Appendix A. Discussion of TDR Probe
Offset and a Simple Laboratory Method
for Calculation
A.1 Discussion of Probe Offset
Probe offset accounts for the segment of the TDR probe rods that is part of the
probe head and is not exposed to the media surrounding the probe rods. The
location of the beginning of the probe that is calculated in the TDR100
operating system is the point along the cable where the transition from the 50
ohm cable to the TDR probe impedance occurs. The distance from this
transition to the point where the rods come out of the probe head is constant
and can be accounted for.
The TDR100 operating system uses the following equation to calculate the
ratio of apparent rod length to actual rod length, La/L. This ratio is equal to the
square root of dielectric permittivity,
ε .
L
ProbeOff
V
start end
L
La
p
−
−
= [A1]
La apparent length (m)
L actual rod length (m)
V
p
relative propagation velocity (1.0)
ProbeOff probe offset (m)
start distance into window for beginning of rods (m)
end distance into window for end of rods (m)
To examine the sensitivity of La/L to probe offset, multiply equation [A1] by L
and take the 1st derivative of La with respect to probe offset.
1 ProbeOff
V
start end
) d(ProbeOff
d
p
− =
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
−
[A2]
The sensitivity of the apparent length measurement, La, is directly related to
probe offset. A probe offset difference of 0.005 changes La by –0.005. The
water content error for saturated soil is 0.16% volumetric water content. In
very dry soil the error is 0.20%. There is a slight dependence of probe offset
on water content but the amount is within the resolution of the water content
measurement.
A-1