Campbell TDR100 Manuel d'instructions télécharger pdf (Page 27)

TDR100

6.2.5 Parameter 5: Relative Propagation Velocity

Relative propagation velocity is the ratio of actual propagation velocity for a

transmission line to the propagation velocity in air. A TDR system is typically

comprised of components with different signal propagation properties. The V

for a particular component depends on transmission line characteristics such as

the dielectric constant of inter-conductor insulating material. Setting V

= 1.0

and using apparent distances simplifies system setup. The displayed position

of a waveform is apparent distance. The value chosen for V

does not affect

water content or electrical conductivity measurement. The selected V

value

does affect waveform display.

6.2.6 Parameter 6: Points

The number of waveform values used to calculate apparent lengths or collected

as a waveform file. A value of 251 is recommended. A higher value can

provide higher resolution but can also be problematic in noisey environments.

The first data point is a relative zero and corresponds with the value of Cable

Length. If 250 spatial increments are desired to span the desired Window

Length, a value of 251 should be chosen.

6.2.7 Parameter 7: Cable Length (meters)

Cable length is used by the waveform analysis algorithm as the start position to

search for the probe beginning and end. It is an apparent length.

The value for cable length is easily found using PCTDR. Select the probe

using the Multiplexer feature in the Settings menu. Then adjust the cable

length and window length to position the waveform so the distance to the

beginning of the probe can be read from the x-axis. Subtract 0.5 meters from

the beginning distance and use that value in parameter 7 of datalogger

instruction 119.

It is important that the V

used during determination of value for cable length

be the same V

used during subsequent measurements with PCTDR and

datalogger instruction 119. A Vp value of 1.0 is recommended for soil water

measurements.

6.2.8 Parameter 8: Window Length (meters)

Window length specifies the length of waveform used in water content

determination and the distance for a saved waveform. The waveform analysis

algorithm begins at the distance specified by cable length and uses data

contained for the value specified by Window Length. It is important that

Window Length be large enough to contain the entire probe waveform at the

highest expected water contents. See Table 4-1.

6.2.9 Parameter 9: Probe Length (meters)

Physical length of the exposed portion of the TDR probe rods.

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 55 56

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Campbell TDR100 Manuel d'instructions Page 27

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