Campbell Scientific TDR100 TDR100 Time Domain Reflectometry Systems - Page 30
Parameter 8: Window Length meters, Parameter 9: Probe Length meters, Parameter 10: Probe Offset
View all Campbell Scientific TDR100 manuals
Add to My Manuals
Save this manual to your list of manuals |
Page 30 highlights
TDR100 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 Vp used during determination of value for cable length be the same Vp used during subsequent measurements with PCTDR and datalogger instruction 119. A Vp value of 1.0 is recommended for soil water measurements. 6.3.8 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. 6.3.9 Parameter 9: Probe Length (meters) Physical length of the exposed portion of the TDR probe rods. 6.3.10 Parameter 10: Probe Offset (meters) Most TDR probes have a block of epoxy or other material which holds the rods rigidly spaced and houses an impedance matching transformer (balun) if used. The algorithm in the TDR100 uses changes in the reflection coefficient to identify end points of the probe. The transition from the coaxial cable to the TDR probe occurs inside the probe block and causes an increase in the reflection coefficient if the probe impedance is greater than the cable impedance. However, this is not the same physical position as the beginning of the probe's exposure to the soil being measured. Therefore, a correction must be made for the distance which the signal travels inside the probe block. This offset is determined empirically. For the CS600 probe, the value is 69 mm (enter 0.069 in parameter 10), and for the CS605 and the CS610, the value is 85 mm (enter 0.085 in parameter 10). The offset for probes not obtained from Campbell Scientific must determined by the user. Contact Campbell Scientific for guidance. 6.3.11 Parameter 11: Input Location Input storage location for measurement result. If waveforms are chosen for output, this location is occupied by the first data point. 6.3.11.1 Reflection Waveform Values to Input Storage Saving reflection waveforms or reflection waveform plus first derivative using Instruction 119 requires manual allocation of datalogger input storage. The Insert Block function of the Edlog input location editor is used to allocate input locations for the waveform array header elements (see Table 6-1) and waveform values. The array header contains nine elements. The minimum number of locations that must be allocated is the Points value defined in Instruction 119 parameter 6 plus 9. An array size allocation of 9 more than the number of waveform points the minimum. 24