56
ERDC/CRREL TR-02-14
As a periodic performance check, a portable TDR system (consisting of a
Tektronics 1503 TDR) was connected to each probe, and the image was recorded
on a laptop computer. The 1503 TDR generated an impulse signal rather than the
step signal of the 1502 TDR. The change in pulse characteristic altered the
response. A dielectric interface (a new sediment layer) appears as an inflection
point in the image from the 1503 TDR, whereas it appears as a slight change in
slope in the image from a 1502 TDR (Fig. 36).
In conjunction with daily TDR monitoring of bed elevations, the data acqui-
sition system recorded 15-min-interval values of water depth (hydrostatically),
water temperature, and ambient air temperature. A 6-psi Druck pressure trans-
ducer with a 4-20 mA output was installed in a standpipe immediately adjacent to
the river and coupled to the river's water level. The resulting water pressure
converted to equivalent stage data is plotted in Figure 37. For instrument redun-
dancy a pair of thermistors was used to monitor the water temperature. The
thermistors were housed in a robust container placed below the water level at the
riverbank. An air temperature thermistor was placed in a housing attached to the
shaded side of the instrumentation shelter located on the site's north bank. The
water and air temperature data are plotted in Figure 38.
Determination of the distance between the probe top to the watersoil inter-
face at the bed is based on the propagation velocity of the TDR pulse in water.
Top of Probe
Bottom of Probe
Reflected signal using
Tektronics 1503
Reflected signal using
Tektronics 1502
Water-Bed
interface
Propagation Time
Figure 36. Comparison between the reflected signals using a Tektronics 1502 step pulse and
a Tektronics 1503 using the impulse signal.
Previous Page
